[Amps] Passive grid stability calculation for a tetrode?

[Fuqua, Bill L]

  Here is the thing.
You are making some assumptions that the output is tuned and already matched. However, while tuning the plate capacitor, loading capacitor and maybe even the pi network inductor not to mention possibly an antenna tuner, you may come up with some very odd possibilities. Not only the resonant frequency may temporarily be way off but the Q may also be high at that resonance. 
A good swamping resistor (non-reactive) assures stability. 
  The output impedance of the transceiver may not be purely resistive off frequency while one begins tuning up.
  One other question, is the grid plate capacitance measured by the tube manufacturer done using the standard socket or in a special test fixture?


73
Bill wa4lav

________________________________________
From: peter chadwick [g8on at fsmail.net]
Sent: Friday, November 15, 2013 1:17 PM
To: Fuqua, Bill L; Carl; donroden at hiwaay.net; Amps
Subject: Re: [Amps] Passive grid stability calculation for a tetrode?

Actually Bill, the formula was derived using tuned circuits, where Rp = QXLp and Rg = QXLg. As you move off resonance, the impedance and gain naturally drops. However, it is worth keeping the value of H around 1 to 1.25 maximum.

The input impedance should be figured as the swamp resistor in shunt with 121pF, although as that is a lower impedance, it will actually increase stability. Whether or not the driver can stand the SWR (and be stable!) is another matter.....

73

Peter G3RZP