[Amps] Alpha 76A - 120hz hum on 160m only

[Colin Lamb]

Jim has isolated the problem to the amplifier.  He has operated the amp into a dummy load with still the same problem - but only on 160.  He has had others listen and hear it and also used a different exciter.

I now think the issue is similar to the problem that direct conversion receivers have when operating on an ac powered power supply.  Doug DeMaw talked about it, and I recall the 120 cycles is a giveaway.  Basically, the signal from the amp goes back into the power supply, past the diodes and electrolytic cap then superimposes the ac onto the signal, then goes back the other way to modulate the oscillator (in the direct conversion receiver).  This would explain why it is only happening on 160 meters, where bypassing is less effective.  

I googled "direct conversion receiver hum modulation" and found this website.  I am sure there are others:  http://www.hamuniverse.com/w4jbm/12voltps.html

I suggest this as a starting point.

73,  Colin  K7FM