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.

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

I will instead concentrate on the more basic functionality of output transformers. Power transformers are pretty basic and the things that are necessary (read “basic knowledge.” There is more but not necessary to fully know to make a good power supply. This is covered in the power supplies section) to understand about them is turns ratio and power handling (for example, the voltages of the secondaries versus the current).

However, in audio circuits, a good understanding of the two types of output transformers is necessary to make a good choice for the best sound and efficiency.

WHAT IS A TRANSFORMER?

As the word suggests, a transformer transforms one thing in to another. In this case we are transforming one level of voltage and current to a different voltage and current. With that we are also transforming one impedance to another. Before I go on it is necessary to remember that transformers work with AC only. Nothing will happen if a constant DC is used. The AC induces a varying magnetic field into a coil of wire that also reinduces a current into the wire. This is known as self inductance. However, the coil can also induce a current into another coil.

A transformer is made using many windings of copper wire that is coated with acrylic to allow for insulation while allowing the wire to be as close as possible to itself in the winding. This only makes one coil of the transformer. One needs a second coil of wire with properties (number of windings) that make it convert the first coil’s characteristics. In other words, if the first coil has 5000 windings, and the second coil has 1000 windings, then it is said that the transformer has a turns ratio of 5 to 1 (5:1). That means that if I put an AC voltage of 100 volts on the first coil, also known as the primary, then the induced voltage on the second coil, also known as the secondary, when placed next to the primary will have a voltage induced to it that is 1/5th the size, or 20 volts AC. This is known as loose coupling.

The thing that allows transformers to conduct from one coil ot the other is the sharing of flux. This is known as mutual inductance. The air is the conductor of magnetic flux in this case. One can only get a certain amount of power this way. Air has a permeability (allows magnetic flux to conduct) of 1. Iron, on the other hand, has a permeability of about 1000 or more (steel goes up to about 5000). If I were to place a piece of iron in the center of the coil, then the lines of flux will be concentrated (because more magnetism can conduct easier through it, like the path of least resistance) and more power could be transferred to the secondary. What happens here is that more flux lines from both coils share the same path, or both coils have common lines of flux. This is what coupling means. If I make that iron core common to both, then more power can be transferred still.

Old style radios (in the 10’s and 20’s) used two separate coils connected by a “U” piece of iron as the common core. But there is an even more efficient configuration that is now used that also makes the transformer more compact. It is the practice of winding the secondary coil directly on top of the primary (although there are still some transformers that have two separate coils that use
a common core). This allows the lines of flux to cut through the secondary directly and more instantly. This is called unity coupling.

So now we know about turns ratio and its direct connection to voltage transfer. But in the voltage transfer there is an interesting thing that occurs. Current also changes. If I put that 100 volts into the transformer of 5:1 ratio the voltage drops to 20. But if that 100 volts came in as 1 amp, the secondary can provide up to 5 amps! This is the conservation of power rule that transformers must follow. Of course, this is not exact, since there are losses that occur in transformers, as will be discussed later. This applies to all transformers and there are of course a few more factors that come into play.

For example, a transformer rated at 12.5 volts output at 100 milliamps only means that the wire is thinner and there are many more windings. BUT the turns ratio is still maintained. So the primary of 125 volts only drains 10 milliamps, because the turns ratio is 10:1, so the input current is 1/10th of 100 milliamps.

IMPEDANCE IN A TRANSFORMER

There is a reflected impedance back to the primary that occurs from the secondary and the source, in this case speaker and the tube. That is why many say that there is a better damping from a triode than a pentode, because the transformer reflects the triode’s plate resistance A.K.A. its impedance. So we do not need to worry too much about that. However, the reflected impedance from a speaker as a load does vary nonetheless because speakers are not constant impedance loads and can vary from less than one ohm to more than 40 ohms. Ideally though the primary impedance is constant and is proportional to the turns ratio by the formula:

Zs/Zp = (Vs/Vp)^2

This states that the ratio of output impedance to input impedance is the same as the ratio of output voltage to input voltage squared. So, if the impedance ratio changes, the voltage ratio changes by a small amount.

