Born just before America’s entry into World-War II, the 6SN7GT dual triode had a precocious childhood, becoming one of the most common tubes by the end of the war. Despite competition from younger, smaller upstarts, the 6SN7GT was widely designed into consumer, industrial, and military equipment throughout the 1950s, and as a replacement tube was extensively manufactured in America until the end of its tube industry in the mid-1980s. Equivalents of the 6SN7GT were widely used in the USSR and China, and are still being manufactured there.

With the revival of tube electronics for audio, the 6SN7GT became a major player again, due to its availability, and, most likely, due to its inherently good audio qualities. Its high power dissipation compared to miniature tubes made it reliable and able to be run hard. The large internal structure compared to miniature tubes contributes to its low distortion. The existence of many variants and many manufacturers of the 6SN7GT over the years gives lots of types to play with – each with its own sonic fingerprint.

Audiophiles may have their own favorite dual triodes, ranging from exotic European types like the ECC40 to the rugged industrial types like the 5687. But the 6SN7GT is arguably the best general-purpose audio triode. Its large octal format is more rugged and less prone to intermittent pin connections than miniature type. Its mu of 20 is a nice compromise between the old low-mu types and low-current high mu types. The plate resistance is low enough to drive choke or transformer loads. The 0.6A heater current, while not as low as the 12AU7, is reasonable, and assures the cathode has plenty of emission reserve. The fact that it is still manufactured and that there are still lots of N.O.S. tubes available make it possible to design the 6SN7GT into modern equipment without fear of obsolescence.

To put the 6SN7GT into perspective, it does have some flaws. There is no shielding between sections, so unwanted interactions can occur. It is more microphonic that the better miniature types, so it can’t be used in very low-level stages. Most types don’t reject heater hum as well as some specific audio types, such as the EF86, 6J7, 12AT7, good 12AX7s, etc. It is rather large, compared to miniatures. The current-production Russian and Chinese types don’t sound as good as American or European ones from the 1940s, 50s and 60s. But, no other tube combines so many good characteristics.

A Note on Nomenclature: The tube type described here is often called the "6SN7". Technically, this is incorrect, since all glass American octal tubes released before about 1955 were followed by "G" or "GT" (plus optionally some other descriptors). An octal type without a G or GT was a metal type, and there were never any metal 6SN7s. In the 1950s, the "GTA" and "GTB" types replaced the regular 6SN7GT. Unless the special characteristics of the GTA or GTB types are important to the discussion, the name "6SN7GT" will be used throughout. Interestingly, some European companies made American equivalents, and often named them simply "6SN7", both for convenience, and more likely, since they didn’t care about the subtleties of American tube naming conventions.

6SN7GT History

According to Ludwell Sibley’s Tube Lore, the 6SN7GT was registered with the RMA on March 3rd, 1941, with data from RCA and Sylvania. RCA was likely the developer, since it had assigned a development number of "A4273B". The immediate predecessor of the 6SN7GT was the 6F8G, developed by RCA in 1937. The 6F8G was a dual version of the 6J5, which was a slightly "hotter" version of the 6C5 of 1935. The 6F8 had the larger "G" (or ST-12) bulb with the troublesome grid cap for one of the triodes. The compact straight-sided GT "Bantam" types, introduced by Hytron in 1938, had become the rage by the early 1940s, and the 6SN7GT was the natural evolution of the unwieldy 6F8G. Another predecessor, at least in nomenclature, was the 6N7, a dual triode intended for push-pull class-B operation. It really wasn’t an electrical equivalent to the 6SN7GT, though, because of its high mu (35) characteristics, high plate resistance, and common cathode connection.

What’s with the "smoked" glass? Most 6SN7GTs made through the early 1950s had a graphite coating on the inside surface of the glass bulb. This served to drain excess accumulations of electrons that could affect the operation of the tube. Why this was dropped isn’t clear, but probably was due both cost-cutting and the use of a different type of glass that was not prone to electron accumulation.

Sylvania and Raytheon came out with a loctal version, called the 7N7, which, interestingly, was registered in 1940, well before the 6SN7GT. Primarily due to the non-acceptance by RCA (the Microsoft of its time), the loctal line, though technically superior to the octal types, withered away in the late 1940s.

The 6SN7GT came just in time for America’s involvement in World-War II. The heavy usage of the 6SN7GT wasn’t in conventional radio equipment, but rather in radar. Radar had many pulse-handling circuits where the low plate resistance and compact footprint of the 6SN7GT was ideal. Most of the wartime American radar sets used dozens of 6SN7GTs. By the way, many 6SN7GTs made during the early years of the war were marked only "VT-231", the Army Signal Corps designation.

The glut of war-surplus parts after the war included incredible numbers of 6SN7GTs. Just as the war wound-down, two brand-new industries became heavy users of dual triodes: electronic computers and television. 6,550 out of the 18,800 tubes in the ENIAC computer were 6SN7GTs. Early televisions had up to six 6SN7GTs each. And, of course in audio applications, from movie theater amplifiers to mixing boards to P.A. amplifiers to the early HiFi amplifiers, the 6SN7GT was widely used.

By the end of the war, it became clear that the regular receiving tube construction and manufacturing procedures were inadequate for military use. The military wanted both ruggedness and high reliability, as well as tightly-controlled electrical characteristics. One of the first military types was Sylvania’s 6SN7W with its unique metal shell base. (Note the lack of "GT"!) This had a stiffening rod added to the otherwise conventional structure. By the mid-1950s, most tube manufacturers made a 6SN7WGT type with the extra stiffening and often thicker micas, as well as a low-loss micanol base. The last versions were basically receiving tube construction that were tested and sorted to tighter specifications.

In 1948, RCA came out with the "Special Red" line of ultra-reliable tubes. Rather than just a slightly modified receiving tube, the Special Red types were designed from the ground-up with high purity materials, extra rugged construction, premium processing, and thorough testing. The 5692 was the Special Red version of the 6SN7GT, and was made by RCA and CBS-Hytron. Only the RCA tubes used the red base. Although the construction is impressive, the audio quality is nothing special. Don’t forget – ruggedness does not correlate to good sonics!

Several variations of the 6SN7GT were developed by RCA. A 12 volt heater version, the 12SN7GT came out in 1941 and permitted use in 12 volt vehicles and in 0.3A series-string sets. This was later upgraded to the 12SN7GTA, to match the power dissipation increase in the 6SN7GTA. Some mobile and aircraft radios during the war were designed to be run with a plate supply of 28 volts, eliminating the need for a dynamotor or vibrator high-voltage supply. However, conventional tubes behaved erratically at this low voltage. In 1946, the 12SX7GT was introduced as part of a series designed for 28 volt supplies. These were the same design as the conventional types, but were specially processed and tested for low-voltage operation.

The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though. The combined maximum plate dissipation was upped to 7.5 watts in 1950 in the 6SN7GTA. In 1954, the 6SN7GTB added controlled heater warm-up time to make it more reliable in series heater-string TV sets. In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT. The 6CG7 was unique in having a shield between the two sections. This was removed as a cost-cutting measure in the 6FQ7.

The 6SN7GTB continued in high production in America as a replacement tube. As the tube companies started dropping out one-by-one, production was concentrated in fewer factories. The last known American production was the 6SN7WGTA by Philips ECG (formerly Sylvania) in 1986. Production of 6SN7 types in Japan and Western Europe also wound down by this time. The 1980s were the "dark ages" for tube technology – it was considered hopelessly obsolete, and the production machinery and tooling was simply scrapped. A replacement market still existed, and some 6SN7GTs came out of the USSR, marked as "Made in Holland" or "Made in England" to get around cold-war import restrictions.

Although the 1980s were the dark ages for tube production, new interest was growing in tubes for high-end audio amplifiers. Enthusiasts started searching for N.O.S. tubes at ham radio swap meets and at garage sales. Then China opened up to the west and the Berlin wall fell. Both Russian and Chinese tubes, still in production for military purposes, started to come into the Western markets. As the demand for audio tubes rose, the former cold-war factories concentrated on the popular types, and the 6SN7GT was one of them. Most new 6SN7GTs are from Russia or China, although some are made in Eastern Europe as well.

Bibliography

Barbour, Eric, "6SN7: Driver of Choice", Vacuum Tube Valley, Issue 11, Spring 1999. Extensive description of the history and different variants of the 6SN7.

Sibley, Ludwell, Tube Lore, privately published, 1996. An excellent compendium of American tubes, especially the obscure ones.

