[AMRadio] Modulator feedback

James M. Walker chejmw at acsu.buffalo.edu
Fri Mar 17 21:14:35 EST 2006

I'll bet you a nickel the bias supply has a problem, the plate supply
for the 2A3s is not normal the plate supply is achieved by putting a
negative voltage on the 2A3s cathode to raise hte plate to 360 volts.

Sounds like the bias supply voltage is off. Check the manual and then
the components that provide voltage to the 2A3s

----- Original Message ----- 
From: "Rick Brashear" <rickbras at airmail.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Friday, March 17, 2006 6:10 PM
Subject: Re: [AMRadio] Modulator feedback

> Thanks Mike.  I just made a discovery about the 2A3 A-F drivers.  I had a 
> set of matched NOS RCA tubes in there.  The feedback seemed to be coming 
> from the area of those tubes more so than anywhere else.  So, I exchanged 
> them for another pair I have and it seemed to help quiet the feedback. 
> Obviously, microphonics in the 2A3's is a factor.  Next I looked for 
> shields for the tubes, but have none, so I used a "poor man's shield", a 
> couple of pieces of aluminum foil.  It helped considerably!  As a matter 
> of fact, I could then advance the audio of the speech amp enough to 
> achieve good modulation.  So, I am now on a quest to find some way of 
> shielding the 2A3's.  I'm afraid my temporary foil shields would cause the 
> tubes to over heat, plus it's not the best way of doing it.  Have you had 
> any experience with this?  I wonder if the 100th's are also part of the 
> problem.  I really think the "talk back" is coming from the modulation 
> transformer, which I can live with.  I just have to get the feedback out 
> of it.
> Thanks for your assistance and advice,
> Rick/K5IZ
> Mike Sawyer wrote:
>>    The UTC may work so don't discount it. Those I and H model 
>> transformers were only about half the size as the D and E models. I can't 
>> think of the company that made them but they were substantially different 
>> then the earlier models. As always, your mileage may vary.
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