For example, let’s use the common impedances of 5K for the primary and 8 ohms for the secondary of a single ended amplifier. Lets use 320 volts for the power supply. Now assuming typical values for the output tube, lets assume that 20 volts will be dropped across the tube/cathode resistor during normal full output swing. So we have 300 volts peak to peak, which translate to 150 volts peak. Multiplying 150 by .707 we get the RMS voltage at the primary. This comes out to 106.5 volts RMS. With this info we can substitute the variables and derive the secondary voltage with impedances as they are. (We will assume that the plate resistance is high enough not to affect the primary impedance too much so we use the 5K as is. Real world values will vary due to plate resistance and load). So we now plug in the known values to the formula and do some algebra to arrive at the secondary voltage (sqrt means square root):

8/5000 = (Vs/106.5)²

sqrt(8/5000)=Vs/106.5

[sqrt(8/5000)] X 106.5 = Vs

0.04 X 106.5 = Vs

4.26 = Vs

Now that we have the secondary voltage out we can check this by plugging all the now known values to the
original formula:

8/5000 = (4.26/106.5)²

0.0016 =  (0.04)²

0.0016 = 0.0016

From this we can see that if a four ohm speaker were put into the eight ohm output the primary impedance will change by the proportions involved. Lets see how that works, assuming that the voltage ratio’s don’t change, since the voltage ratio is directly proportional to turns ratio.

So we have the known values 4 ohms, 106.5 volts and 4.26 volts (this gets hairy!):

4/Zp = (4.26/106.5)²

sqrt(4)/ sqrt(Zp) = (4.26/106.5)

sqrt(4) = (4.26/106.5) (sqrt(Zp))

sqrt(4)/(4.26/106.5) = sqrt(Zp)

2/0.04 = sqrt(Zp)

50 = sqrt (Zp)

2500 = Zp

So, interestingly it seems that the proportion one would expect occurs when the impedance of the secondary changes. A direct proportion occurs. Going from eight ohms to four ohms (halving) actually lowers the primary impedance by the same factor, namely in half. This keeps the power transfer the same, because halving the primary impedance causes the effective current to double. But reflected impedance could still go as low as 625 ohms if the speaker impedance goes down to 1 ohm for a given frequency.

This is pretty much true for solid state also. Since the voltage levels are constant, the current is what changes. Good old Ohms law!

PARALLELING TUBES

So now, what if the opposite were true? What if I double the output tubes? Some have argued that doubling the output tubes have no effect on output power. Let’s take a look at this mathematically.

Let’s assume a plate resistance of 22500 (for a 6l6GC. My current pet output tube) in parallel we use the ole parallel formula. Actually it’s easy because it is merely half for two tubes. So we get about 11250. In parallel with the primary, though we get a different result. This is the formula for paralleling two unlike
values:

R1XR2/R1+r2

11250X5000/11250+5000

5625000/16250

3461

The Primary impedance now becomes 3461 ohms. So the reflected output impedance is about 5.2 ohms. Now, judging from this, what is the power? The voltage still remains at 106.5, so using a little ohms law we get a current of 106.5/3461, or 30.7 milliamps. Not a lot, is it. 106.5 times 30.7 milliamps is 3.2 watts. Not much. Let’s
see what it was originally.

With the above assumptions, namely the 6L6 (22500 ohm plate resistance) and a 5000 ohm primary we use the parallel formula and get a primary impedance of 4090 ohms! Pass 106.5 across this and we get 26 milliamps! So we have a power output of 2.7 watts!
Not a doubling of power, but an increase of half a watt, but an increase none the less.

However, it is noteworthy to add that I am calculating these levels based on an unbypassed cathode resistor in a cathode biased amp. Bypassing the cathode resistor decreases the plate
resistance, hence impedance.

