Help needed with sudden 120Hz hum in Mission Amps 5e3

[loudthud]

I think Bruce uses a brass grounding plate around the pots and input jacks. There may be corrosion causing a bad ground to the chassis. It would be a good idea to loosen and re-tighten the pot and input jack nuts. Get them nice and tight. For the pots I use a "deep" socket to clear the shafts. Be careful not to let the back of the pot rotate and break a connection or one of the terminals on the pots.

[rdh006]

@Enzo

Thankyou.

Power section grounds
The PT CT is connected to the 1st Filter cap, which is connected to a ground lug at the PT. The 2nd filter cap is connected to the same place by a separate wire. The power tube cathodes are connected to the 1st filter cap.

Preamp grounds
The 3rd filter cap is connected to the preamp ground at the input jack. The preamp cathode grounds are also connected here. The pots and input jacks are connected to a brass plate, as loudthud suggests.

This ground scheme was working well until the hum suddenly appeared.

Any additional clues here?

[rdh006]

@loudthud
Thanks for the help. There is indeed a brass plate. I did check those areas for continuity, I looked for looseness and found none, but I didn't actually take it apart. I will break it down and clean and re-tighten, as you suggest.

[Chuck H]

We might be overthinking this...

It's not mentioned if the amp works normally, with a normal amount of hum, with a guitar plugged in. I mention this because an oxidized or broken jack switch ground could cause your problem. This depends on how the amp is wired and it's odd that the hum remains with either preamp tube pulled, but not both. But please report on the hum level with an instrument plugged in and the instrument volume at zero.

[rdh006]

Thanks for the input, Chuck H -- I should have been clear about that in the beginning. No such luck, I'm afraid. The hum is there with or without the guitar plugged in. I have tried with the guitar plugged in to each of the 4 inputs in turn and the instrument volume at zero - the hum is the same whatever.

[rdh006]

...it's odd that the hum remains with either preamp tube pulled, but not both...

I just re-checked this. I can confirm that the hum is present with either V1 or V2 plugged in alone, and the hum is gone if I pull both V1 and V2.

-- However, with V1 plugged in alone, the hum is less than with V2 plugged in alone.

Both V1 and V2 are 12AX7, not the typical 12AY7 for V1.

As the two channels are interactive on the 5e3, due to being connected to a common tone control, would this be suggesting that the problem is in the V2 area? That the lower hum on V1 is coming from the V2 area and appearing on V1 because the two channels are linked at the tone control?

[eschertron]

I just re-checked this. I can confirm that the hum is present with either V1 or V2 plugged in alone, and the hum is gone if I pull both V1 and V2.

-- However, with V1 plugged in alone, the hum is less than with V2 plugged in alone.

Both V1 and V2 are 12AX7, not the typical 12AY7 for V1.

As the two channels are interactive on the 5e3, due to being connected to a common tone control, would this be suggesting that the problem is in the V2 area? That the lower hum on V1 is coming from the V2 area and appearing on V1 because the two channels are linked at the tone control?

This is what confused me the first time. Unless Bruce's 5E3 is different from the tweed model (and I don't think it is), there's only two 12A_7 tubes in the amp. Can you confirm this? Pulling V2 in this amp (from the schem I'm looking at) takes the PI out of circuit, not simply extra inputs. Pulling the PI should silence the amp, unless the problem lies in the power supply modulator cicuitry, i.e., the power tubes and related components. I'm with those above who've suggested something heater-related.

edit: Can you take the heater CT off the power tube cathode point and take it to signal ground?

[Dave H]

My amp is dead quiet with V2 removed. One of the filter cap grounds could have broken. I'd get a cheap meter and check that all the filter cap -ve terminals are connected to chassis. Then check that everything that should be grounded is connected to chassis. It's a simple amp. It won't take long.

[rdh006]

@eschertron
Yes, you are correct, there are only two 12A_7 tubes. Pulling V2 does take the PI out of circuit. Pulling V2 on its own does not silence the hum, although it does make it quieter, as above.

...unless the problem lies in the power supply modulator cicuitry, i.e., the power tubes and related components.

Does that also point back to the filter caps? I wasn't able to make the ripple measurement suggested by Jazz P Bass above because I'm not sure exactly how to do that and I didn't get an answer to the questions I asked to clarify.

Can't get to the amp right now.

[rdh006]

@Dave H
Thanks for the help. I can check that a little later.

[Chuck H]

I'm on board with a ground fault in the power supply filtering being the most likely culprit.

[Enzo]

If hum remains with the PI pulled, then solve that first. If putting the PI in increases the hum, that means a second hum source is present.

[rdh006]

@loudthud
Ref your suggestion (thanks...) I measured the input jacks and pot grounds for continuity to the chassis. All showed a connection and all appeared tight. I went ahead and did as you suggested, to be sure -- loosened all of them, checked/cleaned and re-tightened. I can confirm that they all show a good connection to chassis, and are all good and tight.

The hum is unchanged.

[rdh006]

@Dave H

I have checked the grounds with a meter, as you suggest

the filter cap -ve terminals
220k power tube grid resistors
power tube cathodes
preamp tube cathodes
input jacks and pots

All are showing a connection to the chassis.

Anything else I am missing?

The hum is unchanged.

[rdh006]

@Enzo
Thanks for the info. Not good to hear, but I appreciate the help.