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Re: [Amps] GS-35B

Subject: Re: [Amps] GS-35B
From: R.Measures <>
Date: Mon, 2 Aug 2004 07:44:05 -0700
List-post: <>

On Aug 2, 2004, at 5:44 AM, 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) ;-)


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Richard L. Measures, AG6K, 805.386.3734.

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