So this puts to rest the argument that double the tubes gives double the power. However, in order to take advantage of the doubling of tubes, one needs to half the input or primary impedance of the transformer. Since power through a transformer must be identical on both sides (barring intrinsic losses), this is possible without increasing voltage. So assuming that both tubes combined allows 30.7 milliamps through at 106.5 volts each, the output power will be 6.4 watts. But this means that the primary impedance must be 2500 ohms. The turns ratio will be different, allowing for the current ratio. Current ratio is inversely proportional to turns ratio. In other words:

Np / Ns = Vp / Vs = Is / Ip

Where Np and Ns are the turns in the primary and the secondary respectively, Vp and Vs are voltages and Ip and Is are the currents. If we really want to go crazy, we could calculate this all the way (impedances, voltages, etc.), but I think you can use this as a practice example for yourself.

TRANSFORMER LOSSES

As I said before I will leave core and magnetism theory to the textbooks. But I will mention that the core materials, winding practice, and wire quality all come into play where sound quality is concerned. That is why there is (relatively) so much of a variety in manufacturers and within manufacturers as to the transformers cost and type.

One of the losses that occurs in a transformer is known as eddy currents. What happens is that the current that flows through the coils induces magnetic currents within the core material. This in turn induces a small current back into the coil. It is very similar to self induction. This however causes two of the problems. One is to lower the current transferred to the secondary, so this is a current loss, and it generates heat within the transformer. This is heat loss. This is one of the reasons why the transfer efficiency is 80-90%.

Another loss is stray capacitance. As with any conductor, if it runs close to another conductor in parallel, they will have a capacitance between the two. A transformers windings are many parallel wires. Then there is capacitance from primary to secondary, and from both to the core, which in most cases connected to the chassis, or ground. This limits high frequency response.

There is also a phenomenon called hysteresis. This is a delay that is caused by the time it takes for the core to magnetize and release its magnetism. Different formulations of iron or steel have different degrees of hysteresis. Unfortunately the formulations that have less hysteresis also do not perform as well. So a balance must be maintained. Hysteresis causes phase distortion in the low frequencies. This is not the same as the hysteresis used in some circuits to speed up their switching time. That is a form of positive feedback.

SINGLE ENDED AND PUSH-PULL TRANSFORMERS

The single ended transformer is an interesting beast. Talk about compromises! There needs to be a balance of power and frequency response here. Make a single ended transformer produce a full range, and one needs to sacrifice some power. Make an SE transformer for power and frequency response suffers. There have been some very good posts on the news group rec.audio.tubes about the technical aspects of this relationship.

The frequency response of an SE (low frequency) depends on the primary winding inductance. In order to get a large inductance one needs to add windings. This can lead to saturation easier, though. It also leads to a higher impedance and lower current. Lower power. To lower the ill effects, less windings are needed, but this reduces low frequency output. What a dilemma. So we need to focus on a happy balance.

In the SE design, one needs to take into consideration the saturation point of the transformer. If one biases the output tube past the midpoint, then they run the risk of saturating the core, causing distortion, similar to clipping. The reason for the precaution it because there is always a single direction DC current making the transformer into a single magnet. I am using the word single because in a push pull transformer there are two flows of current, but more on that later. So in essence we have a fluctuation of magnetism from midpoint to full to none, which corresponds to the midpoint flow of current, to full current to none. The same is the case with voltage. The saturation problem is minimized by several techniques. There is winding, core material, and a small gap. This gap is put between the I bar and the E portion of the transformer. (Fig. 1)

Then there is the reduction or elimination of the sweet sounding even order harmonic distortion. I have yet to find out why this occurs, except to surmise that the even order components are in phase, while the odd order components are out of phase. So the even order components cancel out in push pull while the odd order reinforce each other. I think in the strictest sense that the real effect has to do with the speed that the components reach saturation and cut-off as opposed to pure Fourier analysis, but that is discussed on the distortion page.

Here is a couple of posts by Mike LaFevre of Acrosound about some particulars of transformers. I think it is pretty interesting and in depth.

