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got a '73 bassman 50 head and when plugged up under 5 it's fine but turn it up a little past 5 and you get this high pitched noise. you can still play over it but of course that's unbearable. could this be a bad pre amp tube? Thanks
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definitely could be a bad preamp tube. try swapping them. i think it could also be a bad coupling cap. but try the tubes first, if they're old then you should change them all anyway. -
That amp uses a negative feedback circuit (from the OT secondary back to the plate of the third preamp tube, by way of an R/C network.) You could temporarily disconnect this (it goes to either pin 1 or pin 6 of the tube right before the 12AT7 phase splitter) and see if your high pitched sound goes away. If it does, then swap your OT primaries, the blue and brown wires going to pin 3 of each power tube.
After that you can reconnect the neg fdbk line or leave it out, depending on your preference re the tone.Comment
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could be the NFB but i don't see why unless someone worked on it and and unhooked the ot then hooked it up backwards. first thing i would try is tubes since you just pull them and replace. next i would think would be a coupling cap. one of those .01's probably. i doubt its NFB only because it's not a new build. also you should consider blackface converting it.Comment
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