>
>I think it likely this problem has been there all along, but was never
>noticed because I was operating outside the range of parameters that
>make it take off.
>
>The bias is fed through a 150 ohm 2W carbon comp resistor with a
>.001 uF feedthrough (perhaps not enough for effective LF bypassing) on
>the cold end. I try never to use a choke on the grid.
>
A couple of other things to think about:
1. Cathode bypassing. In some ways, DC-grounding the screen is only
exchanging a screen bypassing problem for a similar problem at the
cathode.
You may need to use multiple disc ceramics at the cathode itself to
bypass the signal frequencies, and a mylar film capacitor of say 0.1uF
to prevent low frequency instability. However, the two types of
capacitor should not be connected directly in parallel because there is
a serious risk of forming a HF parallel-resonant circuit between the
ceramic capacitors and the internal inductance of the mylar cap, which
would lead to the cathode becoming completely un-bypassed at the
parallel resonant frequency. To avoid this, the two different types of
capacitors need to be separated from each other by a heavily damped RF
choke. The choke will damp out the parallel resonance, but it also has
to be capable of carrying the DC cathode current.
2. A surge limiting resistor ("glitch resistor") in the B+ line has the
additional advantage of helping to suppress parasitic LF resonances
involving the series inductance of the HV supply and return wiring.
Of course, the problem in your particular amplifier may be "none of the
above"... but those are two more possibilities.
73 from Ian GM3SEK
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