[MRCA] modification to the AM-20 military amplifier

[Patrick J. Jankowiak]

The following simple, invisible, and reversible modifications to the 
AM-20 amplifier are worthwhile. While the system is intended mainly 
for voice, a turntable and a radio input are there for a reason. 
Unfortunately, the amplifier does not reproduce music well.

1. Adding a 3.3 megohm resistor from pin 4 (the 250 ohm output) of the 
output transformer to pin 4 of V3B the 6SL7 phase inverter will 
provide a small amount of negative feedback to improve the sound 
quality of the amplifer. No kidding, the thing has no negative 
feedback at all and it needs some.

2. A 40uF/450V capacitor can be added across C17 to reduce distortion 
at high power levels, especially following a loud percussive sound 
that might drain C17 the 4uF capacitor. When this happens, the whole 
amp starves for power and it takes a short time to recharge the 
capacitors for the low voltages used by the preamp and drivers. The 
result is a 'chopped off' sound following the crescendo, or in the 
case of a sustained note, an annoying modulation by power supply hum.

3. While you are at it, add a 40uF/450V capacitor across C18 too.

4. Adding a 10uF 450volt capacitor to pin 4 of V5 the 6L6 screen grid 
will help reduce distortion at high volumes.

5. Don't add anything across C19, or the high voltage will increase 
too much.

6. Use 6L6GC tubes. The high voltage is 400VDC, a little too much for 
the 360V 6L6G. In place of the 6L6, The 5881 may also be used. 7027's 
and 6CA7's should not be used.

Patrick Jankowiak