I agree, Carl, but I doubt it's the case here.
The hum peak at 120Hz from center is only 6dB down from the carrier when
driven at 1805 kHz, and 14dB down at 1995 kHz. It's not a "little bit"
of hum. Both exciters tried are current tech powered by choked SMPS's,
and show no 120Hz in the output down to the phase noise around the carrier.
Whatever it is, it's broken. I just haven't found it yet. I'll have some
time today to test it with one or the other tubes pulled. That should
settle the cathode-filament short question, at least. I don't have parts
on hand to shotgun the bypasses yet, which I'll order once I determine
the tubes can't be the source of the fault. (every time I order HV
ceramics the price seems to double and availability halves. I am
apparently cursed)
73,
Jim N7CXI
On 10/18/2013 5:32 AM, Carl wrote:
On AM Ive found that GIGO is often the reason for hum. A bit of PS
ripple in the xcvr that isnt noticable on CW/SSB is pronounced on AM.
Lots of older rigs have leaky caps; Kenwood in particular with those
ELNA brand are big leakers.
I cant imagine a problem in the 76PA that involves caps and the circuit
board that hasnt already been discussed; I use mine on AM often
including 160.
Heater- cathode leakage of a 8874 is possible but I dont see why it
would affect 160 only. You can try running with just 2 tubes at a time
of various combinations; the tank circuit is the same in the 2 tube models.
Carl
KM1H
----- Original Message ----- From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Friday, October 18, 2013 12:35 AM
Subject: Re: [Amps] Alpha 76A - 120hz hum on 160m only
On 10/17/2013 7:43 PM, Jim Barber wrote:
160M AM, (100W carrier, 400W PEP) into two different antennas and all
said the hum was very loud. Move to 75M or higher and it
Are you running AM on the other bands too? If it's not there (or not
strong) on SSB, I'd suspect filter cap(s) in the HV string. I don't
work AM, but on CW, I've had exactly that symptom, with exactly that
cause.
73, Jim K9YC
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