THE CATHODE RESISTOR METHOD

This is the method that is best for hobby techs and do-it- yourselfers. It is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. It is performed by reading the cathode current through each power tube. The cathode current is composed of the plate current *plus* the screen current. Plate current can be identical on two tubes (tubes are matched by plate current readings) while one tube is drawing more screen current; with this method, the readings will appear to indicate a mismatch when such is not actually the case. Since the cathode current will always be higher than the actual plate current, the readings obtained with this method will tend to make you set the tubes a little colder than your calculations will indicate that they are. This promotes slightly more conservative operation, which is beneficial to tube life. If you feel that the inaccuracy is significant, or if you’d just like to know how large it is for your particular amp, you can always read the voltage drop across each screen resistor, figure the screen current (I = E/R) and then subtract it from the cathode reading. Note that these instructions assume that your amplifier is biased by applying a negative voltage to the control grids; cathode-biased amplifiers cannot be adjusted other than by changing the value of the cathode resistor(s) so this method does not apply to them. BE AWARE THAT THE ACCURACY OF THE RESULTS YOU OBTAIN WITH *ANY* BIASING METHOD WILL BE DIRECTLY AFFECTED BY THE QUALITY OF YOUR TEST EQUIPMENT, AND YOUR SKILL IN USING IT. If any part of the following instructions doesn’t make sense to you, seek help from someone with more experience. Tech support is available from NBS Electronics AFTER 1:30 PM Denver (Mountain) time, Monday through Saturday. The number is 303-778-1156; ask for Lord Valve.

WHAT YOU WILL DO:

• Replace the ground wire on each power tube socket with a 1-ohm resistor.
• Read the voltage drop across this resistor (in millivolts) with your DMM.
• Read the plate voltage.
• Use the above readings to calculate the static dissipation wattage.
• Adjust the bias to obtain the best tone, while keeping the tubes within specifications.

A SUGGESTION: You may want to practice taking these readings and making these adjustments with your old tubes still in the amp, or with a spare (used) set. That way, you won’t fry your new tubes if you make a mistake.

On some sockets, the pins are numbered on the bottom (terminal) side; it is sometimes difficult to tell which pins the numbers go with. The best way to tell which pin you are looking at is to count clockwise from the notch on the locator “keyhole” in the center of the socket, with the first terminal clockwise from the notch being pin ONE. This assumes that you are looking at the sockets from the BOTTOM, or UNDERSIDE.

Most guitar amplifiers use output tubes which have the same (or very similar) basing. (”Basing” refers to the order in which the internal elements of the tube are connected to the pins on the bottom of the tube.) The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. are common guitar amplifier power tubes, and are all easily biased with this method. If you have an amp which uses power tubes which are not listed above, you will need to consult a spec manual (the RCA RC-30 Receiving Tube Manual is one of the best) for the basing, and adjust these instructions accordingly.

You’ll need a 1-ohm resistor for each power tube in the amp. All of the tubes listed above have their cathodes on pin EIGHT, which will be grounded to the chassis. On some amps, such as Marshalls, pin ONE will be tied to pin EIGHT, and both will be grounded. On older Fenders, pin ONE is usually used as a tie-point for the 1.5K grid-stopper resistor, and the negative bias voltage will be on this pin. DO NOT GROUND PIN ONE ON A FENDER AMPLIFIER, or you’ll get a big surprise. (Expensive, too.

REMOVE the ground wire from pin EIGHT on each tube, and REPLACE it with a 1-ohm resistor. On older Fenders, the ground wire is a piece of copper braid; unsolder it from the socket pin but *don’t* cut it off where it attaches to the chassis. Solder the resistor to pin EIGHT, and attach the free end of the resistor to the ground braid you unsoldered from pin EIGHT. Repeat this for all of the power tube sockets. I prefer to use 2-watt resistors because they have thicker leads which will take more abuse, but half-watters will work just fine. The accuracy of your measurements will be directly related to the tolerance of these resistors; precision 1% (or better) types are suggested.

A WORD ABOUT TAKING READINGS: It is vital that your probe tips make *good* contact with the pins you’re attempting to read. Tube-socket terminals often have a residual coating of non- conductive flux on them, and it is necessary to push the probe against the terminal hard enough to break through this coating. Most test probes supplied with today’s meters are fairly blunt; if you can come up with a set of “insulation piercing” test probes, these will help solve this problem. Don’t go overboard when pressing the probes against terminals, however…the probe tip may suddenly slip off the terminal and slide down against the chassis while the side of the metal prong is still touching the terminal. This will result in a dead short from that terminal to ground, and if you’re reading plate or screen voltage the resulting spark (and loud popping noise) might make you jerk back reflexively, pulling the chassis off your workbench and into your lap, injuring you or breaking your tubes. BE CAREFUL! Also be aware that amp chassis surfaces are usually dirty or corroded, so the advice above goes double for touching a probe to the chassis (ground). You may even want to take a small flat file and scrape a nice shiny spot on the chassis in a convenient place, to aid you in making a good ground contact.

Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of any power tube socket. You should see a negative voltage in the -35 to -50 volt range if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s, 6L6s, or KT66s. KT88s, 6550s, KT90s, and KT100s can have bias ranges that go as high as -100 volts. Amps which use 6V6s will usually have bias supplies which produce voltages that are similar to EL34 amps…but not always. Note that you should *not* have any power tubes installed in your amp yet.

First, locate the bias trimmer. (Possibly a little square blue thingy with a screwdriver-adjust slot in the center, or a round black thing that stands on three legs, or, for an old Fender, a full-sized pot with a screwdriver-adjust slot on both sides; newer PCB-type Fenders use three-leg horizontal trimpots, if they have a bias-adjust pot at all.) Next, adjust the bias control until you have MAX NEGATIVE voltage on pin FIVE. (In other words, rotate the bias trimmer until you obtain the highest negative voltage that the bias supply is capable of delivering.) Note that on some really old amps, the bias supply may be controlled by the standby switch; if you see *no* negative voltage on pin FIVE, you may have one of these amps. This is a poor design; see a tech about having the bias supply moved to the “hot” side of the standby switch. Install your tubes (the amp is still on STANDBY, remember) and wait a few minutes for them to warm up. Take the amp off STANDBY and make sure your DMM is still set to the highest DCV scale; take a reading between the chassis (ground) and pin THREE on any power tube socket. Remember, the BLACK probe always goes on the CHASSIS. Write this voltage down; you’ll need it later.

Now, set your DMM to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor(s). (This reading can be interpreted directly in milliamperes, because one millivolt across one ohm equals one milliamp. Ohm’s law says so, and you ain’t gonna argue with *that*, are ya? It’ll be pretty low, because you have the bias trimmer set to max neg voltage.

Adjust the bias trimmer (”pot”) until you get a reading across the 1-ohm resistor(s) somewhere in the 30-40 mV range, for everything but 6V6s or EL-84s. For 6V6s, you’ll want to start out at around 20 mA and work upward from there. Note that the polarity of this reading is unimportant; only the numerical value means anything. (If you put the black probe on the side of the resistor that is grounded to the chassis, you will get a POSITIVE reading.)

MULTIPLY the voltage you read on pin THREE earlier by the reading you just obtained from the 1-ohm resistor. (Example: 450 Volts times 35 milliamps, or .035 Amperes.) This will give you the STATIC DISSIPATION WATTAGE at which the tube is operating. (It’ll be wrong, but more on that later.) The above example gives a static dissipation of 15.75 WATTS, which is well within specs for an EL34 (fairly cold, in fact) or a 5881/6L6. See TABLE “A” (at the end of this article) for suggested static dissipation wattages for most of the common octal-based tubes discussed here. To sum up what this calculation is, PLATE VOLTAGE times CATHODE CURRENT equals STATIC DISSIPATION (IDLING) WATTAGE. It is important not to exceed the tube manufacturer’s specification for this parameter, because tube life will be shortened. At extreme settings, tube life will be measured in MINUTES…be advised.

Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the plate voltage will drop somewhat. Multiply this new reading by the value you measured across the 1-ohm resistor(s); this will give you the idling (static) wattage. The cooler you run the tubes, the longer they’ll last. If you dig the way the amp sounds when the tubes are idling at only 12 watts, fine…don’t worry about it. (6V6s, though, will be running fairly *hot* at 12 watts.)

Remember, each time you adjust the bias control, you’ll have to take a new reading from BOTH the 1-ohm resistor *and* the plate (pin THREE) and multiply them to see how hot the tubes are running. You can play your guitar through the amp each time you adjust the bias, and see how you like it. You can even adjust the bias by ear, and then take readings as outlined above to see if the tubes are being operated within their ratings. If you find that you only like the tone when the tubes are operating near their limits, you may decide to trade some tube lifetime for the tone you seek. If you like the tone with the tubes running cold, you’ll obtain significant extra tube life that way. It’s *your* call.

If you see a few milliamps difference between the readings on the 1-ohm resistors, don’t sweat it; this could be due to poor matching (not a factor if you bought ‘em from *me* , differences in screen current between the tubes, or differing leg impedances in the output tranny’s primary. (All of those things are fairly common in guitar amps.) Note that for an amplifier which uses four (or more) power tubes, balance between the two sides is more important than having identical readings from socket to socket. You should add the readings for each pair; if the left pair is close to the right pair, things are fine. If the left pair reads 32 and 34 milliamps (total = 66) and the right pair reads 35 and 31 milliamps (total = 66) then you’ve got a nicely balanced output stage, even though some of the tubes are running slightly hotter or colder than others. Having the currents balanced on the two legs of the tranny helps eliminate 120 Hz power-supply ripple from the output. Note that you can swap the tubes around to obtain the best current balance, since you can take individual readings on each socket. If you see a large difference between them (say, 8-12 milliamps) this means you need to find out why this difference exists. One thing you can do is SWAP the tubes into the opposite sockets and take new readings. If the bogus readings are consistent on the SOCKETS, then you’ll need to look at the circuitry to find out the cause. If the readings MOVE with the TUBES, you can be fairly sure you have a poorly-matched pair/quad.

