Can’t Stop the Hum!!

[Chuck H]

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.

+1

But nearly all of these amps have been worked on. And not every repair tech is stringent about stock arrangements and wiring. These amps are spaghetti plates inside.

[Chuck H]

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.

That's the whole story right there and it can't be disregarded as a matter of convenience. Something was changed and now the amp hums. (<period)

[Chuck H]

Before that comes off wrong... What I'm saying is that whatever is different from stock now, and certainly something is, should be identified and corrected if possible. And whatever is different was almost certainly made that way when that last guy worked on the amp.

[words453]

Agreed. Something was changed that caused the hum, but for the life of me, I can’t find it. And why is the hum only injected into the grid of V4b? I can remove V4 and play through the normal channel with no hum. So it seems to me that would eliminate the filter caps as an issue.

[words453]

At this point, I think my only option is to rewire the amp. Or at least the preamp section. I guess it could be a conductive board.

[mozz]

If you dont have a scope, you can measure the DC voltage on the filter caps, then measure the AC voltage at the same points. Post here and they will tell you what's acceptable AC ripple. (I myself can't pull numbers out of thin air unless i have it on my bench)

I was always under the impression if the filter cap was leaky, and you paralleled another cap across it, it was useless as the leakage was still there. One way or another i would disconnect the suspect cap, even if i had to cut a trace with a razor knife, easily repairable.

[words453]

I just talked to the owner and he said that the amp had a slight hum when he got it back from the tech, but the hum got worse as time went on.

[words453]

If you dont have a scope, you can measure the DC voltage on the filter caps, then measure the AC voltage at the same points. Post here and they will tell you what's acceptable AC ripple.

Will do thanks!

[Chuck H]

At this point, I think my only option is to rewire the amp. Or at least the preamp section. I guess it could be a conductive board.

You shouldn't do that. You've already been making random changes chasing the problem. Many, many times here we've seen attempted shotgun repairs end up in additional complicated symptoms that turn amps into dust collectors due to frustration. The solution to "hum in amp" has never technically been "rebuild the preamp". The problem probably isn't even in the preamp, per se. More likely how something is grounded, a miswire of the master switch circuit or possibly a change to the reverb tank jack ground isolation scheme. If you cannot identify any miswire now you would only recreate it with a rewire. Much better to stay focused on identifying the problem first.

[Helmholtz]

I don't think it's likely that the grounding scheme was accidentally changed during a repair. Detailed pictures might help.

Also make sure that all cable shields are well grounded at one end.
And as nosaj suggested, measure ripple voltages at supply nodes.
Further I would remove and clean the reverb cable jacks.
Is the heater voltage symmetrical wrt ground?
Sometimes hum problems vanish if the chassis is covered by a grounded metal plate.

[Enzo]

Typical Fenders had multiple ground connections, not a star ground. The scheme need not change, but if ONE of those ground connections failed, problems could result.

If grounding grid of V4b kills hum it only means the source of hum was at OR BEFORE that grid.

[Helmholtz]

but if ONE of those ground connections failed, problems could result.

Yes, that's what I call a ground contact/connection problem as opposed to a ground loop problem. It might show in increased ripple voltage.

[words453]

Main filter 465.5 vdc
3.24vac
Screen cap 455.7 vdc
3.19vac
PI cap. 431,2 vdc
0.069vac
Preamp cap 374.7 vdc
0.012vac

[words453]

You shouldn't do that. You've already been making random changes chasing the problem. Many, many times here we've seen attempted shotgun repairs end up in additional complicated symptoms that turn amps into dust collectors due to frustration. The solution to "hum in amp" has never technically been "rebuild the preamp". The problem probably isn't even in the preamp, per se. More likely how something is grounded, a miswire of the master switch circuit or possibly a change to the reverb tank jack ground isolation scheme. If you cannot identify any miswire now you would only recreate it with a rewire. Much better to stay focused on identifying the problem first.

Ok, I’ll keep at it. I have disconnected the signal lead on the reverb jacks entirely and the hum persists. But I’ll try to isolate the ground on them as well.

[words453]

Typical Fenders had multiple ground connections, not a star ground. The scheme need not change, but if ONE of those ground connections failed, problems could result.

If grounding grid of V4b kills hum it only means the source of hum was at OR BEFORE that grid.

All grounds are intact.