CONCLUSION

So, to SE or not to SE, that is the question. I personally prefer push pull, but I have made a SE amp and find it to have some desirable sonic qualities as well as surprising power.

There are quite a few manufacturers of quality power and output transformers available. Here are a few manufacturers and vendor of them. Note: they are in no particular order:

Manufacturers:

One Electron
Lundahl
Magnequest
Hammond

Vendors:

Antique Electronic Supply
Triode Electronics Corporation
Acrosound
Communication Jute

Designing a SE output transformer is the imposition of the transformer’s gods’ fury…. look at the formula’s for AC flux density vis-a-vis DC flux density. What we find is that if we want to keep the AC flux density low then (all other things equal) we would want a large number of primary turns. But, conversely, if we want the DC flux to be low (all other things being equal) we would want to decrease the number of primary turns.

In part of the design process for SE… you must calculate what your AC flux Consumption will be (at full power and lowest frequency of interest) and then subtract this amount from the maximum flux capacity for the specific core material that you are using. The amount left over tells you what the maximum DC flux could be…. if you wish to stay below the knee. And this, provided only as a one approach to design, assumes that your willing (or feel it smart) to use all of the flux available. And there are good arguments against this.

Another approach as explained in detail in Rueben Lee’s book on magnetics is that to maintain a high degree of inductive consistency… then your DC flux should be(I think it was… short of looking up the reference) 3 to 4 times the magnitude of your AC flux. And remember in a magnetic component with both an AC and a DC flux component the effective perm is a function of the absolute values of each as well as the relative (to each other)ratio of fluxes. As just one example… on one of our part’s (NLA in the DIY community btw) the DC flux was approximately 9850 gauss while the AC flux was approximately 3000 gauss. Rueben Lee demonstrates that if these conditions hold then the deviance in primary induction will be approximately 20% or so.

Push-pull coils can (and perhaps should be) mirror-imaged around a centerline (both geometric and electrical)… so that each half has the same leakage inductance and each half has the same capacity. And given the “equal but opposite” (at least in theory)nature of the two halves of a PP transformer the coil design and it’s geometry will be (or do I mean should be?) different than a SE output transformer design would be.

The SE output has one end of it’s winding at AC ground potential and the other end at “AC HIGH”…. for push-pull we have two ends at “AC HIGH” and (let us say informally) the center (the B+) is at AC ground. This is a fundamental difference…. if I am making this clear enough in my writing….. so if we follow the AC voltage gradients in a PP transformer we will find that they are “equal but opposite” in a mirror imaged way between the two halves of the coil.

SE outputs don’t offer us this “degree of symmetry” (or at least not on first blush)…. here one end of the winding is at AC ground and the other end at AC HIGH. So that as we traverse the coil, through each primary winding turn, the AC voltage potential is changing… at our interstices (junctions between primary and secondary windings) we then have certain voltage gradients… the gradients will be different across each interstice… and the capacities different. So if you have a PP output that was optimized for a different voltage gradient and a different voltage potential at the interstices… then using it as a SE device the unit will not achieve the intended optimization that the transformer designer had wished to achieve.

There is a way to do a SE output transformer as a quasi-symmetrical design…. wherein you design it as a PP coil geometry but then reverse wind one half of the primary and put it in parallel with the other half. Using this technique a design can achieve the same level of “symmetry” in terms of the capacities being equal about a coil geometric centerline….

POST 2

>Mike, please excuse my technical misunderstandings in advance. In your
>post, considering symmetric winding geometries for SE transformers, you
>described a technique of taking two primary windings, as used in a PP,
>reversing one winding, and connecting them in parallel. Question: are
>the two windings not bucking each other, if paralleled out of phase?
>If an ordinary PP transformer was reconfigured this way, would DC
>currents in the paralleled windings be nulled out?