Once you have everything adjusted to your taste and you’re sure the tubes are being operated within specifications, leave the amp fully powered up for three or four hours. Eyeball the tubes every fifteen minutes or so, to make sure the plates aren’t turning red. You are doing this to let the tubes “settle” into their new operating con- ditions; at the end of the settling period, take a final set of readings to make sure everything is still OK. If any readings have drifted significantly, readjust the bias accordingly. Note that the incoming line voltage directly affects all of the voltages in the amp; you may want to read the line voltage occasionally to see if this is happening. Line voltage will drop a bit around supper time (lots of juice being used for cooking) and also after sunset. If the line was 120VAC when you completed your biasing procedure and it’s 117VAC when you take your final readings after the settling period, expect to see a corresponding small drop in your measurements.

You may decide to purchase a “bias-probe” type device; this is a gizmo which consists of an “interruptor” socket/plug assembly that goes between the tube(s) and the amp’s socket(s). This test adaptor will have a couple of testleads hanging out through a hole in the side, for connection to your meter(s). If you do get one of these, there is no need to install the 1-ohm resistors on the tube sockets as outlined above. You can use the readings obtained from the adaptor sockets in place of the readings normally taken across the one-ohm resistors. BE AWARE THAT THERE ARE TWO TYPES OF THESE ADAPTORS COMMONLY AVAILABLE. One type *breaks* the cathode connection, and instructs you to connect the testleads to the CURRENT jacks on your meter. The other type contains a one-ohm resistor in series with the cathode pins, with the testleads connected to either side of the resistor; this type instructs you to connect the testleads to the VOLTAGE jacks on your meter. It has been my experience that some amps (especially old Marshalls) do not react well to having several feet of wire inserted in series with the power tube cathodes, and will oscillate like crazy. Therefore, if you decide to get a set of these test adaptors, get the ones which use an internal 1-ohm resistor.

REMEMBER…THERE ARE VOLTAGES PRESENT INSIDE EVEN THE SMALLEST TUBE AMPLIFIER WHICH WILL KILL YOUR ASS JUST AS DEAD AS A HAND GRENADE WILL!! If you’re not familiar with high-voltage safety, seek guidance from someone who is. BTW, an oven mitt or a pot-holder (real men like me use welding gloves) will come in handy for handling hot power tubes if you need to switch sockets; you don’t want to let the tubes cool off too much while you swap them before taking new readings.

THE OUTPUT TRANSFORMER SHUNT METHOD

This is the way many pro techs measure plate current. A *good* quality DMM is required for this measurement. (When it comes to good DMMs, you have three choices…Fluke, Fluke, and Fluke.) This section assumes that you know a bit more about your amp, and how to use your testgear. If any of it is unclear, DON’T TRY THIS.

NOTE… Marshall amps have output transformers which have a very low DC resistance in the primary winding. If your meter’s internal current-measuring shunt resistor is a relatively high value (~ 10 ohms, for instance) it will induce significant error into a transformer shunt measurement. This is because when such a meter is connected in parallel with half of the output transformer’s primary, a significant portion of the current is not flowing through the meter, and can’t be read. For this reason, unless you’re *sure* you have a meter with a low internal current-sensing resistor (~ 1 ohm) the shunt method is *not* recommended for use on Marshall (and other low DCR) output transformers. Fairly good results can be achieved on Fenders, though.

WHAT YOU WILL DO

• Read the current flowing through each leg of the output transformer’s primary.
• Read the plate voltage.
• Use the above readings to calculate the static dissipation wattage.
• Adjust the bias to obtain the best tone, while keeping the tubes within specifications.

For this particular reading, you’ll need to change your test leads to the CURRENT input jacks, and select the 200 mA DC range. The two probes are applied to the center tap and either of the ends of the output transformer’s primary. (On a Fender, for instance, the center-tap is RED, and the two plate wires are BLUE and BROWN. On a Marshall, the center tap is usually BROWN, and the plate leads are usually RED and WHITE.)

On some amplifiers, the easiest way is to put one probe on pin THREE of either socket (or of either of the two sockets on each side) and the other on the center-tap, which will be located at some distance from the socket. Some amps (like the Marshall JCM 900 series, for instance) have all the wires soldered to terminals on the bottom of the output transformer, conveniently sticking up right where you can reach them.

The current that would normally flow through half of the transformer’s primary winding is “shunted” through the meter, and thus measured. A small amount still flows through the part of the winding you are shunting, but the transformer’s resistance is much higher than your meter’s internal resistance. (See “NOTE” above.) Nearly all of the current flows through the meter.

SAFETY ADVISORY

BE WARNED…for all practical purposes, a meter set to measure CURRENT is equivalent to a STRAIGHT WIRE. This means that as soon as you touch either probe to the high voltage circuitry, THE OTHER PROBE NOW CARRIES THE SAME VOLTAGE. If you drop the probe and it lands on your arm or leg, you could be electrocuted. If it lands on the chassis (or anything else that is at earth or circuit ground potential) a huge spark will be generated, along with a noise like a small firecracker. (Please don’t ask how I know this. The probe tip will be partially melted, and at the very least, the meter’s internal fuses will blow. At worst, the meter will be history. Shorting the HV to ground isn’t especially good for the amp either, and may blow the amp’s fuse or damage the circuitry. You can easily kill a rectifier tube this way. BE ESPECIALLY CAREFUL NOT TO LET A PROBE SLIP OFF A TERMINAL AND HIT THE CHASSIS WHILE YOU ARE TAKING A READING! BE *EXTRA* CAREFUL TO MAKE SURE YOUR FINGERS DON’T SLIDE DOWN THE PROBE AND COME INTO CONTACT WITH THE METAL TIP!! And make DOUBLE DAMN SURE you know which two points in the circuit you are supposed to touch the probes to, because if you accidentally touch the bias supply and the plate supply at the same time, you won’t *believe* what happens. IF YOU’RE NOT *SURE* WHAT TO PROBE, *DON’T* PROBE IT!!
Once you’ve obtained the current readings from both sides of the output transformer’s primary, you’ll need to take a plate voltage reading so you can calculate the static dissipation wattage (as outlined above in the CATHODE RESISTOR method) and decide whether you need to increase or decrease the plate current. Note that if you are using the OPT shunt method with an amplifier which uses more than one tube per side on the transformer, you will need to divide the current reading on each side by the number of tubes used. Example: you read 88 mA on one side of a Twin Reverb’s output tranny; that’s 44 mA per tube, since there are two on each side. (4 total.)

REMEMBER TO REMOVE THE TEST LEADS FROM THE CURRENT MEASURING JACKS, AND TO SET THE METER TO THE HIGHEST DC VOLTAGE RANGE BEFORE YOU TRY TO READ THE PLATE VOLTAGE!! If you attempt to read the plate voltage with your meter still set up for a current reading, the results will be spectacular (as outlined above.) Since you may need to take several plate CURRENT and several plate VOLTAGE readings before you are finished setting the bias, you will need to be extremely vigilant about changing the meter settings (and the test leads) each time you take the different readings. Most pro techs use TWO METERS for this procedure, leaving one set up for current and one for voltage. (I use a handheld meter for the voltage reading, and a bench meter for the current.)

Once you have the necessary readings, the procedure is the same as for the CATHODE RESISTOR method: read, multiply, listen, adjust, read, multiply, listen, adjust, read, multiply, etc. Don’t neglect the “settling” period, either. BE CAREFUL!!

TYPES OF (FIXED) BIAS CIRCUITS

Many amps which use “fixed” (negative grid) bias have provisions for adjusting the negative grid voltage upward or downward. Making the grids LESS negative will cause MORE current to flow through the tubes. Some amplifiers don’t have a bias-adjusting control (pot) but instead use a fixed resistor to set the voltage. If you encounter one with a fixed resistor, the best thing to do is convert it to an adjustable type. Most of the time, the fixed resistor will be in parallel with the bias capacitor; the lower this resistor’s value is, the lower the bias voltage will be. If you can locate and identify this resistor, you can replace it with a simple network consisting of a (lower value) resistor in series with a potentiometer. What you’ll be shooting for is a range of adjustment that goes from LESS voltage to MORE voltage than is set by the (existing) fixed resistor. Take the value of the fixed resistor and divide by two; pick the closest standard value to your result, and put it in series with a pot which is as close to the original resistor’s value as you can find. Example: the existing resistor is 33K; use a 15K resistor in series with a 25K pot to replace it. The original resistor was 33K; you now have the ability to adjust the value from 15K to 40K. This should provide you with sufficient adjustment range to set any plate current you wish. If not, use a different value pot or resistor.
Some amps have a “balance” type bias adjustment, which allows you to vary the negative grid voltage between the two halves of the output stage; this makes a “matched” set of tubes less crucial to good performance, although it can’t compensate for tubes that are wildly different. If you encounter this circuit, the easiest way to adjust it is to simply “tune” the control for minimum 120Hz hum on the output. This type can be modded to the *best* type, which is not only variable from side-to-side, but adjustable up-and-down, too. Usually, this circuit will have the “balance” pot’s wiper connected to a resistor which is grounded at the other end. You can replace this resistor exactly as outlined above (half the value, add a pot, etc.) and have the best of both worlds.

