On Aug 2, 2004, at 5:44 AM, JMLTINC@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
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