Tracking Down 120Hz Hum

[FourT6and2]

Got a Marshall Super Lead here that I've modified with an additional 12AX7 for five cascaded gain stages and a post-stack master (single input). I've done this mod a few times with great results. Very quiet, only a tiny bit of hiss as you turn the volume up. This particular amp has some 120Hz hum that I can't seem to figure out. I've done some tube pulling and it mostly goes away when V2 is pulled. Someone else did a filter cap job recently (F&T). The cap job looks fairly decent, but they grounded the preamp filter cap right to the chassis via a cap clamp mounting bolt. In the other ones I built, I grounded this cap via a longer wire over to the preamp ground buss. This is the only thing I can think of that might be causing this hum. I've gone through the rest of the amp as best I could. I tried different preamp tubes, etc. Haven't tried different power tubes yet, though.

The rest of the amp's grounding is how Marshall typically did it. PT CT and HT CT grounded to first cap's (-) (the cap over in the far corner by the AC Mains. Bias ground connected to preamp buss and rectification ground connected to screens cap (-) then to chassis ground. Preamp grounded through V1a cathode ground and then to input jack and then to ground buss.

I want to move that cap's ground, but the genius who did the cap job used a short piece of solid wire to solder the cap's ground tab right to the mounting bolt. There's no way to get to that connection without completely removing the turret board, which I really don't want to do. But I guess that's the only way now. Unless anybody has other ideas?

[Phil_S]

Picture of the subject cap? I am thinking, it may be more expedient and less expensive for the customer to clip the cap (if you can,) and mount it in a different location. For example, use a radial cap with a clamp mounted to one of the vertical sides of the chassis. Then, see if you can clip and remove as much of that offending ground as you can without making more work than necessary. Maybe not tidy, but maybe worth doing. Maybe I have no idea what I'm talking about?

[cdemike]

they grounded the preamp filter cap right to the chassis via a cap clamp mounting bolt. In the other ones I built, I grounded this cap via a longer wire over to the preamp ground buss. This is the only thing I can think of that might be causing this hum.

Definitely could be the source of the hum, especially with it getting better with V2 out. To clarify, what's now V2 is what would have been V1 in a stock 1959? Relatedly, where does the added triode get its B+? Did the amp have a hum that was there before the modification that got worse after adding a new gain stage?

In terms of testing the filtering ground as potentially culprit, I'd check to see if the leads to and from that can cap accessible from the top of the board. Potentially also having to remove the board would also frustrate me, but I'm thinking it might make sense to unsolder the leads running from the node to that cap filters. You could also check to see if the leads are original, since if they were replaced, there could be issues with lead length. With it out of the circuit, you could try alligator clipping a known good cap to that node with the ground alligator clipped directly to the signal ground lug like in a star grounding scheme.

If those leads aren't accessible, you could also just try lifting the ground for that cap and alligator clipping in a good cap for the affected node the same way as above.

With an improvised grounding scheme like that, it could also be worth checking the solder joints to make sure everything that would have touched a soldering iron after the amp left the Marshall factory is still connected.

[FourT6and2]

Picture of the subject cap? I am thinking, it may be more expedient and less expensive for the customer to clip the cap (if you can,) and mount it in a different location. For example, use a radial cap with a clamp mounted to one of the vertical sides of the chassis. Then, see if you can clip and remove as much of that offending ground as you can without making more work than necessary. Maybe not tidy, but maybe worth doing. Maybe I have no idea what I'm talking about?

The cap can't be clipped. That's what I'm saying. It's an external, radial dual filter cap 50uF + 50uF. Just like any other Marshall. To remove the cap, its ground connection must be severed. But usually these caps are wired with flying leads, so you can unclamp it from the top of the chassis, pull it out, and disconnect. But whoever installed it, soldered it directly to the mounting bolt on the inside of the chassis, underneath the turret board. So the entire board needs to come out to get to it.

[FourT6and2]

To clarify, what's now V2 is what would have been V1 in a stock 1959?

Correct. What used to be V1 is now V2.

Relatedly, where does the added triode get its B+?

Former V1 node. So current V1a, V1b, V2a (former V1), and V2b all share the same node. I thought about tapping new tube farther upstream, at PI node (like how a 1959T Super Trem does it), but ultimately chose V1 node because of how some well-known modders historically did it. I also did it this way in a few other builds that were incredibly noise free. But yes, I'm aware that this can cause noise in some cases. Another option is to tap it farther upstream, add another interstage decoupling/dropping resistor and another filter cap. Or even do it in series after former V1.

Did the amp have a hum that was there before the modification that got worse after adding a new gain stage?

