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[Amps] Runaway Plate Current

Subject: [Amps] Runaway Plate Current
From: John Lyles <>
Date: Mon, 30 Dec 2013 22:58:24 -0700
List-post: <">>
This is what I do in amplifiers at work. One tetrode runs with 1200 VDC on the screen grid all the time, and 12 kV on the anode. Control grid is normally -250 VDC for quiescent current for linear amplification. Run the control grid at -600 or more and the tube is completely cut off. A bigger tetrode does same thing, running 1500 VDC on screen grid and 23 kV on the anode. Grid is -380 for normal operation and ~-700 for cutoff.

Both of these tubes have pyrolytic graphite grids. There is no secondary emission from the screen grid. And no negative current as read by a meter in series. Never.

But we have another tetrode (an RCA) that does have screen voltage turned off while G1 bias is the same all the time. It runs 1500 on screen and 15 kV on the anode. There is a series MOSFET switch (several in series) that pulses the screen voltage. A large shunt R also loads the screen. This older design with wire grids fdoes occasionally run negative screen current, but it mostly from heating the screen and making it a primary source. It shows up in pulsed waveform where screen current starts out positive and drifts below zero to negative, at high power.

Happy New Year

Message: 2
Date: Sun, 29 Dec 2013 13:05:33 -0800
From: Bill Turner <>
To: Amps <>
IMO, putting a tetrode into standby by removing screen voltage is bad design
for the reason the original poster mentioned. Depending on the tube, even
zero screen voltage may not put it all they way into cutoff anyway.

Much better to leave screen voltage applied and just bias the control grid
into cutoff.

Opinions may vary, but that's how I would do it.

73, Bill W6WRT

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