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Re: [Amps] Super Cathode Driven Tetrodes

To: amps@contesting.com
Subject: Re: [Amps] Super Cathode Driven Tetrodes
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Mon, 29 Oct 2018 17:38:45 +0000
List-post: <mailto:amps@contesting.com>
Bill,

Can anyone give me a reason why a cathode driven tetrode with the control
grid connected directly to
the cathode be unstable. The screen grid is connected directly to ground.

While I'm definitely not a tube guru, let alone with such nonstandard configurations, I will risk a pure guess:

The inductance of grid and screen connections is probably about the same, or at least within the same order of magnitude. But the plate-screen capacitance is very much larger than the plate-grid capacitance in the normal RF-grounded screen, driven grid configuration. So the resonant frequency of the screen structure might be low enough to become a problem. It is often very close to being a problem in conventional circuits - hence multiple screen connection, screen rings, screen bypass caps built into sockets, and so on. Add just a little more phase shift, and you get a power oscillator.

If the same tube is run with both screen and grid grounded, the total capacitance between plate and the grounded parts is not much increased, but the inductance is halved, and the grid-cathode capacitance might help stabilizing the circuit. So this might be more stable than joining the grid to the cathode.

The (in)stability of such an amplifier must depend a lot on the characteristics of the tube used, and construction quality. Short, direct, multiple grounding connections to a chassis, input on one side, output on the other, are the conventional techniques. More modern techniques involve a complete phase/amplitude analysis over the whole frequency range and at all possible tuning settings.

Warning repeated: I'm just guessing. I have never used, let alone worked on such a super cathode driven amplifier.

Manfred


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