[Amps] GS-35B

[R.Measures]

On Aug 2, 2004, at 5:44 AM, JMLTINC at aol.com wrote:

> Hi Dave-
>
> Yes, you are correct, a grid driven triode will make a good oscillator 
> and running the tube in G-G is an easy way to prevent such 
> oscillations. However, neutralization of the grid driven configuration 
> is possible. To what extent this tube can be neutralized and made 
> stable is unknown to me and that is why I posted the question.
>
Eimac's "Care and Feeding of Power Grid Tubes" shows no way of 
neutralizing a single-ended grounded-grid amplifier.  However, even if 
there was a way to do so, neutralization circuits work only at the 
fundamental freq., but not at the self-resonant freq, of the 
anode-circuit -- and, in a HF amplifier, that is where a VHF parasitic 
oscillation usually begins.  Thus, where neutralization is possible, 
such as in a grid-driven, AB1 amplifier, one still needs a VHF 
parasitic-oscillation suppressor even though the amplifier is 
neutralized at the fundamental.
cheerz, John

> Thanks for the thoughts-
> John N9RF
>
> Dave, VK3HZ wrote:
> Hi John,
>
> The GS-35b is designed for G-G use (the grid ring doubling as a chassis
> mounting point), and for a reason.  If you try to use a GS-35b (or any
> triode) in a grid-driven configuration on 2m, you'll almost certainly 
> end up
> with a nice 2kW oscillator suitable only for cooking the Christmas 
> turkey
> :-)  The reason is that the feedback (output to input) capacitance 
> within
> the tube is much higher in that configuration.  In grid driven, this
> capacitance is formed by inter-electrode capacitance between grid and 
> anode,
> which is of the order of 5pF.  In G-G, the grid acts as a capacitance
> shield, so the output-input capacitance (anode to cathode) drops
> substantially to around 0.1pF or less.
>
> So, for VHF-UHF use (and, I would say, HF), G-G is really the only 
> option.
> However, it is a very good tube.  I use one on 70cm and it performs
> extremely well - around 13dB of gain in G-G.
>
> Perhaps you need to build a 4CX250 driver for it (with extremely well
> regulated screen supply) ;-)
>
> Regards,
> Dave
> VK3HZ
>
>
>
>
>
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>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org