Several of the responses to my post thought that the example used a bilfilar wound primary winding. Although perhaps it could, the illustration does not rest on it being bifilar wound. And the language I used was imprecise enough to lead to this confusion.

In the example…. what is happening is that you wind the whole primary twice… using three guages smaller than normal. for instance suppose the primary needed 2,000 turns of #30. then what you would do it wind 2.000 turns of #33 in P1 and P3 and another 2000 turns in P2 and P4. The example I had in mind derives from the Acrosound patent… and is a modification
of that patent in that P2 and P4 are reverse wound and then placed in parallel with P1 and P3.

Barry concerns about phasing… remember, and I always have to think this through and on some coils with tons more interleaving it becomes a real puzzle to keep the reverses straight…. when you reverse a winding the physical start becomes the electrical finish. And the reverse physical finish becomes an electrical start. so if you go through these reverses…. and their placement in parallel with the non reversed windings (say P1 an P3) then the electrical
polarities of the reversed windings… you’ll see (I hope) that they are not bucking….

>Question: Can an SE
>winding symmetrically configured give true symmetrical cancellation of
>stray reactances, as reflected to the secondary? Its not clear to me how
>symmetrical, out of phase components could be set up in an SE winding,
>in order to effect a true cancellation. I hope there’s a cogent question
>in here.

I used the term quasi-symmetrical moreso as an analogy to describe the physical configuration of the windings and their electrical polarities in response to a question about the capacitances within a coil and etc. In a single ended output…your right…. no differential phases (the push and the pull) are present…

So the strategy in illustrating what I called a quasi-symmetrical coil geometry in a SE output is to achieve a minimum number of different voltage gradients across the interstices of primary to secondary and to achieve a “sameness” of voltage potentials across these interstices. Wish you (and everyone who is interested in this topic) had a copy of the Acrosound patent and perhaps a diagram showing the winding sequences and etc. Then it would be easier to explain…

Just another note…. the illustrations of coil geometries should not be construed (on the basis of my choosing it as an illustration) as the method that I use on our own products. I actually use several different strategies for interleaving in our own products… but to fend off any misconceptions or speculations… I just wanted to put this disclaimer in here….

POST 3

I have been threatening Joe Roberts to do some more “Core Issues” in Sound Practices for quite some time and this is one of the articles that I would like to do.

Anyone have access to the old SP where I did an article on “how to pick a power transformer”? In it there was a discussion of current capability and how to evaluate or, minimally, establish a context for assessing qualitatively the numbers (the current ratings) which mfgr’s spec out.

Part of the problem in power trans is that the “current” number must be provided in some sort of context if it is to have meaning as a useful basis of discriminating btwn products. My article tried to show that to compare “raw numbers” from one company against “raw numbers” from another was often like comparing “apples to oranges” as opposed to “apples vs. apples”. My sense is that the same difficulty applies to especially the comparison of raw DC current numbers which manufacturers use for SE output transformers. But…. least I get a head of myself…

Steve, I don’t mind putting up some of my ideas and research on
“AC versus DC characteristics in SE and PP” as you stated above…. but I want to figure out how to structure the response so that it goes in some logical order. And I would be grateful for constructive comments and criticisms since I want to turn some of this stuff (that I have done over the past week or so) into an article for Sound Practices or Positive Feedback.

so give me a day or two or three or four or….. yep…. probably never get it done… so let me throw out a teaser and see if any of you RAT’s can “tell” me about this formula and what it means and what the variables are and what is unique about some of the variables… it is one of the building blocks of designing a SE output trans…. a focus and understanding of it…. leads, IMO, to clearer sight on some of the other issues that come after it.

energy factor equals L times (Isubdc) squared divided by core volume

core volume equals L times (Isubdc) squared divided by energy factor

L is primary inductance — unit measure is henries

Isub dc is the magnitude of dc current (and it is squared) — unit of measure is ADC

core volume is the volume of the magnetic core being used — unit of measure is cubic centimeters

Excerpt from Rec.Audio.Tubes newsgroup
By Mike LaFevre