[Amps] Tuned Input - IMD and efficiency

[Bill Turner]

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.

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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.

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