If the simple mods outlined above (and the reasons for making them) don’t seem perfectly clear to you, DON’T TRY THEM. A schematic (and the expertise with which to interpret it) will go a long way towards helping you do them correctly. You can have the mods performed by a tech, and then do your own biasing from then on, if you wish.

TABLE A

Suggested MAX static dissipation wattages for common guitar amplifier tubes. You can exceed these (although I wouldn’t do it with a 6V6) at the cost of some tube lifetime. The colder you run ‘em, the longer they will last. Remember, as long as you don’t run the tubes hot enough to damage them, there are *no* rules about how much current to set them for. If you like the way your amp sounds when your 6L6s are only pulling 14 watts, bully for you… you probably won’t need to retube it for 10 years. I know that many sources for biasing information just specify plate (or cathode) current settings; telling you to bias your 6L6s at “35 milliamps” is nonsense. Unless you take the plate voltage into consideration, a current specification is meaningless. For instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts is 20 watts… TWICE as much. In both cases, the current is the same. Amps vary; two identical amps can have plate voltages which differ by as much as 20%. Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC, don’t expect to see that voltage when you take a measurement. TAKE the reading, don’t assume the voltage will be as specified. Trust your meter. Most of these suggested MAX wattages have been arrived at through my own experience. NOTE THAT THESE FIGURES ARE NOT TARGETS, BUT MAXIMUMS. This is important. You are not looking to set your static plate dissipation to the values listed here, but to set it at a level which produces the tone you are looking for without exceeding them.

KT88, KT90, KT100 can be treated as 6550s, although all three of these tubes are supposed to be able to take more current. The ultimate test is to view the tubes’ plates IN THE DARK, after they have been powered up for 15-20 minutes. If you see any red spots, back the current off a bit. One exception to this is the NOS 6V6; some of these will show a slight red “stripe” down the center of the plates even when they’re set fairly cold. I’ve seen them run for years in this condition. *Large* red blotches, or even the entire plates turning red, is what you want to watch out for.

Entire contents copyright Lord Valve, 08/24/01

Essentially, you want to bias your amp such that the plate dissipation rating of the output tubes is not exceeded. Plate dissipation is measured in watts (but don’t confuse this plate dissipation rating with the audio output power of your amplifier, which is also measured in watts). By Ohm’s Law, power in watts is given by the following:

P = E * I

So, in order to determine the actual plate dissipation of a given output tube, you multiply voltage by current. In this case, E represents the actual plate voltage, and I represents the total current draw of the tube. But, although we can easily measure plate voltage, we need a way to determine how much current a tube is actually drawing.

In order to determine the actual current draw of an output tube, we can introduce a resistance in series with that tube. Because current is common in a series circuit, the current draw of the resistor will be identical to that of the tube. But we can easily determine the current draw of the resistor. By Ohm’s Law, current in amperes is given by:

I = E / R

In this case, E represents the voltage found across the resistor, and R represents the value in ohms of that resistor. We can easily measure the voltage across this resistor. And, if we make the value of this resistor 1 ohm, the total current becomes E divided by 1, or simply, E. Since the resistor is in series with the tube, the current drawn by the resistor is identical to that of the total current drawn by the tube.

Putting this all together: in order to determine the actual plate dissipation of an output tube, we need to solve P = E * I. E is simply the plate voltage, and now we can determine I by simply measuring the voltage across resistor R.

Here is an example. The maximum plate dissipation rating for the 6L6GC is 30 watts. Suppose I want to bias my amp such that my 6L6 output tubes run at a conservative 20 watts. Suppose also that the plate voltage of my amp is 400 volts. Recall that P = E * I. In this case, 20 = 400 * I, so we simply solve for I. A quick calculation gives 0.050. So, I simply adjust the bias of my amp such that the voltage across R reads “0.05″ to reflect a total current of 50 mA.

In order to implement this method in your amplifier, you’ll need to install a 1 ohm, 1% resistor between ground and the cathode of each of your output tubes, as illustrated in the diagram below. Although I have heard of people using 1/2 watt resistors in this position, I recommend using a higher wattage device.

For reference, here are the maximum plate dissipation ratings for some popular output tubes:

• 6L6GC / 5881 — 30 watts
• 6BQ5 / EL84 — 12 watts
• 6CA7 / EL34 — 25 watts
• 6V6 — 14 watts
• 6550 — 35 watts

This is a recipe to construct some very simple diy speaker cables. I use these cables with my DIY Hi-Vi 3-Way Speakers. The following materials were used for these simple DIY Speaker Cables:

The materials for this project are available from Parts Express. Add banana plugs or spades of your choice to terminate the wires.

Photograph 01: Parts for DIY Speaker Cables

Photograph 02 below shows a test weave for the speaker cables. The idea here was to use multiple 16 gauge wires to lower resistance. Weaving the wires helps reduces asymmetrical field interactions since the wires are not on either the inside or outside of the cable more than any other wire.

Photograph 02: Test Weave of Speaker Wires

Once I was happy with the test weaves, I put together a short Test Speaker Cable.

Photograph 03: Short Test Speaker Cable

The pictures below show the final speaker cables.

Photograph 04: Simple Home Made Speaker Cable

Photograph 05: DIY Speaker Cable

After much frustration I found someone on the Internet to help me with some specifics on building one. At least a good starting point. I want to thank P.G. Doynov who helped me greatly with his expertise. He gave me a good starting point on building my output transformer and answered many questions that I had on the subject.

This man knows his stuff and has a good supply of rare tubes and transformers and other valuable tube related components, for anyone who is interested.

With just under $5 worth of materials and an evening of work I was able to successfully build my output transformer. I wanted one for putting into my Fender Deluxe guitar amp (5D3) clone that uses 2 6V6 outputs tubes in push pull. This amplifier is about 15 watts.

Above is the schematic of the output transformer. The primary wire could be from 0.16 to 0.18 mm in diameter, enameled copper wire and the secondary can be 0.69 to 1 mm diameter. Mr. Doynov told me that thicker wire will result in better bass response but it will depend on if you can fit it on the core OK or not. I used the dimensions in the above drawing and it had good bass response for a guitar amp in my opinion. I used a core that was 1″ square in “E” and “I” laminations.

The Winding Plan

Following is how I wound my transformer. This is something that I am just learning, so I know there are many improvements that can be made and I am sure that I will eventually write up a sequel to this article as I learn more about it and gain more experience. I first want to point out that in winding the transformer it is very important that things are isolated well as if things are done in an unsafe matter not only can it ruin your amplifier, it can also kill you!

I started with a plastic bobbin that was made for the 1″ square core for this transformer. Although interleaving is recommended in output transformers, they are used less extensively in guitar amplifiers as compared to “hi fi” tube amplifiers. Interleaving is where you wind part of the primary windings and then some of the secondary windings over it and then some more of the primary winding on that and so forth to where the primary and secondary windings are interspersed in sections within each other instead of just winding the primary windings all together and the secondary windings on top of that. Interleaving the windings is to help give the transformer a better high frequency response. In my transformer I did a minimum of interleaving. I first wound 66 turns of the secondary windings. I wound it neatly side by side and tight and then before I started the the next row I painted the windings with hot wax and then wrapped some very thin insulation paper around the layer before continuing the next layer. I got the thin insulation paper or capacitor paper at a transformer supply place. After wrapping a layer of paper tightly around the coil and gluing it together (kind of like rolling a cigarette) I started the next row of the secondary in which I was able to finish the 66 turns. I then ran the ends of the wires outside the bobbin giving them about a 6 inch length. (This will also be where I make my 4 ohm speaker tap. I was now ready to start the primary windings. Before starting the primary windings I put a layer of thicker transformer paper around the first section of the secondary windings. It is extremely important that this is done right and that the primary is insulated well from the secondary windings. I then started the primary windings which use wire that is a lot thinner. Ideally it is good to wind the thinner wire in a neat row but because of the crudity of my setup I didn’t get it perfectly wound side by side so I basically did the best I could and then after about 200 turns of the primary I then painted it with the hot wax and covered that layer with the thin paper before starting the next row. After 7 or 8 rows of the primary I got up to 1500 windings which is where the center tap of the primary coil is. I then ran a loop of wire out for the future center tap and then continued on with the next half of the primary windings. After the primary windings were done, I then wound the remainder of the secondary windings over that making another tap where the 8 ohm speaker connection will be. I then connected this in series with the first section of the secondary windings. After I was done all the windings I covered it again with the thick transformer paper. I then got each wire and soldered a different colored stranded wire to it as leads that will run out of the transformer. After taping them separately to the transformer to secure them individually and away from each other, I then put one more layer of paper around the whole thing to further secure the leads and to protect the transformer coils.

