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

[Jim Barber]

Thanks, Colin.
Since it's the only one standing, I'm going with your theory.

This isn't a known (or heard of) issue with the design, so I'm going to 
choose to assume that there's a component failure somewhere that's 
causing the hum to appear only on 160m.

One possibility might be a cathode-filament short in one of the tubes, 
but if that were the case I can't see where it would show up only on 
160. (I've checked for inter-element shorts with an ohmmeter also)

Shorted turns in the plate choke and possibly the resultant overcurrent
condition in the bypass caps might account for it. I'll inspect the 
choke, and probably order and replace the bypass caps "just because".
I've had the amp for 20+ years, but don't use it much and haven't been 
active on 160 until fairly recently. It's possible the condition has 
existed for a long time and I was just blessedly ignorant...

If one or more of the filament bypass caps opened or headed in that 
direction, I wonder if that could leak drive power back into the 
filament circuit? I haven't inspected the area under the tube deck yet 
either.

The above seem to me at least to be most likely, but other suggestions 
are definitely solicited.

Thanks,
Jim N7CXI


On 10/17/2013 9:13 AM, Colin Lamb wrote:
> 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
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