Hi
Bill,
Thanks for the info. I understand that very low values of swamping resistor
will probably result in a stable design.However, it looks like the capacitive
feedback in a 4CX5000 is so low that just about any value would do, even higher
than 50, higher by lots.Still commercial designs (from which these tubes and
components were pulled, Marconi from 1967) use neutralization.
So how do I calculate the stability criteria for this tube?
Thanks!
Markku
> From: dezrat1242@yahoo.com
> To: amps@contesting.com
> Date: Fri, 15 Nov 2013 00:42:59 -0800
> Subject: Re: [Amps] Passive grid stability calculation for a tetrode?
>
> 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
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