Above is an illustration of the top view of the coil that hopefully will make things clearer. Part of the secondary is wound first and then the primary is wound on top of that and then the rest of the secondary is then wound over the primary. The 2 secondary windings are then connected in series. This is a very basic interleave. (Important) Always wind each section in the same direction. If not, some coils will cancel out other coils reducing voltage.

My Simple Winding Setup

I made a simple but effective winding setup using a hand drill and a digital counter that I built. I like using the hand drill because I feel I have a fair amount of control as I can alter the speed of the turning at my own wit plus with my other hand control the tension. Although a more complicated machine could probably do it faster and probably more consistently, the hand drill works and I can wind things quickly. I got this idea from Mar Hammer for also winding guitar pickups. The thing that took time doing the transformer was not the winding but taking time to paint each layer with hot wax and cover each layer with the paper. The actual winding of a few thousand turns is actually quite easy and fast. I bought a hand drill on the street for under $10 that worked find. I did realize later the it is important to make sure the hand drill you get works smooth and that there is no wobble in the axle.

I simply screwed the hand drill to a board where I also mounted the digital counter I built.

Above shows my very crude assembly. The counter has 2 wires that go to a reed switch that is placed close to the chuck of the drill. The drill chuck has a little magnet glued on to it so that everyt ime the drill makes one turn the little magnet will pull on the reed switch making a contact and triggering the digital counter.

Here is the transformer bobbin in the chuck. I just glued a reed switch on the little piece of wood with a glue gun to hold it in place near the chuck. The bobbin is held with a little square block of wood pressed into the center and a thick nail with the head cut off in the center of the block of wood which is inserted in the drill chuck. The trick is to get the nail well centered and straight so that it will turn evenly while winding. The lead wires coming out of the bobbin are temporarily taped up out of the way while winding. It is important to test out the counter well before starting, to make sure that it is counting correctly and that there is no false triggering. I am sure that this can all be done better and that this method can be improved but it worked good for me and I am inspired about the possibilities.

Here is the digital counter section and circuit board that I just glued on the wooden board. I also have a reset button for resetting the counter. I didn’t take time to make it fancy in any way. Maybe later!

This is the same circuit that I showed in my pickup winding project that I also used for this project. This is a digital counter that can be duplicated for as many digits as desired. It is cheap to make and easy to find the parts. I made 3 digits on mine which was practical and enough for me. I don’t have a PCB layout for this but I just drew one up and surface mounted the parts. You can probably find some ready made perforated boards or other similar ready made boards that will probably work fine for making this. This is the basic circuit excluding the reset switch and the bouncless switch.

Here is the bouncefree switch that I used. This is essential in preventing any false triggering in the counter as there has to be a small amount of time between each pulse before it can be triggered. I used a .01uf capacitor for C1 to get about a max of 10 turns per second triggering properly. I don’t think that I would wind anything faster then that. I replaced the push button with the reed switch.

Here is how to make the counter reset to zero which is not indicated on the first circuit. All the pin 2 and 3’s of each 7490 IC should connect together.

Here is a suggested power supply for the Digital counter. I would use a transformer that is 500 mA or larger. I would also advise putting a small heat sink on the 7805 voltage regulator.

This illustrates how the magnet will close the reed switch every time the drill makes one turn. I glued on the magnet to the drill chuck.

The Core

Above shows the E and I laminations that I used. For a “Hi Fi” output transformers it is common that the laminations are assembled in such a way as to create an air gap in the core. This creates less distortion but causes a slight decrease in the output power of the transformer. I will explain more about this later but in this transformer that is used for a guitar amp, I laminated it without an air gap but by staggering the laminations as in most power transformers.

How Does It Sound?

I was very pleased with the sound when I tried it in my amp. I still have to try it compared to other transformers but I have no complaints in the way it sounded and worked. I plan to also install bell caps and supports when permanently installing it in my amp. I Played for a while loud and soft and visually I can see that the tubes were working properly and there were no apparent problems. The test will be to take it on a live performance for a longer period of time and see how it does. I consider this project a success and I am inspired about making and experimenting more. I would like to try and make another one using more interleaving to see how this might effect the sound.

There is a lot schematics on the internet, but finding a good one is pretty hard. I want to build something like that:

• Good power: 10 Watts seems to be a good starting point.
• Something simple. I want to build an amp, I can fix by myself. So I decided to build a Single Ended one. (Not a push-pull) Push Pull provide far more power, but need more components and harder to build.
• Low cost. The best issue for this kind on amp, is the price. You need to buy some output transformers, big power supply transformer and choke. And of course, when we talk about HIFI this cost a lot of money.
• Look’s good. Something which is important is the way it looks. So I will have to take really care of that.

For people who have never seen this kind a stuff here a little pic I ripped out.

Tubes

For this design, I decided to use some EL34. Check out DrTube[0] for additionnals data. They offer good output power for a decent price.
The reflected load permits to use a KT88 instead if this doesn’t provide enought power.

For the preamp, I think I gonna use a 6SN7 in SRRP. This provide a good gain, and very good linearity.

Output transformers

EL34 has a reflected load of 2KOhm, so I need to find coresponding transformers. Of course this cost a lot of money, Hammond HIFi (not guitar) output transformer for 20w cost something like 99$ each. And
even worst, I unable to find 2.0K only 2.5 are currents.

I found this one:

• EuphoniaAudio[1] (205$ the pair)
• Hammond[2] (99$ each)
• Electra Sud Ouest (86Euros)
• Elettronocanovarria[3] (75Euros)

Despite, Elett.. offer good price, I unable to buy them here, cause the vendor speack only italian.. and that sucks. Electra sud has a good price too, so I guess I will buy them here if I don’t find anything else.

Update: I found a matched pair of 2.5K / 20W transformators on eBay.

Power Supply

This part isn’t really clear for me right now. Singled Ended amps doesn’t need regulated PSU because they work in pure class A, so the current is fixed (throught the tube bias) but It need to be really stable to avoid distortion.I don’t want to buy a large transformer because they cost a lot of money and doesn’t feet with my needs. EL34 use a quite low voltage (~250v) but need quite a bit of current ~100ma. I think, a good isolating transformer can do the job. Something like 230*230 at 200Va doesn’t cost a lot of money.

Schematic

This is really a hard part. But I found 2 good example:

• first[4]
• and second[5]

To be continue…

Links

• [0] DrTube: http://www.drtube.com/tubedata.htm
• [1] Elettronocanovarria: http://www.elettronicanovarria.it
• [2] Hammond: http://www.tubetown.de/ttstore/index.php/cat/c18_Hammond-SE.html
• [3] EuphoniaAudio: http://euphoniaaudio.netfirms.com/ea/nfoscomm/catalog/product_info.php?products_id=28
• [4] http://perso.orange.fr/michel.terrier/radiocol/detail2004/ampli-el34-se-stereo.htm
• [5] http://web.mac.com/scress1958/iWeb/Steve%27s%20Tube%20Trials/EL34%20Amp.html

In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in several specialized applications such as guitar amplifiers (also called a valve amp outside the U.S.) and high power RF transmitters, as a display device in television sets and in microwave ovens.

Operation

Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.

The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal “plate” inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. In it’s simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction. A third element called a “control grid” can be added to the design which provides the ability to amplify a signal. Other configurations are also possible including the Pentode, a tube with 5 active elements providing an additional amplification factor. There are a large number of tube varieties and uses. This is only a very brief overview and we suggest consulting additional resources if you are interested in additional information. Some Information provided by wikipedia.org.

Diode	Triode

Why Use Tubes in Guitar amps ?

Most good guitar amplifiers use tubes rather than solid-state components. Why tubes ? The amplifier is a critical element in achieving the sound the musician desires. Tubes provide the tone that musicians want. Tube amps are warmer, richer and have a more desirable tone than solid-state amps. The distortion and speaker-damping characteristics of a tube amp with an output transformer matched to the speaker load is hard to replicate with solid-state devices. Tube amps are particularly popular with serious musicians. Many musicians prefer to play vintage Fender, Marshall and Gibson amps. Replacement tubes and transformers are readily available for these amps however there are many boutique amp manufacturers making new tube amps with a vintage sound.

Amplifier classes

Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device.

What’s a Class A Amp ?

In a class A amp 100% of the input signal is used. The amplifier is passing current at all times even when you are not playing. The instant you strike a note it’s immediately fed to the speakers resulting in a “fast” sound. Class A is very inefficient but usually gives very low distortion and is generally a better sounding amp at low volumes. Class A amps are often more expensive boutique amps. Some of our Divided by 13 amps are Class A.

What’s a Class B Amp ?

A class B amp uses 50% of the input signal. Class B is different from Class A in that there is no current flowing when the output is at idle and turn on from zero current when a signal is present. In a push-pull Class B amp design each of the output circuits produce one half the audio waveform with each circuit not producing any current flow when the other circuit is operating. Class B designs tend to have more crossover distortion and require a less beefy power supply. Many popular guitar amps use class B designs including Fender and Gibson amps.

What’s a Class AB Amp ?

As the name implies class AB amps exhibit some characteristics of class A amps and some of class B amps. In a class AB amp design, more than 50% but less than 100% of the input signal is used . If an amp uses class A mode for a portion of it’s output then has to apply additional circuitry for the remainder of it’s output then it is considered a class AB Amp. Class AB amps are also more efficient than a straight class A therefore does not require as large a power supply.

