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

[Carl]

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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