[AMRadio] Modulator feedback

Rick Brashear rickbras at airmail.net
Fri Mar 17 18:10:29 EST 2006

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,

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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