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As some of you know, (and were kind enough to give me advice), I was having a hum problem with my DIY SV83 amp. On and off for the last week and a half I have been unsoldering and resoldering every ground wire on this amp. I have tried every groud scheme I know of and all the variations I have read about on this forum to no avail. I tried adding capacitance to the filter and tried different filter configurations also but this didn't help either. Today I noticed that when I tied the safety ground to the circuit ground the hum increased but I had no idea why. Disregarding this I proceeded to install a hum pot in the filament circuit just for the heck of it. So I ground the wiper to circuit ground and I then tie safety ground to circuit ground and 90% of the hum disappears. The thing is I don't have a clue why and was wondering if any of you with lots more experience than me could tell me why this happened. Listening through a 105db horn I can't hear any hum at a distance of maybe 1 foot from the speaker now. I used to be able to hear it from a listening position 6 feet away! By the way, adjusting the hum pot has absolutely no effect on the remaining hum. Thanks,
Jim
Follow Ups:
before you installed the pot? With or without a hum pot, referencing the filament voltage to ground is essential to minimize hum. Otherwise, the filament circuit assumes a high common mode impedance that picks up all sorts of junk and couples it into the cathode.
Yes the center tap was grounded. The amp did have instant hum as soon as you turned it on but the hum got louder as soon as the tubes started to conduct. Thanks so much for responding!
Jim
That wouldn't be resolved by changes to filament circuitry...
Jim
I think you partially ansewered your own question..Typically on a conventional tube amp like a mac mc60 they installed a hum pot in the filamemt circuit going to the heater of all the tubes..It makes no difference where you set the pot as long as you dont have any heater to cathode leakage.It sounds as tho you may have a bad tube that you unaware of and even if its new,they can still have leakage.Try a tube and if it takes the other 10% hum away,you have solved the problem.
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