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:
Rick,
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.
Mod-U-Lator,
Mike(y)
W3SLK