What is an Amp Transformer ?

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. It consists of a minimum of two coils, the primary and the secondary, wound on the same core. An alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits. This is useful in converting the voltage from a wall outlet, typically 120 or 240 volts, into a higher voltage required by tubes in tube amps . typically 400V or more, and a lower voltage for the tube filament, typically 6.3 or 12.6V. There are several transformers used in tube amps. Some information provided by http:// en.wikipedia.org /wiki / Transformer

Output transformer – An output transformer is used to match the low impedance of a speaker voice coil to the high impedance of a tube output stage. Output transformers consist of at least two windings, a primary and a secondary. Some output transformers have multiple impedance taps on the secondary side, to allow matching to different speakers, typically 4, 8, and 16 ohms

Power transformer – A Power transformer converts the incoming line voltage to a higher or lower value for use in the guitar amplifier. Typically, the power transformer will have at least one primary, but sometimes two or more, to allow use at 120V or 240V. In an amp the power transformer will generally increase the voltage to 400 volts or more for the tube plate. There will also usually be a 6.3V filament winding. There is also sometimes a 5V. winding for use with a tube rectifier.

Choke – Another term used for an inductor, most commonly an inductor used as a power supply filter.

Fender Transformer Chart

Dating Fender Amps – Using the Fender Tube Chart

Look inside the amp (but don’t stick you hand in there, even after being unplugged the amp may retain a dangerous electrical charge), there should be a tube chart on most amps. On this chart there is a hand stamped date code consisting of 2 letters. For example AD would be April 1990 and DG would be July 1954.

Dating Marshall Amps

In 1969 Marshall introduced a date coding system. Some of the older Marshall amps have an inspection sticker on the top of the chassis which usually has the day, month and year the amp was actually made or inspected. Here’s a chart with date codes for Marshall amps. Note, “A” Date Code ran for 18 months (July 1969 to December 1970) so the “B” date Code was never used and has been omitted. Use the serial number to determine the date code. The serial number is generally located on the back of the chassis but from 79 to 80 it was on the front panel. From 1969 to 1983 the date code was after the serial number. From 1984 to 1992 the model number was first, then the date code, then the serial number.

A=1969      A=1970
C=1971      D=1972
E=1973      F=1974
G=1975      H=1976
J=1977      K=1978
L=1979      M=1980
N=1981      P=1982
R=1983      S=1984
T=1985      U=1986
V=1987      W=1988
X=1989      Y=1990
Z=1991      Z=1992

Early Marshall Model Codes (approx Mid 69 to Late 83)

/A = 200 Watt
SL/ = 100 Watt Super Lead
SB/ = 100 Watt Super Bass
SP/ = Super PA
ST/ = 100 Watt Tremolo
S/ = 50 Watt
T/ = 50 Watt Tremolo

Later Marshall Model Codes (approx Early 84 to Late 92 )

A/ = 200 Watt
SL/A = 100 Watt Super Lead
SB/A = 100 Watt Super Bass
SP/ = Super PA
ST/A = 100 Watt Tremolo
S/A = 50 Watt
T/A = 50 Watt Tremolo
RI = Reissue

Early Marshall Date Code Example
EXAMPLE: SL/A 25353 E
SL/A = Model Code
24523 = Serial Number
E = Date Code
This amp would be a 100 Watt Super Lead 1973

Later Marshall Date Code Example
EXAMPLE: S/A S 24523
S/A = Model Code
S = Date Code
24523 = Serial Number
This amp would be a 50 Watt 1984

Common Tubes used in Vintage Amps and modern replacements

The vacuum tube as amplifier

All tube types except diodes have the potential to work as amplifiers. A voltage change on the control grid will cause a corresponding change in the plate current, so basically, the tube is a voltage amplifier, but the input voltage signal is changed to a current signal. The trick when building an amplifier is then to utilize this current signal in a suitable way, either by using it directly or by changing it back to a voltage signal.

Early methods

The first tube amplifiers used transformer coupling. A transformer basically handles current, so by letting the plate current pass through a transformer, you utilize the current signal very well.

Another advantage of the transformer is that the winding ratio can be made to adapt the signal to the next stage; if the next is to be another amplifier tube, the transformer can be a step-up transformer, effectively giving up to 10 times extra gain (higher ratios tend to run into various practical problems). If the next “stage” is a loudspeaker, on the other hand, a step-down transformer can be used, matching the low impedance of the loudspeaker. Once radio technology came out of infancy, tubes became better and cheaper, so in AF amplifiers, the transformer coupling only lived on in the output stage, where low impedance loudspeakers would otherwise have been difficult to drive with the relatively high-impedanced vacuum tube. In RF amplifiers, transformers are simple and cheap and not only have they stayed the tube era out, but they live on in even the most modern solid state circuits. We will return RF amplifiers later.

Resistance coupling

Transformers for AF are bulky and expensive, so as the gain-factor of tubes became better, the resistor coupling became the method of choice for most AF amplifiers and even some RF amplifiers. A resistor is inserted in the plate circuit instead of the transformer. As the grid signal changes the plate current, obviously, the voltage drop over the plate resistor will change accordingly, giving raise to a voltage signal on the plate. This signal will be in opposite phase compared to the grid signal; when the grid signal voltage swings in the negative direction, the plate current will fall, thus the voltage drop over the plate resistor will be smaller, making the plate voltage rise, whereas a positive grid swing will increase the plate current and lower the plate voltage.

The plate signal will have a rather large DC component: It will swing around an average plate voltage, typically at around two thirds of the B+ voltage. To keep this voltage from entering the next stage, we use a coupling capacitor, or in older terminology, a blocking capacitor (because it blocks the DC, but lets the signal pass). Obviously, unpleasant side effects may occur if this capacitor fails and shorts out, especially if the next stage is a transformer coupled output stage: The output tube will get a positive bias making it draw an extreme amount of plate current, which may in turn overload and damage or destroy any or all of the following parts: Output tube, output transformer, rectifier tube(s), mains transformer. Some restorers of old radios for this reason prefer to replace all coupling capacitors as a preventive measure.

A look at the details

The values in the schematic are for a stage using Ѕ 12AT7 (ECC81). At 100V plate voltage, this tube will draw 3mA plate current with a grid bias of -1V, so to make the stage autobiasing, we need a cathode resistor R2 of 330 ohms. The decoupling capacitor C1 needs to have an impedance comparable to R2 at the low frequency limit, lets say at 20Hz, thats
1 / 2p *300 * 20 ~ 33MF (so this will be an electrolytic). The grid return resistor R1 is there because electrons leaving the cathode will occasionally hit the grid and if they are not led away from the grid, a negative charge will accumulate, eventually cutting off the plate current. This resistor just needs to be some large value, usually 1meg is used. The plate resistor R3: The setup with the bias used calls for the tube to have a plate voltage of 100V, and if we assume a B+ of 150V, this means that the voltage across R3 must be 50V, and at 3mA, that gives us 17k (standard value: 18k). A configuration like this, with about 1/3 of the B+ across the resistor will give a near optimum signal swing (when saturated, a vacuum tube has a considerable voltage drop over it, so a 100V swing is all we can hope for). Finally, the coupling capacitor C2 needs to have a low impedance compared to the input resistance of the next stage at the low frequency limit. Assuming 1meg and 20Hz, we get 0.01MF (easily within polyester range).

So, what gain can we expect from this stage? The conductance for the 12AT7 at this bias point is 5.5mA/V, so a 1V signal will ideally give a 5.5mA change in plate current. We might expect this to result in a 99V signal across the 18k, but life is not like that: A triode basically acts like a resistor with its value controlled by the grid. If this were an ideal resistor, we would expect it to be 100/3=33k, but the plate voltage has an influence on the resistance of the tube, so the dynamic impedance of the tube is only 16.5k (according to the data sheet). This impedance has to be calculated in parallel with the 18k plate resistor, giving 1/(1/18+1/16.5)=8.6k, so the actual voltage swing on the plate will be 8.6*3=26V, giving our stage a voltage gain of 26. Then there is the load of the next stage, but 1meg load will not make a world of difference.

Output stage

As I mentioned, one place the transformer coupling has stayed in use is in AF output stages. The signal impedance of even a powerful output tube is still in the kiloohms range, but loudspeakers typically have impedances of 4-8 ohms. The transformer is perfect for this, and at the same time provides for a safe isolation between the voltages inside the set and outside circuits like external speakers.

Above is a full diagram of a typical single-tube output stage cabable of delivering 5-6 watts of output power. Here, as mentioned, the transformer is virtually indispensable. A stage like this is biased for a quiescent current of half the full swing; this is called class A, a naming convention that has led some people to assume that such a stage is in some way better than a stage that runs in class B (discussed later in this article). We find another electrolytic here, again at the cathode resistor. If this capacitor shorts out, the stage will draw a lot of current, possibly with the effects described earlier, but the more common fault for an old electrolytic is that it dries out and looses most of its capacity. In this case, a negative feed-back occurs over the cathode resistor, which will reduce the amplification in the stage considerably, probably resulting in a low output volume.