I don't remember if there was this hum before. I actually didn't even notice it until someone else pointed it out. I was like oh damn... how did I miss that hum haha. I do believe the hum was there before, though. But memory can be tricky. The added gain stages certainly made it more apparent if that was the case.

In terms of testing the filtering ground as potentially culprit, I'd check to see if the leads to and from that can cap accessible from the top of the board. Potentially also having to remove the board would also frustrate me, but I'm thinking it might make sense to unsolder the leads running from the node to that cap filters. You could also check to see if the leads are original, since if they were replaced, there could be issues with lead length. With it out of the circuit, you could try alligator clipping a known good cap to that node with the ground alligator clipped directly to the signal ground lug like in a star grounding scheme.

Fantastic idea! I do have another cap somewhere I can use.

But yes, the leads for the new cap that's in there now are also quite short. I would need to remove/replace them along with the cap for sure. But they are too short to allow remove of the cap while in circuit. But those are easy to desolder from the turrets and remove/replace. It's the cap's ground that's preventing me from removing the cap. But I will try disconnecting the cap's (+) and clipping in another cap temporarily and ground it to another location! Thank you for that suggestion!

f those leads aren't accessible, you could also just try lifting the ground for that cap and alligator clipping in a good cap for the affected node the same way as above.

I'll do it like you said above. I can't get to this cap's ground connection, that's the point. It's soldered to the mounting bolt under the turret board. Can't reach it.

[FourT6and2]

Oh one other question, does the heater twisted pair wiring's direction of twist matter? Clockwise vs counter-clockwise? And relatedly, if it's not consistent, can that induce hum? e.g. Marshall wired it counter-clockwise all the way down the chassis, but when I added the new tube, I did it clockwise. I can always redo it counter-clockwise.

[FourT6and2]

Ok I disconnected that cap's (+) leads from the top of the board and temporarily attached a new cap with alligator clips and grounded it to the preamp buss. I think that worked! Most of the noise is gone.

Thank you for that suggestion.

Now I have to figure out how to remove the cap and ground it to the correct place. Looks like I might just have to remove the turret board. Ugh.

[TUBEDUDE]

It seems to me the bias circuit should be grounded with the power amp ground, not the preamp.

[martin manning]

Oh one other question, does the heater twisted pair wiring's direction of twist matter? Clockwise vs counter-clockwise? And relatedly, if it's not consistent, can that induce hum? e.g. Marshall wired it counter-clockwise all the way down the chassis, but when I added the new tube, I did it clockwise. I can always redo it counter-clockwise.

I don't think that matters. There is some argument for running the same side of the heater winding to the same pins going down the line so the signal is flipped wrt heater polarity each time, providing all stages are inverting.

[Phil_S]

Now I have to figure out how to remove the cap and ground it to the correct place. Looks like I might just have to remove the turret board. Ugh.

Dremel with a cutting wheel? Try to think out of the box. There's no set obligation to configure it the way it came to you -- I'm talking about the can cap.

[R.G.]

Oh one other question, does the heater twisted pair wiring's direction of twist matter? Clockwise vs counter-clockwise?

Nope, doesn't matter.

[FourT6and2]

Now I have to figure out how to remove the cap and ground it to the correct place. Looks like I might just have to remove the turret board. Ugh.

Dremel with a cutting wheel? Try to think out of the box. There's no set obligation to configure it the way it came to you -- I'm talking about the can cap.

I don't think I understand. Can you explain how to get a Dremel UNDER the turret board to cut the ground connection of a cap that can't be accessed unless you remove the turret board? Maybe it there was a long 90-degree wire cutter with a head maybe 1-2 inches long, that might be able to get under there to cut the ground lug off the cap. But I've not been able to find such a tool.

[FourT6and2]

It seems to me the bias circuit should be grounded with the power amp ground, not the preamp.

And where is the power amp ground?

[TUBEDUDE]

It seems to me the bias circuit should be grounded with the power amp ground, not the preamp.

And where is the power amp ground?

It should be on a bolt to the chassis near the power transformer, with the power tube plate and screen supply caps negative terminals. But you want it to be connected to the negative terminal of the caps supplying the power tubes, wherever that is. The preamp ground should be as far across the chassis from there, usually near the input jack

[FourT6and2]

It seems to me the bias circuit should be grounded with the power amp ground, not the preamp.

And where is the power amp ground?

It should be on a bolt to the chassis near the power transformer, with the power tube plate and screen supply caps negative terminals. But you want it to be connected to the negative terminal of the caps supplying the power tubes, wherever that is. The preamp ground should be as far across the chassis from there, usually near the input jack

What do you mean by power amp ground? Are you referring to the power transformer high tension center tap?