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Been chasing down a hum in my bread boarded mono 26/845 for two days now, and would have been pulling my hair out if I had any. I sort of thought it was a 120 hz hum as it was higher pitched than a 60 hz, so I spent a lot of time checking out the psu, but found nothing wrong there.
Tonight I decided to go back through and check all the voltages again and I found that one of the 26's had a plate voltage that was half again as much as it neighbor for no apparent reason. After ruling out everything else I decided to try a different tube even though this one was working in the circuit. Wouldn't you know the plate voltage came in just a volt different on the second tube and the hum disappeared. I have never had that happen before, and was wondering if anyone else has? Also wondering if anyone has an idea what caused the hum? The plate voltage was at about 145 vdc instead of the 105 it should have been with the plate resistor I am using.
Edits: 09/27/16 09/27/16 09/27/16Follow Ups:
Hey Vinnie
26 tubes are tricky at best. But everyone seems to enjoy the sound. Check out DIY audio I think it was Andy's thread on a 26 pre amp it goes on for ever. Lots of ideas.
Enjoy the ride.
Tom
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 😀
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.
now put the humming tube back in....still hums?
Yes, and when it did it went in the dust bin.
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