On Aug 9, 2006, at 10:20 AM, Bill Turner wrote:
> ORIGINAL MESSAGE:
>
> On Tue, 08 Aug 2006 22:44:54 +0100, you wrote:
>
>
>> As an aside, adding some capacitance with low inductance leads from
>> cathode to grid won't only help linearity/efficiency, it might
>> improve
>> vhf stability too.
>
> ------------ REPLY SEPARATOR ------------
>
> This makes perfect sense. Rich recommends a 33pF in series with a ten
> ohm resistor directly from grid to cathode with short leads. That is
> what I used in my 8877 amp and it is perfectly stable without any
> anode parasitic suppressor at all. Do not leave out the resistor; it
> lowers the Q of any parasitic resonant circuit and thereby lowers the
> gain.
The RC network also attenuates VHF signals feeding back to the input
from anode-circuit ringing at the output.
>
> I think where people run into trouble in the grid area is with long
> leads running to the input circuit, be it a switch or relays. It
> doesn't take very many inches of wire or coax to create a VHF
> resonance of about the same frequency as the anode resonance, and then
> you have a classic TGTP oscillator, whether grid-driven or
> cathode-driven. If you can move the grid resonance up way higher than
> the anode resonance, the amp will be stable, and the 33pf/10 ohm will
> help damp any fed-back VHf energy in the first place.
>
> I wish there was an easy way to quantify all this, but each amp is
> different. This is as much art as science, IMO.
>
> Bill, W6WRT
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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