>
>> 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!
? SB-220s typically exhibit grid-resonance at approx. 90MHz. To
observe this, one needs a dipmeter. A fun experiment is to directly
gnd the grid and re-measure the resonant freq.
>
>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,
? By Mr. Bill Orr
>it actually makes a
>mess of the feedback and makes the IMD get a bit worse........
- later, Tom
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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