Peavey VTM-120 Possibly bypassed Heater Elev ckt?

[Just Mike]

Someone had hardwired the 6.3VAC windings directly to the output board heaters which does connect thru the ribbon cables to the rest of the heaters. But in tracing the circuit, I see that there is a DC elevation ckt on the PS board as well as the 8A fuse. I don't think the elevation circuit or the fuse will work like this and I'd like a 2nd opinion on my thoughts.
Also, I was trying to trace down some hum the amp has. Whoever did this heater job also replaced the main 100u filter caps so I'm assuming they're OK. The only other caps would be the odd value 200u/75v bias caps and a 1000u/50v heater elev. cap. How critical are these cap values? So far, they're impossible to find.

[Just Mike]

For reference, I've attached the schematic. I get why the 6.3v winding was wired directly being that the molex connectors tend to burn up.

[Enzo]

If they wired the transformer to the power tube board, yes it skips the fuse. But if the amp works, then that means the ribbons are getting the heater current over to the pramp tubes. SO the question remains: is that 6v wiring still connected to the output side of the power supply board? In which case, the DC elevation is likely intact. SImple test: measure and find out if the heaters have a DC offset on them.

Cap values are hardly critical, this is only a guitar amp. Look for standard values. 63v instead of 50v, 200uf is not a standard, but 220uf is, so search those. Can't find 75v? Try 80, 100, or whatever comes next.

If they did indeed kill the DC offset, that alone could be your hum source.

Hum? First, is it 60Hz or 120Hz? That tells us about the source. And know that hum is not generic, there are multiple sources of hum, and each has its own cure.

[Just Mike]

The 8A filament fuse was blown and I don't happen to have one on hand. I don't know if I should risk a 10A fuse here.

[Enzo]

I thought it was already hard-wired past the fuse.

[Just Mike]

It WAS. I'm restoring the original layout and at this point, I've removed the output tube board direct filament connections and soldered them to the original J7 molex pins on the power supply board.

[tedmich]

As Enzo has stated (possibly more than a few times...) the cap values are all non critical. This era Peavey did use all big axial caps which are going the way of the dodo, but with careful mounting radials or even snap ins can be made to work (a daughter board sometimes helps with the latter). I'd opt for a good name brand radial from a real electronics dealer.

By all means put the wiring back to original with close value caps! The 100 ohm / 5W SGRs can all use an upgrade to 470-1k before they blow as well IME.

[Just Mike]

I've restored the heater circuit and it works. The hum balance doesn't remove the hum I'm describing. It does bring in &out more of what I'd call a buzz, so I'll assume that works as it should and call it good. Now, on to the 120hz hum. It's present as soon as I take the amp out of standby. It's got all new JJ tubes. I also replaced the bias ckt 200µ filter caps to no effect. The 4-100µ filter caps have been replaced by someone before me, they are F&T 100µ's and I assume they're pretty new. But if they're not, they're wired in series for an effective value of 50µ, so I think I'm ok with testing with a F&T dual 50µ can? and in doing so, I see that there are bleeder resistors across each 100µ cap. I assume I can just leave these in series across 1 50µ cap with no ill effect?

[Enzo]

A dual can cannot be wired in series, it can be wired parallel.

They wired them in series to get higher voltage rating in sum.

You can measure ripple on the B+ ssupplies with a scope or good meter. Find out if there is a problem there.

120Hz means power supply ripple. The bias supply is half wave and so makes 60Hz ripple. The B+ might have a weak cap, but can just as easily have a grounding issue, especially if there has been any actions in the inter-board wiring (the ribbons).

[Just Mike]

Yes Enzo I know that the 2 100µ in series is 50µ@700v. My 50µ cans are rated at 500v. That was my concern. But you are correct in suggesting the meter scope approach first. Thanks again!

[The Dude]

I think what Enzo is getting at is that it's not possible to wire a dual can cap in series because there is only one common ground for both caps in the can. The two caps already have their negative leads tied together in the can. At any rate, if you replace 2 series caps with a single cap rated at higher voltage and half the capacitance, you wouldn't need the totem pole resistors, but where are you going to find caps rated at 700V?

[g1]

I don't think he wants to wire the two 50uF's of the can in series. I think he wants to tack one 50 across the stock totem pole (two 100's in series) as a test.
Just Mike , is the 500V sufficient voltage rating there? If so, yes you can do it and you don't need to worry about the bleed resistors being in place.
But if it's less than 500V there, I'm not sure why they would have bothered with the 700V totem pole arrangement.

[The Dude]

I agree, g1. I was just clarifying Enzo's point and also pointing out the potential voltage problem using a single cap instead of a totem pole arrangement.

[Just Mike]

I got it all wired up properly and replaced all of the Ecaps, but I'm still getting the 120hz hum when I take it out of standby. It's pretty low in level, but I need some help in isolating it. The filament DC offset is exactly 27V. Tubes are new JJs.

[Enzo]

SO isolate the problem. Does it do it with no guitar plugged in? If so, then leave the input jack empty.

Do ANY of the controls affect the hum in ANY way? Level, tone, whatever?

Turn your guitar to zero and plug it into the FX return. Still hum?