[Amps] Passive grid stability calculation for a tetrode?

[Bill Turner]

ORIGINAL MESSAGE:          (may be snipped)

On Fri, 15 Nov 2013 07:32:39 +0000, WW1C wrote:

>
>All
>I am back on my 4CX5000 project and as it not done yet, I am still contemplating the circuit to be used.Earlier this year I got solid info from this reflector for GG operation and that look like a pretty good solution.However, I was wondering about passive grid, grid driven also.
>The question is:  How do a calculate the stability?  If I simply do math for tube voltage gain I get something like 25.Then if I calculate the feedback capacitive voltage divider, Grid to Cathode 120 pf, Plate to Grid 0.7 pf, I get less than 0.01 for the ratio. If the beast needs the voltage gain times feedback gain to be above 1 to oscillate, to me it seems this tube is stable no matter what.  Where did I err if I did?
>Thanks!
>MarkkuWW1C 		

REPLY:

The thing that makes the passive grid design stable is the very low swamping
resistance from grid to ground, typically 50 ohms. This, together with the
very low anode to G1 capacitance of a tetrode means the feedback gain is
much less than 1 and that's why it is stable. 

Of course it's very important to use a non-inductive resistor of adequate
power handling capability. I like the Caddock resistors. Mouser carries then
and no doubt others do too. 

It's a simple, effective design. Go with it. 

73, Bill W6WRT