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What causes an EL-34 to arc inside the envelope? I turned on my restored ST-70 last night and heard a bunch of popping. Noticed that one of the output tubes was arcing near the bottom. Turned it off and switched tube positions with the same results. The tube is a SED =C=, probably a year or so old.
Usually when it arcs many times its because there are mass particles that get lodged between the elements and if the tube doesnt glow cherry red,I have seen this condition correct itself many times.Take the tube and tap on it a bit and shake it up and see if that corrects it.
Thanks MIchael. Tapping it for a couple of minutes did indeed yield a bunch of small grayish particles. A few questions come to mind:
Where do these come from and what are they?
If I tap the tube to gather them at the bottom, can they short the pins where they enter the envelope?
Will the tube continue to worsen?
Now that I think of it, I do remember 3 or 4 times hearing a loud pop while running the amp, usually shortly after power up but all was well after that. So apparently the warning sign was there but I did not recognize it.
I have a quad, now a triplet, of SED winged Cs that used to run in my ST-70. Same thing happened to one of mine. I think it is just a QC problem.
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