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In Reply to: RE: These here tubes can be tricky little buggers! posted by vinnie2 on September 27, 2016 at 18:25:29
Tubes draw current based on the negative grid bias voltage relative to the cathode. It sounds like your first tube was defective. Yes, I have had defective tubes 😀
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but did they cause a hum? That's what my real question is; why would the tube drawing more current cause a hum?
Edits: 09/28/16
"why would the tube drawing more current cause a hum?"
You said anode voltage was much higher than the other tube. That would indicate it was drawing less current. A number of things could cause that, including misalignment of the grid (as someone else mentioned) or a defect in the filament. If the filament is AC heated, all bets are off. :)
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Buy Chinese. Bury freedom.
All bets are off then. Forgot to mention that all though the 845 has dc on the filaments, I left the 26's ac.
Any ideas what might cause it with ac heated filaments?
Edits: 09/28/16
It's difficult to guess, almost anything that unbalances the filament-grid relationship could do it. Did you happen to check it for shorts in a tester? Could still be bad if that shows OK, but not likely vice-versa.
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Buy Chinese. Bury freedom.
NO, I just tossed it once I saw that it was causing the hum problem. It was a used pull from a hamfest, so the odds are there was something wrong with it.
The grid n the bad tube is miss-aligned which is causing the bias problems. The grid could be way too close to the filament and be coupling with it. But take any DHT and go way past the current ratings for it and it will likely hum, kind of like the scream of torture.
Ok. Thanks.
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