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Tube failure mode: gassy?

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June 17th 06, 03:35 PM posted to rec.audio.tubes,rec.radio.amateur.homebrew

Ken Scharf

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Tube failure mode: gassy?

wrote:
Pooh Bear wrote:

wrote:

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.

Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.

I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).

One thing I want to eliminate as a possibility is that something in my
homebrew amp did this tube in after just a few dozen hours of service:
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

Tim.

swap the tubes around and see if the problem moves with the tube or
stays in the socket. But I bet it's the tube. The getter only
absorbs oxygen. However other gasses can be outgassed from the
metals in the tube and cause problems perhaps.

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