[Top] [All Lists]

Re: [Amps]  Runaway Plate Current

To: Paul Baldock <>,
Subject: Re: [Amps]  Runaway Plate Current
From: peter chadwick <>
Date: Mon, 30 Dec 2013 17:28:28 +0100
List-post: <">>
You can never bias a tube completely off with a cathode resistor, although you 
can reduce the plate current to a very low value - no cathode current, no drop 
across the resistor, hence no bias. With a high mu tube that needs a low 
voltage to turn it off, a 10 or 20k ohm cathode resistor will reduce most of 
them to a milliamp or less - which may be adequate for the requirements. A 
tetrode could be dealt with the same way if the screen voltage is fixed: if 
derived from a higher supply through a resistor, the screen volts would 
increase and could be outside the ratings. This technique was sometimes used in 
receivers to extend the grid base when using 'straight' pentodes in the RF 
stage, where generally they could offer lower noise figures than variable mu 

Grid biasing is much nicer....

73 es HNY

Peter G3RZP

 Message Received: Dec 30 2013, 03:35 PM
 From: "Paul Baldock" <>
 Subject: Re: [Amps]  Runaway Plate Current
 Can you bias a Tetrode off like a Triode with a large resistor in the Cathode?
 - Paul
 At 02:03 AM 12/30/2013, you wrote:
 >I agree that biasing the control grid off is better.
 >Because of negative screen current, an active (rather than VR tubes 
 >or zeners) shunt regulator is best, and not difficult to do, 
 >although the heavy bleed current approach worked.
 >Question: Why do these modern ceramic tubes show negative screen 
 >grid current under some conditions? What is the mechanism that 
 >causes it? Straight thermionic emission from the screen seems 
 >unlikely, especially as a gold sputter would effectively prevent it.
 >73 es HNY
 >Peter G3RZP
 >Amps mailing list
 Amps mailing list

Amps mailing list
<Prev in Thread] Current Thread [Next in Thread>
  • Re: [Amps]  Runaway Plate Current, peter chadwick <=