Mesa Tremoverb Hum

[AdmiralB]

I'm working on a '98 TOV that has a serious hum problem...actually, two of them.

It has some hum - enough to be a problem - whenever it's on. None of the controls have any effect on this hum, except for the loop master (and only if the loop is engaged). Pulling V5 kills it, so it appears to be localized around the reverb return. Hum's there whether or not a reverb pan is attached; it's worse on the red channel and in the higher-gain modes of the two channels.

It also has hum that is post-gain but pre-tone stack (i,e. the tone controls modify it and the master kills it). Pulling V2 kills this hum, and it seems to be worst on the orange channel in clean mode - although it's there on all modes/channels.

I've gone after all the usual suspects - tubes, grounds, etc. No good. I did find a post here (in the cache) from a guy named 'Stan' about a year and a half ago with the exact same problem, but he had only one reply - from a guy with the same issue whose solution was to sell the amp. Stan didn't leave an email so I'm unable to contact him to see if he ever got it straightened out.

Any ideas?

[drewl]

I recently had a bizarre oscillation problem with a Tremoverb that was in the fx return stage.
The only way I cured it was to lower the gain of that stage.
So that's where I would look.

[AdmiralB]

It's not related to the effects loop. If I switch the loop out, the hum is still there.

I think it's related to the NFB - since it is affected by the selected channel and mode, the only place post-master volume that is changed by those selections is the FB loop.

[AdmiralB]

Well, I fixed the post-master hum - it was a ground loop in the reverb recovery circuit; the stock setup has each channel's master volume, presence, and reverb pots sharing a ground bus. I removed the reverb pots from the bus and grounded them at the grid reference resistor for the post-master triode.

Now I'm left with the pre-tone stack hum. Now that the reverb ground problem is fixed, the hum gets worse as you climb the gain structure (i.e. red channel on high-gain is worst). Grounding the grid of V2A silences the hum; I believe this is also a grounding issue but it's a lot harder to trace. I played around with the grounds on the gain pots, but didn't really get anywhere. I've fiddled with the ground points of the cathode resistors for both halves of V2, again no joy. I even replaced LDRs 8 and 9.

[AdmiralB]

Problem solved. Design issue. Somehow the 470K in front of V2A causes the hum; bypassing it eliminated the hum but changed the tone (made the amp a lot brighter).

The 20pF snubber on the grid of V2A wasn't fitted to this amp (location was on the board, though). I added it, then gradually increased the value to trim the extra highs off. I stopped at 250pF; amp sounds like it should and is very noise-free.

Gotta love Randall Smith's "F7ck with it, hope it works, when it's 'close enough', sell it" design philosophy.

[Roberto Lasco]

Thank you so much Robert ! Just got a Dual Rect (old version) on the bench with this very same V2 hum. Bypassing the 470k worked like a charm...Sign me in for the Mesa Haters Fan Club !

[g1]

Mesa seems to suffer a fair bit from mounting components directly over unrelated traces, often high voltage ones.
I wonder if lifting the 470K off the board a bit would also offer some hum relief?

[Roberto Lasco]

Valid call, but nah...Disassembled it again and tried mounting a 470k away from the board, no help.

[diydidi]

The inside of these chassis are also painted. The tooth washers on all the pots don't always cut through to earth the pot casings to the chassis. I loosen all the pots and dremel some of the paint away on the inside and refit the pots. Nice 0ohm connections. Also move all wiring connected to the 120 Sandblock resistor (if your amp has it) far away from the pcb.

[AdmiralB]

Thank you so much Robert ! Just got a Dual Rect (old version) on the bench with this very same V2 hum. Bypassing the 470k worked like a charm...Sign me in for the Mesa Haters Fan Club !

Nice to 'see' you, Roberto - it's been a long time. Hope you're well!

[Roberto Lasco]

Nice to 'see' you, Roberto - it's been a long time. Hope you're well!

Hi Bob ! Sure it is ! Old Weber forum...It's all good; hope you're well too ! take care !