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Tube failure mode: gassy?

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June 18th 06, 05:29 PM posted to rec.radio.amateur.homebrew

Bill Janssen

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Tube failure mode: gassy?

wrote:

On 17 Jun 2006 10:20:03 -0700, wrote:

Doug Smith W9WI wrote:

semi-dumb question: where are you measuring the bias voltage? (directly
at the tube, or on the other side of the grid resistor?)

I'm wondering if a bad coupling cap is causing the bias at the tube to
drift towards 0v or even positive.

Well, I WANT the bias to be circa -20V or -25V. There's a negative
supply and some pots which form a stiff voltage divider and supplies
grid bias through a 100K resistor.

On the "bad" tube grid current is so high that it does indeed drift up
to 0V in a minute or so. Yeah, I know, there's not supposed to be grid
current until the grid goes positive. I suspect this is the root of the
problem.

The coupling capacitors are indeed good and moving the tube around it
follows the tube.

Tim.

Tim,

Thats a classic gassy tube. One possible source is small cracks
around the tube pins as it gets hot there and often there is limited
air circulation.

I've been 30 years since I've seen a 807 modulator (acutally 4 in
parallel push pull) and I've had the honor of watching them do
interesting things back when they were easily found at $2.50 each.

Allison

I have used tubes with the "gassy problem. I got the tubes free because
of the problem.
I used the tubes in my transmitter by biasing the tube with a very
stiff bias supply. I used
a choke coil to get the bias to the grid. They were 829 tubes and were
used on two meters.

Bill K7NOM

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