[Amps] Passive grid stability calculation for a tetrode?

[peter chadwick]

Gerald,

Feedback does change the input impedance, both in  terms of real and imaginary components. The equations are certainly in earlier Editions of Terman's 'Radio Engineering', if not the later editions. Obviously, the swamping effect of passive grid helps reduce the variations, but as the magnitude of the variation depends on the gain, then any non-linearity will affect (and probably degrade by Murphy's Law!) the IMD.

73

Peter G3RZP

========================================
 Message Received: Nov 18 2013, 01:42 PM
 From: TexasRF at aol.com
 To: g8on at fsmail.net, wlfuqu00 at uky.edu, km1h at jeremy.mv.com, donroden at hiwaay.net, amps at contesting.com
 Cc: 
 Subject: Re: [Amps] Passive grid stability calculation for a tetrode?
 
 Hi All, a late input here: I have been wondering what the feedback  
 power/voltage does to the input vswr. Seems there would be two standing waves  
 present: one from the input circuit mismatch (if any) and the feedback voltage  
 of hopefully a smaller magnitude.
  
 The resulting combined vswr would depend on the relative phase of the two  
 standing waves, right?
  
 It seems that there would be some good benefits to adding a neutralizing  
 circuit even for a passive grid design.
  
 All of my tetrode experience has been with cathode driven designs. I have a 
  TH347 PA on 1296 MHz that has a fully adjustable input matching circuit. 
 No  amount of tweaking will make the input vswr much better than about 2:1. I 
 always  assumed it was because of the unneutralized feed back from anode to 
 cathode. By  tuning the driver and input match for maximum PA power output, 
 it all seems to  work well so it has been left as is.
  
 73,
 Gerald K5GW
  
  
  
  
 In a message dated 11/15/2013 12:17:40 P.M. Central Standard Time,  
 g8on at fsmail.net writes:
 
 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
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