A look at the details

Let’s start at the input: The grid return resistor R1 is empirically set to 560k (a big grid produces more return current), so the coupling capacitor C1 ends at 0.02MF. R2 is there to prevent high frequency oscillation (in spite being built for AF, this tube will work well into the shortwave band), the 1k value is again empirical. The cathode resistor R3 is 135ohms to give us a -6.5V bias at 48mA, this leaves C2 at 100MF. The output impedance of the stage with this bias is 5.2k. This value is equivalent to the load value we calculated for the triode stage (internal resistance in parallel with an imaginary plate resistor), and gives an output power of 5.7W, according to the data sheet. If the speaker impedance is 8ohms, the winding ratio of the transformer is the square root of the impedance ratio: 26.

With no signal, the full B+ (250V) will be at the plate since the DC resistance of the transformer is low (in the order of 200ohms). 5.7W in 5.2k is 172V RMS, 243V peak, so with a positive signal swing at the grid the plate voltage swings very low, but what with a negative grid signal? Well, the plate current will now start to fall from the 48mA, but the inductance of the transformer will make the plate voltage rise above B+ to nearly 500V, so the full voltage swing at the plate is a teoretical 500V PP! Whether the tube is actually able to swing down to a few volts is perhaps doubtfull, but th upward swing is a real thing, and now watch out: If the load is larger than 8ohms, as when the speaker is open, the voltage will simply rise till the power can be dissipated SOMEWHERE. I have seen the output transformer of a 2W amplifier arch to the iron core of the transformer. So contrary to what you might expect, underloading can be more damaging to this stage than overloading. An overload, like a 4ohms speaker, will just reduce the voltage swing at the plate, and we won’t get optimal power, but no damage will be done. If the amplifier is built with a high negative feed-back, the situation can be different.

More power

If we want more power, we could use a bigger tube, but the quiescent DC current in the class A stage poses some problems: To cope with the DC magnetization, the transformer has to have a relatively large core; in effect, we only use half the operating range of the transformer core, since it is only ever magnetized in one direction. Also, the power stage always draws power equivalent to full output, and this is really ineffective, since even if we run it at full volume, the average output power in music or speach will be considerably lower than the sound peaks. So a big amplifier will dissipate a lot of unneccessary heat. Instead, we can choose to run two tubes in a so-called push-pull stage, where each tube takes care of one half of the signal:

Right away, you might expect this stage to be able to handle twice the power than the one above, but because the two 6BQ5 tubes dont need to negotiate the high quiescent current, they are actually able to deliver up to 17W. Note the center tab on the output transformer: Not only is the quiescent current lower, but the bias current to the two tubes cancel each other out in the transformer. As a result, the transformer for our 17W amplifier need not be much larger physically than the one for the 5W class A stage. This class is called a class B stage, signifying that the quiescent current is at a level considerably lower than the mean signal bias. The tubes need two input signals, in opposite phase, so there has to be a driver stage that supplies this. There are several methods of doing this, they are discussed below.

Details

The obvious way to get a split-phase signal is of course to use a transformer:

This kind of construction can be seen in old sets, but again, transformers are relatively expensive, and also their frequency transfer charateristics are less than ideal, so there are good reasons to try and avoid them if possible. Interestingly, this configuration later had a revival: Transistor output stages used it extensively during the first 15 years or so of transistor radios. But for tube sets, other methods came in use. Some amplifiers use a simple phase splitter stage:

The cathode- and plate resistors are the same value, so equal signals, but in opposite phase, are found on cathode and plate. Note the voltage divider to the grid: Since the cathode resistor is far larger than in a normal self-biasing stage, the grid must be “lifted” to obtain a suitable bias. There is no overall gain in the stage, so a previous stage is usually needed to provide the neccessary gain. Also, as the output impedances of the two outputs are different, some imbalance can occur, especially at the extreme ends of the frequency range. This next driver construction, while slightly more complex, is better balanced:

The idea is simple; one stage is a normal amplifier, the next one is too, but a voltage divider at the input lowers the signal back to the level before the first stage, then it is amplified back up, and the phase is inverted. Note that the stages have no decoupling on the cathode resistors: This gives us precise control of the gain which will simply be equal to the ratio between the cathode resistor and the plate resistor.

Finally, the cheapie:

The idea is, of course, that if we feed signal to one output tube, there will be a nice inverted signal on its plate, which can then be fed to the other tube. This stage is inherently poorly balanced, because if tube no. 2 gets too much signal, the stage will oscillate, so to be on the safe side it has to be underbalanced. Obviously, the only advantage of this configuration is its simplicity, and it is probably quite rare, but does exist.

Bias considerations

As mentioned, the bias conditions for push-pull stages are normally called class B, but most tube amplifiers really run in what modern amplifier designers would classify as AB. The reason is that the bias is fairly high compared to class B solid state stages. This means that cross-over distortion is not too big a problem, but it is also a product of neccessity: Normally, the stage will be self-biasing, so to back the quiescent current way off, you would need a quite large cathode resistor, so the normal 15-30W radio or HI-FI output stage will run with a bias current of about 25% of max current. If bias is to be lowered to the 2-5% level of modern amplifiers, a negative supply is normally needed.

RF amplifiers

One type of RF amplifier found in nearly any receiver is the IF amplifier:

Obviously, this is really a transformer coupled stage. RF transformers are simpler and cheaper than their AF counterparts because they have fewer windings and the bulky iron core can be replaced by ferrite cores or nothing at all. Also, most RF amplifiers are tuned, so the transformer coil naturally becomes part of the tuned circuit. Note that a pentode is used: In a triode, the internal capacity between the grid and the plate acts as a parasitic feed-back path, and because of the phase transitions around the resonant frequency of the tuned circuits, the stage will have a strong tendency to oscillate. The screen-grid in the pentode disconnects this capacity, and makes the stage inherently stable.

At high frequencies, pentodes become a problem. Not only do they have a higher noise level than triodes, but the more complex tube system has to be physically larger, and this limits their performance at high frequencies. So here we are stuck with the triode, and the inherent instability if the triode stage must be overcome in another way. Take a look at this:

This configuration is called grounded grid, for obvious reasons. What controls the plate current is the cathode-grid voltage, so instead of holding the cathode at a fixed voltage (effectively grounding it) and feeding the signal to the grid, it is possible to ground the grid and feed the signal to the cathode. The grounded grid effectively screens the plate from the signal input, which is now the cathode, and thus the stability is vastly improved. The draw-back is that the stage has a low input impedance, but since we are using a transformer coupling this is not too big a problem. The impedance matching in the stage shown is achieved by feeding the stage from a tab on the input coil. The grounded grid configuration is widely used for RF stages at all frequencies above some 50mHz. For the real high frequencies, special triodes exist that can work effectively well into the gigahertz range.

The low input impedance makes the grounded grid configuration unfit for AF amplifiers, except for the special uses, where the low input impedance is desireable, like dynamic microphone inputs, etc. To get the best of both worlds, the cascode configuration can be used:

This set up, at the expence of an extra triode system, has the high input impedance of the grounded cathode stage, an excellent input-output separation, and a high gain, making it very well suited for tuned amplifier stages. It is rare in commercial receivers, not only because of its complexity, but also because it is difficult to AGC regulate.

Special amplifiers

We have looked now at two basic amplifier couplings, grounded cathode, and grounded grid. A third exists: Grounded plate:

As you can see, the plate is of course connected to B+, not to ground, but signal-wise, B+ and ground are the same (to signify this, one of the B+ filter or decoupling capacitors is shown), so what happens is that no signal occurs at the plate, instead the signal is taken from the cathode. The max voltage gain from this stage is 1, but it has a high current gain, or, in other words, it has a low output impedance. The output impedance is much lower than the cathode resistor, because a high negative feed-back exists within the stage. Any change in output voltage because of a load will change the cathode-grid voltage and be countered by a current change, so the output impedance equals the cathode resistor divided by the gain factor of the tube. The input impedance, on the other hand, is very high: The grid return resistor may be 1meg, but it is not referenced to ground, it is referenced to a point close to the cathode, where there is almost the same signal as on the input. Therefore the input impedance is equal to the grid resistor multiplied with the ratio between the two cathode resistors, if we assume it is in this case 10, it will give us an input impedance of 10megs. More sophisticated input back-off or bootstrap schemes can be employed taking the input impedance into hundreds of megs. The grounded plate, or cathode follower circuit is used whereever you need a high input impedance and/or a low output impedance.

We can couple a cathode to another and get this, the differential amplifier:

The circuit has here been drawn symmetrical, and what really happens is that the two tubes divide the current from the cathode resistor between them. At small signal levels, this current can be considered constant, which means that what really gets amplified is the difference between the two grids. Plate signals are in opposite phase. The signal between the two grids is called the differential mode signal, any signal that is the same on both grids is called common mode, and common mode signals are suppressed, how much depends on how well the stage is balanced and on how close the cathode current is to a constant current. The differential amplifier has high input impedances and can have medium to high gain. The circuit is mainly used in instrument circuits, but can also be seen as driver for push-pull output stages.

