Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vintasonic on September 29, 2021, 04:58:27 pm
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Sound weird but this is the first time I noticed.
I have an amp on my bench that had some intermittent problems associated with the output tubes and the bias - which I seem to have sorted out. However, when I was adjusting the bias I noticed that one of the tubes bias current changed pretty dramatically when I changed the chassis position from flat to sideways - from about 32 ma to 22 ma. When I returned it to the flat position the bias returned to 32ma. I swapped tube sockets and it followed the tube so it's definitely a problem with the tube.
I have a bunch of used tubes I keep around for testing and tried the same thing with some of them. Sure enough, at least one other one exhibited the same problems. I threw another used pair in and didn't see any change, so it's something to do with the specific tube.
Can anybody clue me in to what's going on? I understand there is some mechanical thing happening, some element must be moving but I can't imagine what might cause it to act that way without seeing other, bigger problems. I have replaced hundreds of output tubes and don't recall ever noticing this before
FWIW, EL34s in a Blackstar HT60