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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