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
types.
Grid biasing is much nicer....
73 es HNY
Peter G3RZP
========================================
Message Received: Dec 30 2013, 03:35 PM
From: "Paul Baldock" <paul@paulbaldock.com>
To: amps@contesting.com
Cc:
Subject: Re: [Amps] Â 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@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|