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

[Jim Barber]

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 at audiosystemsgroup.com>
> To: <amps at 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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