Can’t Stop the Hum!!

[Jon Snell]

120HZ (twice mains frequency) is the clue. I would investigate main smoothing issues first.

[Mick Bailey]

Does turning the reverb control to zero affect the hum? What about the 470K resistors that connect to the grid of V4b - are they good? Measure from the wiper of the reverb pot to the pin 7 of V4b. BTW, grounding pin 1 or pin 6 of a tube isn't really a good idea.

[Chuck H]

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[Chuck H]

The hum is there with the channel volume turned to 0. It seems to be picked up by pin 7 of V4. I can ground pin 1 of V4 and the hum is still there. But if I ground pin 6 or pin 7, the hum stops.

I hope you're not grounding the actual tube pins 1 or 6 on V4? You would be grounding the B+ supply.

[words453]

What happens when you ground either side of your 3.3M reverb mixing resistor?

When I ground the V2 side, no change. Of course, when I ground the V4 side it stops the hum. I have lifted both the 3.3M and the parallel cap and it makes no difference.

[words453]

120HZ (twice mains frequency) is the clue. I would investigate main smoothing issues first.

I have replaced all filter caps.

Does turning the reverb control to zero affect the hum? What about the 470K resistors that connect to the grid of V4b - are they good? Measure from the wiper of the reverb pot to the pin 7 of V4b. BTW, grounding pin 1 or pin 6 of a tube isn't really a good idea.

Yes, the hum is there either reverb pot at zero and increases as the pot is turned up. The 470k resistors have been changed. The pot has been bypassed using a 50k resistor and the hum is still there. I am grounding pins 1 and 6 only after the coupling cap.

[words453]

Thanks Chuck H, that is the correct schematic.

[dmeek]

I have often come across 12AX7s that hum on their own. Once I had a new EH/Sovtek that was humming, replaced it with another new EH/Sovtek, still humming, replaced it with a new JJ. No hum

[Chuck H]

Just thinking out loud now...

What if it IS a ground loop and the 120Hz is percieved because it's a harmonic of a 60Hz hum? How was the 120Hz frequency determined and do you know that there is no 60Hz component to the hum?

[Chuck H]

What happens when you ground the the juction of the 470k and 1k resistors located here:





Please run through other test criteria with this node grounded, like engaging the master vol switch, etc.

[words453]

What happens when you ground the the juction of the 470k and 1k resistors located here:





Please run through other test criteria with this node grounded, like engaging the master vol switch, etc.

Grounding at that junction kills the hum with boost engaged but the hum is present with the boost disengaged. FYI, I have lifted that connection as well and the hum is still there.

[words453]

Just thinking out loud now...

What if it IS a ground loop and the 120Hz is percieved because it's a harmonic of a 60Hz hum? How was the 120Hz frequency determined and do you know that there is no 60Hz component to the hum?

The hum is between the B and Bb on the low E string of a guitar. There definitely could be a 60Hz component to the hum. For some reason, I feel like it is a ground loop but I have checked and rechecked the grounding scheme and don’t see any loops.

[Helmholtz]

While amps often have ground contact problems, they can't develop additional ground loops by themselves. So if the grounding scheme hasn't been changed, a ground loop problem would have existed from the first day and would be common to all these amps.

[words453]

I don’t see any evidence that the grounding scheme has been changed. However, someone else worked on it several years ago and replaced output tube sockets and filter caps and when the owner got the amp back, that’s when he noticed the hum. I have checked the work done previously and it all looks good. I did correct some filament wiring on two of the sockets but that made no difference in hum.

[nosaj]

I have replaced all filter caps.

Even so, you can still have a bad cap, checking the ac ripple will answer whether it is functioning correctly or not.

nosaj