Nov

24

How To Make Audio Output Transformers

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Basic calculations enough for building transformers with examples. (single ended amplifiers)

 

R
 loudspeakers impedance Ohms
N
 tube power in W W
Ra
 plate load impedance Ohms
Ia
 plate current A (mA)
F
 core area cm2
n1
 winding per Volt winding
Up
 AC on primary V
np
 primary winding winding
p
 prim/sec turns ratio  
ns
 secondary winding windind
I
 secondary current A
d1
 wire diametar - primary mm
d2
 wire diametar - secondary mm

 

WARNING: be careful of high voltage on primary!

1. Calculations for necessary transformer dimension (core area)

(1)

17.april 2000.  suggestion Damir S. - CroatiaBetter calculations for transformer dimension:
(EI core area)
F=20*SQTR(N/fd)

F = core area  (cm2)
N = tube power in W
fd = the lowest freqency that you wont to accomplish

 

2. Winding per Volt

(2)

3. Calculation for primary winding (np)

a.) Before calculation for np - calculate AC on primary (Up),

(3)

b.) and primary winding (np) is:

(4)

4. Calculation for secondary winding (ns)

a.) Before calculation for ns - calculate
primary/secondary turns ratio (p)

(5)

b.) and secondary winding (ns) is:

(6)

5. Wire diameter: dp (primary) and ds (secondary)
 

a.) primary: For  Ia  (tube data) - see  TABLE example: if Ia=50 mA -  wire diametar is  0,16mm or 0,18 mm with  isolation (row 11)

b.) secondary: current of secondary (Is)
 =

(7)

and see TABLE

example: if Is=1,3 A - wire diametar is 0,8 mm or 0,85 mm with isolation (row 31)

E & C core 


Isolation (gap) width highly effects final results
For that reason, specify the width experimentally (for a start: 0,2 mm )
Perform the necessary measuring with instruments (frequency response) and by listening !


Allways put a thin isolation material (resistant to high temperatures) between primary  rows.
Secondary is not that critical, but wouldn't hurt.
EXAMPLE 1e

Output transformers for triode 300B (plate voltage 400 V, DC gird voltage -89V)

Class A (single ended)

R ( loudspeakers impedance) = 8 ohms

N (tube power in W)= 11,5 W

Ra (plate load impedance)= 3000 ohms

Ia (plate current) = 50 mA

1.) F=2sqtrN     F=2 sqtr11,5 = 2 * 3,39 = 6,78 cm2

2.) n1=70/F      n1= 70/6,78 = 10,32

3a.) Up= sqtr(N*Ra)   Up = sqtr (11,5* 3000) = 186 V

3b.) np = Up * n1      np= 186 * 10,32 = 1919

4a.) p=sqtr(Ra/R)     p=sqtr( 3000/8 ) = sqtr375 = 19,36

4b.) ns=np/p      ns=1919 / 19,39 = 99

5a.)Primary wire diametar

For   Ia=36 mA (tube data) see TABLE ( row 11)

dp = 0,14 mm - with isolation 0,16mm

5b.) Secondary wire diametar
Is=sqtr(N / R) = sqtr ( 11,5 / 8 ) = sqtr 1,43 = 1,19 A

For Ia=1,19 A see TABLE  (row 31)

d = 0,8 mm - with isolation 0,85mm

EXAMPLE 2e

Output transformers for triode 211 (plate voltage 1250 V, DC gird voltage -80V)

Class A (single ended)

R ( loudspeakers impedance) = 8 ohms

N (tube power in W)= 19,7 W

Ra (plate load impedance)= 9200 ohms

Ia (plate current) = 50 mA

1.) F=2sqtrN     F=2 sqtr19,7 = 2 * 4,43= 8,87 cm2

2.) n1=70/F      n1= 70/8,87 = 7,9

3a.) Up= sqtr(N*Ra)   Up = sqtr (19,7* 9200) = 425 V

3b.) np = Up * n1      np= 425 * 7,9= 3363

4a.) p=sqtr(Ra/R)     p=sqtr( 9200/8 ) = sqtr1150 = 33,9

4b.) ns=np/p      ns=3363 / 33,9= 99

5a.)Primary wire diameter

For   Ia=60 mA (tube data) see TABLE (row 12)

dp = 0,18 mm - with isolation 0,19 - 0,2mm

5b.) Secondary wire diameter Is=sqtr(N / R) = sqtr ( 19,7 / 8 ) = sqtr 2,46 = 1,569 A   (1,6 A)

For Ia=1,6 A see TABLE (row 31)

d = 0,9 mm - with isolation 0,95mm



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