Finally, still mainly for instumentation purposes, DC coupling

These days, complementary solid state devices have made construction of DC coupled amplifiers a breeze, actually most solid state AF amplifiers are largely DC coupled, if only to save capacitors, but back in the tube era, getting that zero hertz lower frequency limit was a rather complex undertaking. Here is a simple two-stage DC amplifier:

There is really no getting around a B- supply here, and since the signal partly refers to B-, it has to be a good one, so often stabilizing tubes are used. I have assumed a -90V B- here, as it can easily be stabilized with a neon lamp. The trick about it all is the voltage divider, which gets us back to an appropriate level for the second stage. This of course costs us some gain, as the signal is also divided. The capacitors around the voltage dividers are there to compensate for various stray capacities, most prominently the cathode-grid capacity of the second tube, which would otherwise form a low-pass filter together with the resistirs, and lower the upper frequency limit considerably. The capacitors are quite small, in the MMF range and basically inversely proportional to the resistors, but the exact values will have to be determined empirically, because of the influence of stray capacities. Properly built, this amplifier will be linear from DC to over a megahertz. The method of transferring a DC signal by means of a voltage divider to a negative supply can be applied to most amplifier configurations.

There are quite a few interesting details: We are using pentodes, because they have better gain potential than triodes. The first tube is auto-biasing, this allows us to have the grid at zero potential. The cathode resistor is of course not decoupled, since we cannot decouple for DC, anyway we want control over the gain and we want the linearity from the negative feed-back it offers. The next tube also has a cathode resistor although we could easily bias that via the voltage divider that transfers the signal, but we also want the negative feed-back here. The screen grids need 120V for the bias point we are using; to get that from 250V, we would normally just fit a 56k resistor, which would get us close enough with the 2.4mA drawn by the grid, but as the screen grid cannot be decoupled, our gain would be reduced, so we need to add some stabilizing. A really expensive amplifier would have a separate B+ for this, but here we just use a voltage divider with some extra current running through it to get a more stable screen grid voltage. In a real application, some bias adjustment would be needed to trim away the DC offset that will invariably occur. This amplifier has a total gain in excess of 250, and some negative feed-back might be employed to improve linearity and frequency range.

Guitar amplifiers should always be tested before purchase (most music stores will let you play around a little) to make sure they are compatible with your guitar and produce a good clear sound. If there are any doubts in the music store, don’t’ buy it! Some amps will sound better with certain brands of guitar; bring yours in with you to be extra sure of your purchase.

Finally, guitar amps are all about volume. What’s the point of playing if no one can hear you? As with sound quality, try out a high volume on an amplifier before you commit to buying it. If the sound goes crackly, it’s not right for you. If you were to go on stage at any point, you’d need to push the volume right up to be heard over the rest of the band, and crackling speakers are out. As a simple rule, test out an amplifier before you buy it. If it gives you special effects, controlled sound and good volume, you’ve got yourself a great amp.

Six JAN (joined army navy) tubes, after some 40 years of rest, finally make music…

I started this project with the idea to build a simple single-ended tube amplifier. I didn’t have any experience with the construction of tube amps so I was looking for a beginner’s project. Such a project should be simple (for easy debugging) and it should also be cheap. Nobody likes to blow up a component of 300 dollars by a small lapse of attention…

As far as simplicity is concerned, nothing beats the “Darling” series of amplifiers which are described on the website of New Jersey resident Bob Danielak 1. They employ power triodes type 1626 which can be obtained cheaply from various sources (I bought several for 3 US dollars apiece). Old readers of this page may remember that the 1626 was employed in various transmitters for the American airforce, e.g. the AN/ARC-5 series (BC-457, 458, 459 and 696). Since they were used in military airplanes, 1626s are rugged valves which can stand shocks, vibration and electrical insults. Once I had excessive power dissipation in two 1626s because of a wiring error. The plates became fiery red and blazingly hot and remained in this condition for two minutes before I discovered my mistake. Yet these tubes still sound and measure OK. Apparently, Darling amplifiers make excellent projects for dummies like me!

I decided to make Jeremy Epstein’s DC-coupled variant of the Darling amp with two parallelled 1626s per channel 2. The schematic is shown below:

For my first try, I followed Jeremy’s schematic to the letter, although I used a GZ34 rectifier in stead of a 5U4G. The Hammond 125E output transformers, 1.5 H choke plus the 8532 and 1626 tubes were from Antique Electronic Supply in Tempe, AZ. A NOS power transformer (Prova) supplied 2 x 345, 5 and 6.3 Volts. I acquired this tranny and the GZ34 rectifier from Frits Meuris Electronics (Sittard, Holland). The 12.6 Volts for the 1626 heaters came from an obscure transformer from my junkbox which I bought for one Dutch guilder from Conrad Electronic.

I built the amp in the old style, hardwired on an aluminum chassis in a wooden frame with all tubes, chokes and trannies on top and all electrical wires below. Special parts were not used. I took what was in my junkbox (carbon resistors and styroflex capacitors from the former DDR, electrolytics from Czechia, wirewound resistors from Vitrohm in Germany). After I had corrected my wiring error, all voltages were within a few percent of Jeremy’s values and 1 kHz square waves looked OK. No trace of microphonics. So on for the listening tests!

After a burn-in period of a few hours, I listened to live recordings from twentieth-century classical music (Schцnberg Ensemble), using the multimedia speakers which are described elsewhere on this website. The amp sounded wonderful, as if you were in a very good seat in the concert hall. I was really excited about the “SE magic”. But after a while I became disappointed. The amp sounded excellent with some programme material (chamber music, small combos), but it didn’t sound good at all with large orchestra, grand piano, church organ, or big drums. The presentation seemed then “anaemic”. Especially piano recordings sounded terrible. Measurements indicated what was wrong:

The frequency response of this version of the amplifier was 75Hz…14 kHz (-3 dB). With orchestral music and grand piano, I was missing the lower two octaves. For Jeremy this didn’t cause problems, for he employs his own Darling as the treble amp in a bi-amped system. But as a stand-alone amp, the results were not satisfactory.

Jeremy thought the cheap Hammond transformers were limiting the bandwidth. So I decided to try more expensive output trannies. I replaced the Hammonds with Lundahl LL1664s from Sweden which I purchased from Aqua Blue in Belgium. The (almost fivefold) difference in price certainly resulted in improved specs (see below).

With the Lundahls, the frequency response was flat from 8 Hz…55 kHz (-3 dB). This was a major improvement! Grand piano now had weight and sounded like the real thing. Sibilants were somewhat emphasized in the Hammond 125E version, but sounded natural with the Lundahl LL1664.

So I was very happy for a while. But the perfectionist bug hit me once again. IMHO, the amp still had a major flaw. There was audible hum in the output. With the multimedia speakers, the hum was not objectionable during the reproduction of music. But in silent passages, it became annoying. Additional measurements indicated that the signal-to-hum ratio was only -55 dB with shorted inputs!

First I thought that I had made a grounding error, but this proved not to be the case. Then I thought: maybe I should use DC heating. However, there was no improvement when I hooked up the 8532 heaters to a DC power supply. I noticed that the hum had a frequency of 100 Hz rather than 50 Hz. I concluded that it originated from the B+ supply.

After running some simulations on Ben Duncan’s PSU designer program, I decided to modify the B+ circuit. After the GZ34, I maintained the 47 uF electrolytic capacitor. The stereo channels are connected to this cap via individual LC filters (10 H chokes and 200 µF/500V electrolytics). This modification resulted in a 29 dB improvement of the signal-to-noise ratio. The capacitive load to the GZ34 remains within safe limits. And hum and noise are now completely inaudible.

Specifications of our modified Darling:

Output power: 2 x 1.5 Watts in 8 Ohms

Voltage gain: 8.6 x

Input sensitivity: 400 mV

Frequency response: 8 Hz…55 kHz (-3 dB)

Signal-to-noise ratio (guesstimated): -84 dB (related to 1.5 W output)

This is an excellent amp. It sounds lively, agile, and natural. Makes you tap your feet and sing along with the music.

Verdict after two years of use: We listen daily to this amp on the large Jericho horns, so you can guess that we are pleased with its sound. However, the NOS power transformer (Prova) suddenly broke down (catastrophic damage to the insulation of the B+ windings) and we had to resurrect our killed Darling, using a custom-made transformer (purchased from Kent Electronics). We now used a metal chassis (Hammond) and we replaced the 1000µF 250V electrolytics with 47µF 400V polypropylene capacitors (Audyn-Cap, Intertechnik). This slightly improved the sonics. With the electrolytics, the sound could occasionally be somewhat “hard”, with the foil caps this never happens, not even in difficult passages. This tube amp sounds sweeter than any of my solid-state products. Finally, we added volume controls (100k log). Now, the amp can be driven directly by line-level sources (CD players, FM tuners). A preamp is no longer necessary.

Finally, a picture:

Darling (front view). A protective cover was mounted over the transformer after the testing phase.

References

¹ Bob Danielak, “Darling” and “DC Darling” SE 1626 amps”

² Jeremy Epstein, “My experiments with Bob Danielak’s ‘darling’ 1626 amplifiers”