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#3
June 17th 06, 01:42 PM
posted to rec.audio.tubes,rec.radio.amateur.homebrew
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Tube failure mode: gassy?
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. |
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