Can’t Stop the Hum!!

[words453]

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.

Heater voltage is symmetrical. Also the 100 ohm resistors were replaced.

[words453]

BTW, thanks to everyone for the help. This thing has me running in circles!

[Chuck H]

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.

If grounding that junction kills the hum with the boost engaged then it should also kill the hum with the boost disengaged. Because the node is grounded so is everything coming through the switch from the boost circuit. The other switch on the master pot is downstream of where the problem is detected. Or at least it should be. There may be a miswire of the master volume switch for the boost circuit. I believe that is a shielded lead as well so there could also be a grounding error there.


EDIT: Thinking it over I realized that with the boost engaged you are effectively grounding the reverb tank input circuit. But not so with the boost disengaged. Further, if the hum is mitigated by grounding that junction at the 1k/470k resistors then the problem isn't with V4B because there is a 470k isolation from the V4B grid at that node and only a 1k load forming a 470:1 voltage division.

I think you're definitely looking for a miswire or a ground fault for the reverb tank input circuit (that includes the boost circuit).

[nosaj]

BTW, thanks to everyone for the help. This thing has me running in circles!

Please read this article on high resistance grounds http://tone-lizard.com/high-resistance-ground/
nosaj

[nevetslab]

BTW, thanks to everyone for the help. This thing has me running in circles!

I've run into this same problem on the Silver Face Twin Reverb amps. I've gone thru my service notes on the two we have in CenterStaging's inventory, but didn't find the problem on them, so it was on one of my other client's repairs. That problem, from what I recall, was from a damaged Master Volume Pot, along with connections in the Reverb circuit. The amp has the shielded cable running from the tag board all the way down to the master volume pot, then back again to the input of the phase inverter circuit. You could try unhooking that cable at the send and return ends, and connecting those two points together, taking the pot out of circuit. And doing likewise with the boost circuit that the pull switch engages, to see if the control and it's wiring is causing this hum problem.

I'll resume digging thru my service notes to try and find that one repair that had these issues.

[nevetslab]

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

Looking at the AC Ripple Reading at the Main Filter cap (which feeds the O/T CenterTap) and the next Filter cap, which follows the power supply inductor, feeding the Screens of the output stage, there should be a substantial reduction of ripple at this point, yet there's very little difference here. I'm suspicious of your inductor.

In my service notes on one of the SF Twins I had worked on in 2015, this same supply feed also feeds the primary of the Reverb Tank Drive xfmr. I had constant hum in the reverb circuit of that amp, and adding a 47uF/450V filter cap there eliminated the hum. Which suggested the 20uF/500V Filter cap in the doghouse below the chassis was faulty. It could be that filter cap in your amp, if not the inductor.

[words453]

Nevetslab, I have disconnected the boost circuit but not the MV circuit. So I will give that a shot. It could be the choke. I need to order one for a build, so I’ll try that out. I guess I’ll replace the filter caps, just to cover all my bases.

Good article, Nosaj, but I don’t have a scope unfortunately.

[Enzo]

If you think it might be the choke, just disconnect it and clip in a big old resistor, say 400 ohms. Well 390. Nothing special about the value, 250 or 500 ohms. SOmething, just needs to be a power resistor. We don't care for this test if the voltages downstream are slightly altered, but we do want to listen and see if the hum changed. That might be quicker than waiting for an order. Plus if it does make the difference, you know you need two chokes instead of just one.

[tdlunsfo]

I've run into this same problem on the Silver Face Twin Reverb amps. I've gone thru my service notes on the two we have in CenterStaging's inventory, but didn't find the problem on them, so it was on one of my other client's repairs. That problem, from what I recall, was from a damaged Master Volume Pot, along with connections in the Reverb circuit. The amp has the shielded cable running from the tag board all the way down to the master volume pot, then back again to the input of the phase inverter circuit. You could try unhooking that cable at the send and return ends, and connecting those two points together, taking the pot out of circuit. And doing likewise with the boost circuit that the pull switch engages, to see if the control and it's wiring is causing this hum problem.

I'll resume digging thru my service notes to try and find that one repair that had these issues.

I'll second this one. I repaired a 70's Bassman 100 a few years ago that had 3-4 bad pots. I dissected one of them and was surprised to see cheap plastic and the wiper material completely delaminating.

[words453]

If you think it might be the choke, just disconnect it and clip in a big old resistor, say 400 ohms. Well 390. Nothing special about the value, 250 or 500 ohms. SOmething, just needs to be a power resistor. We don't care for this test if the voltages downstream are slightly altered, but we do want to listen and see if the hum changed. That might be quicker than waiting for an order. Plus if it does make the difference, you know you need two chokes instead of just one.

Will do.

[words453]

I replaced the choke with a resistor( 300 ohm/10 watt) and the hum was substantially lower. It still has a slight hum but the amp would be usable. The hum did get louder with the boost engaged but at least we’re on the right track!

[Enzo]

OK, so the resistor worked better than the choke. SOunds like a choke.

[words453]

OK, so the resistor worked better than the choke. SOunds like a choke.

The new choke fixed it. I still can’t figure out why there was no hum in the normal channel and why the hum seemed to be injecting itself only at v4 but it sounds great now! Thanks Enzo and everyone who offered advice!