> Could have been an HF oscillation for all I know as I didn't have a freq
> counter at the time to measure them. I guess I could have used my MFJ-259
> but didn't think about that until now! Needless to say, without the
> series RC suppressors on the cathode, when keyed with zero drive and as
> the tune C was reduce to minimum, the grid and anode currents would
> increase. Putting the RC networks in, eliminated this problem. Wether it
> was HF or VHF, I can't say, but regardless the tube was stabilized.
I've seen amps unstable at HF stabilized by that technique. The SB-
220 has an RF choke in parallel with silver mica caps. The RF
choke has multiple periodic resonances where it goes from
inductive to capacitive. At one VLF and a few other higher
frequencies, the grids are not even grounded for RF!
The worse thing in the 220 and other GG triode PA's are the use of
mica caps to float the grids. While the engineers were assured that
would add negative feedback and improve IMD, it actually makes a
mess of the feedback and makes the IMD get a bit worse and
change more with frequency since feedback phase changes with
tuning, drive level, and frequency.
73, Tom W8JI
w8ji@contesting.com
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