Hello all.
I have built a Musical Machine valve amp and am having some problems with 100Hz hum coming through the speakers. The hum is absent with the volume turned all the way down then increases as volume is turned up and disappears again at maximum volume.
I have tried taking the driver tubes out and the hum disappears, so it appears to be confined to the driver stage. If I change the channel to one with no input the hum is still there.
The audio circuit is what is in the the original schematic but the power supply is different as I redesigned this to take advantage of more economical parts, so this is suspect to me.
What is the best strategy to begin looking for the source of the hum? I have just bought a basic oscilloscope I'm itching to try out so maybe this would be useful in the search.
Cheers, Sam.
I have built a Musical Machine valve amp and am having some problems with 100Hz hum coming through the speakers. The hum is absent with the volume turned all the way down then increases as volume is turned up and disappears again at maximum volume.
I have tried taking the driver tubes out and the hum disappears, so it appears to be confined to the driver stage. If I change the channel to one with no input the hum is still there.
The audio circuit is what is in the the original schematic but the power supply is different as I redesigned this to take advantage of more economical parts, so this is suspect to me.
What is the best strategy to begin looking for the source of the hum? I have just bought a basic oscilloscope I'm itching to try out so maybe this would be useful in the search.
Cheers, Sam.
If hum is absent when volume is turned down then the humproblem
occurs before the volume pot.
Schematics needed !
Then it´s inductively or capacitively coupled into the wire leading from Volume pot wiper to following grid.
I´ll have to guess some values since you provide no schematic or details, but supposing you have a 12AX7 triode driving a 1M pot which in due turn goes straight to next 12AX7 grid, source impedance driving that wire is zero when wiper touches ground (so zero induced Hum) and about 40k when on maximum volume which absorbs most (not all) induced interference.
But with pot electrically halfway , usually around "6" or "7" setting, source impedance is maximum: 2 500k halves in parallel so 250k , in series with tube plate impedance of 40k , total some 300k.
That will be way less effective at sinking any induced interference, happily sending it to next grid to be reamplified.
Solution is to use a shielded wire instead, with grounding only on next grid ground reference, to avoid creating a new ground loop which was not there before.
EDIT: same thing happens often with Guitars, for the same reason, solution is to invest in *good* Guitar cable; good meaning good shielding and low capacitance, not snake oil.
100 Hz comes from worst filtering of +U and adding chock for 30 Eu will decide most of hmmm problems, or try starting iron chock for luminiscent tubes with R 30 - 40 ohm. /Input tracks and grounding cause 50 Hz hmmm/.
Thanks for all the replies and apologies for the lack of schematic, it's on my old pc but I'll post it tomorrow some time. The inductive or capacitive coupling sounds possible, some but not all of the signal wires are shielded and there could be a gap somewhere. I have checked and rechecked grounding but I'll check again in case I've missed something.
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