Pavey VTM 60 preamp hum

[wormdirt]

So I have a VTM that has developed a terrible 60hz (I believe) hum
Standard maintenance, tube socket + jack cleaning new tubes and filter caps have been preformed
Nothing looks obviously burnt, cold solder joints etc
I believe the problem is pre MV control because raising the pre / post controls raises the hum considerably. Plugging a cord into the effects loop stops it, but only if I ground the + on the cord by half inserting it into the jack. If I plug in and leave it dangling then it gets worse.
When I engage the first DIP switch I get a sizeable POP, this is new. This switch adds an additional 4.7k resistor in parallel to the 6.8k cathode resistor on V1b.
Because this pops I believe I have a ground problem somewhere. Pops in conjunction with switches like this, to my limited knowledge, usually means there is voltage being fed into the ground somewhere

Anyway I'm pretty handy at tube amp repair but the real deal is being able to actually know what it wrong and what to do about it.
I'd appreciate any help as to what my next move should be.

[wormdirt]

Peavey-VTM-120-Schematic.pdf

[Enzo]

Switching the little DIP switches while the amp is running is going to make pops. That would be normal. Switching that resistor in instantly changes the cathode voltage. That results in a pop.

If the PRE control affects the hum level, then the hum comes from before it.

the real deal is being able to actually know what it wrong and what to do about it.

Well that is the whole deal, isn't it? You need to systematically isolate the problem, narrow down where it is and where it isn't.

You have HIGH and LOW gain input jacks, they connect to different things. Plug a guitar into the LOW gain jack, and dial it to zero. Still hum? The V1a circuits are between those jacks.

[wormdirt]

Switching the little DIP switches while the amp is running is going to make pops. That would be normal. Switching that resistor in instantly changes the cathode voltage. That results in a pop.

I'm positive there was no pop before the recent hum creeped up, but when I switched it I might have had the master volume dimed so I'll check that out

If the PRE control affects the hum level, then the hum comes from before it.

You have HIGH and LOW gain input jacks, they connect to different things. Plug a guitar into the LOW gain jack, and dial it to zero. Still hum? The V1a circuits are between those jacks.

I'll do this in the in the morning thanks

I'd like to add that it is at its worst with high gain (duh) but that it is something other than the usual noise floor

[wormdirt]

Still hum either jack as well as with nothing plugged in. Went ahead and ordered some more caps as well

[wormdirt]

Ok new problem, maybe I should make a new topic where I don't misspell Peavey
Amp started to make a new terrible noise. I'll link a youtube video of the amp
Nothing is plugged in and all controls are at zero. PI and power tubes are the only tubes in the amp. If I unplug the PI the noise goes away.
So far I have put new power supply caps and new plate resistors in the preamp.
I reflowed a lot of joints that were not nice and shiny. Tried different tubes in all positions and cleaned / retentioned the tube sockets

https://youtu.be/2oPOBuYCBRk

[glebert]

Does this have stacked (aka mezzanine) circuit boards? Probably not the cause of your problem but if they are stacked make sure those interconnects get cleaned well. I have had Peavey Triumphs (which I think are similar) and poor signal and ground connections are a common problem. I actually have a Triumph right now that has the exact kind of noise you described in the original post and I have never been able to figure it out, I was hoping to get some insights from your fix

[g1]

Did you try a different PI tube?

[wormdirt]

No on the mezzanine board

Did you try a different PI tube?

Yessir

At this point I'm going to order new coupling caps for the preamp. This is a real irritating troubleshooting session.

[wormdirt]

Wait NVM I already replaced most of the coupling caps
I'm going to move onto some of the smaller caps

[SoulFetish]

Try pulling the preamp tubes in order, one at a time, starting with V1. Let us know if the noise goes away, and if so, after which tube was pulled.

[SoulFetish]

I think your looking in the wrong place. I can't think of a time I've ever needed to wholesale change coupling caps in any amp I've ever fixed unless the customer made a special request that I do so. While there's probably no harm in swapping them all out, perhaps we can help you from a lengthy, costly, and frustrating fix.

[Enzo]

I can't say I recall coupling caps causing hum.

[wormdirt]

The sound stops only when the PI is pulled even with no guitar plugged in and all controls at zero

Normally I wouldn't do the "shotgun" approach in troubleshooting an amp but I was already ordering parts and I wanted to get all the high voltage caps replaced

Enzo I don't know if the noise qualifies as a hum exactly
Check this video of it happening
https://youtu.be/2oPOBuYCBRk

[Enzo]

I don't have sound on my computer, sorry.