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100Hz Hum in Valve Amp
100Hz Hum in Valve Amp
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Old 7th December 2017, 09:59 AM   #1
Bassy is offline Bassy  United Kingdom
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Default 100Hz Hum in Valve Amp

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.
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Old 7th December 2017, 10:17 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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A schematic may help.
100Hz hum is from the bridge rectifier, (twice mains frequency as a full wave bridge doubles the base frequency).
Check your decoupling and earthing/ground point. Ensure all decoupling uses the same ground point.
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Old 7th December 2017, 11:06 AM   #3
DF96 is offline DF96  England
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Hum which is maximum at mid volume setting probably means capacitive coupling from something carrying AC to the circuit connected to the volume pot slider. Are your internal signal connections screened?
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Old 7th December 2017, 11:30 AM   #4
Merlinb is offline Merlinb  United Kingdom
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Noise on the heater supply coupling to the grid. Even when the heaters are AC you can get 100Hz hum from rectifier hash coupled onto the AC heater waveform. I encountered this once where literally the only solution was to use DC heaters.

Last edited by Merlinb; 7th December 2017 at 11:42 AM.
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Old 7th December 2017, 11:33 AM   #5
petertub is offline petertub  Sweden
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Quote:
Originally Posted by Bassy View Post
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.
If hum is absent when volume is turned down then the humproblem
occurs before the volume pot.

Schematics needed !
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Old 7th December 2017, 11:41 AM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by petertub View Post
If hum is absent when volume is turned down then the hum problem occurs before the volume pot.
Read OP closely:
The hum ...disappears again at maximum volume.
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Old 7th December 2017, 12:58 PM   #7
petertub is offline petertub  Sweden
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Quote:
Originally Posted by Merlinb View Post
Read OP closely:
The hum ...disappears again at maximum volume.
Ahhh
Still needs a schematic.
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Old 7th December 2017, 01:50 PM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Bassy View Post
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.
Then its inductively or capacitively coupled into the wire leading from Volume pot wiper to following grid.

Ill 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.
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Old 7th December 2017, 03:02 PM   #9
azazello is offline azazello  Bulgaria
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First step is adding chock 5-10 Hn in +U filtering track, after first electrolytic cap.
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Old 7th December 2017, 03:51 PM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by azazello View Post
First step is adding chock 5-10 Hn in +U filtering track, after first electrolytic cap.
That is possibly the most expensive and least logical first step.
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