[Amps] Super Cathode Driven Tetrodes

[Manfred Mornhinweg]

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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