Strange Problem with Peavey Valveking 112 - Squeal when Gain Boost engaged

[Enoch0)))]

The issue is a loud squeal when the "boost gain" button is engaged. It is not merely feedback, because I get no signal from the guitar through the amp while it's squealing. It also squeals even if there is no guitar/instrument plugged in. If I turn the volume all the way down the squeal goes away, but comes back as soon as I turn up the volume at all. The higher I turn up the volume, the louder the squeal.

When the button is malfunctioning the following symptoms occur:
If I turn the boost gain off, the squeal goes away, however I am left with about half volume on the lead channel compared to when it's working properly. While it's malfunctioning, if I turn the gain knob, I can hear it slightly change the tone of the squeal, but I still get nothing but squeal. If I change channels and/or press the "gain boost" and/or "volume boost" buttons on/off enough times, sometimes it will start to work again, giving me proper function of the "gain boost" button and proper volume. The clean channel is unaffected by any problems.

I pulled the amp out and visually inspected everything. No components are visually damaged/scorched (I realize visually isn't comprehensive). The Gain Boost switch doesn't appear to have any damaged solder joints. A couple joints on one of the relays near the switch looks slightly suspect, so I reheated them to no avail.

I've replaced all tubes and that didn't solve the issue either.

I greatly appreciate any help you could provide.

[Jazz P Bass]

I had a similar issue (low volume on Lead Channel) on a different Peavey.
It turns out that the channel switching relay was not going all the way to ground when switched to the Lead Channel.
Schematic at: The Blue Guitar
Monitor V1 's grid resistor, R115.
See that the ground side of it does ineed go "Fully" to ground when the Lead Channel is selected.
If it does not, then the relay is bad.
(When Lead Channel is selected, ground R115, the not grid side, to ground to see if the squeal goes away)
If this is the problem, contact Peavey, they have the relays in stock.

[Enoch0)))]

Thank You Jazz P Bass! I thought a relay was a logical place to look, I just had no idea how/where to go about testing it on this amp to make sure.

I'm no expert at this stuff, merely an adept hobbyist at best. Simple and/or low voltage circuits don't scare me too much, but I was a little nervous to just go probing around without a little guidance.

I'll report back with my findings!

[Enoch0)))]

I have yet to get to the amp, but I'm just curious for the sake of learning: If the relay is the culprit, what would cause the squeal when using the Boost gain button? Maybe just the increased gain upsetting the faulty relay even more, making the symptoms stronger?

[Enzo]

Or faulty relay not completing a circuit path that it is supposed to, leaving an op amp gain loop open, for example.

[Enoch0)))]

Alright, I powered the amp up with nothing plugged into the inputs. I couldn't definitively determine which side of R115 went to the grid and didn't want to arc anything so I didn't end up shorting it to check if it stopped the squeal. Would shorting the wrong side damage something?

All measurements are from the green ground wire bolted to the chassis:

Clean channel selected, no input:
R115 - 0 volts on both sides

Lead channel selected, no input:
R115 - 0 volts on both sides
V1 pin 1 - 184v
V1 pin 2 - 0v
V1 pin 3 - 1v
V1 pin 4 - 0v
V1 pin 5 - 0v
V1 pin 6 - 206v
V1 pin 7 - 0v
V1 pin 8 - 2v
V1 pin 9 - 6v

V2 pin 1 - 196v
V2 pin 2 - 3v
V2 pin 3 - 1v
V2 pin 4 - 0v
V2 pin 5 - 0v
V2 pin 6 - 255v
V2 pin 7 - 9v
V2 pin 8 - 2v
V2 pin 9 - 6v

Voltage on R115 didn't change while changing channels, however touching the Multimeter probes to the grid side did slightly lower the frequency of the squeal coming from the speaker. V1 pin 7, and V2 pin 2, as well as R104 and R102 also changed the frequency of the squeal when probed with the MM. Am I doing/measuring something wrong?

I also tried tapping/probing on the relays and components to no avail.

HERE'S WHERE IT GETS WEIRD: When I breathe onto the vicinity of V1 socket from the inside of the amp, the squeal lowers in amplitute quite significantly. It happens every time. VERY weird am I right? Would this be a sign of anything in particular?

Let me know if you'd like me to measure anything else and I surely will.

[Enoch0)))]

I swear I'm not crazy about the breathing thing. I actually had to stop and ask my wife to stop and down to make sure she heard what I was hearing too. Sure enough, she heard it too, and definitely gave me a funny look like "what the hell did you do to that amp!?".

[Jazz P Bass]

Peavey VK 112 Schematic

O/k.
It seems that I have misled the OP on the function of the relay.
It is not the same as the solid state Peavey that I had repaired.

[Enoch0)))]

Shoot, I was hoping that would be it. Thanks for trying Jazz P Bass

I actually have the 112 not the 212. Although similar, the 112 is 50watts instead of 100watts. Attached is the 112 spec sheet.pv_valveking_112.pdf

Anyone else have any ideas? I would greatly appreciate it!

[exclamationmark]

It sounds like a case of feedback somewhere within the amp, especially with the pitch changing when fiddling with the gain. Another thing to check would see if being in the general vicinity of the tube/wires changes the pitch/amplitude of the squeel (I once made a 6 gain stage design and played some improptu theremin whilst figuring out how to get the damn thing working). I would guess a faulty relay not cutting out part of a circuit it's supposed to, especially since you mention that it 'sometimes works' while constantly spamming it's buttons. It might be sticking or just plain not working.

Generally turning up the master volume increases the amount of voltage anywhere past the MV pot, increasing EMF which can quickly cause the amp to oscillate. Pre-Master gain essentially does the same thing but worse. As to why it changes frequency as the gain level changes, I'm not really too sure (probably something to do with multiple harmonics being present in the distorted output and the gain pot/random capacitors acting as a variable RC filter, thus changing the harmonic ratio).

[Enoch0)))]

Thanks exclamationmark! Do you think I'd be best off just ordering the relay's from Peavey and swapping them in to see if it does the trick? Is there any way of manually grounding a pin/s (or some other procedure) on one of the relays to see if it is indeed the relay that's the problem?

It may be a lot to ask, but does anyone know by looking at the schematic which relay/s may be the culprit?

I realize I'm asking for a lot of hand-holding, I really do appreciate the help everyone!

[Enoch0)))]

Update: I gave up. Brought it into the repair shop. I'll post what the issue was when I get it back.

[Enoch0)))]

Turns out the issue WAS a relay. The tech that worked on it is out of the office until tomorrow so I can't verify exactly which one. It appears it was the one closest to the inputs though. I guess all he did to "fix" it was hammer on the thing until it unstuck. If I at least know which one is the problem, at least I can order a new relay and replace it if it ever happens again.

[olddawg]

The "tech" should order and replace the relay now while the amp is apart and in the shop. Hammering on it is not a fix. Once in the shop the cost of the repair should not be hugely different from the estimate fee unless it is really time consuming.

[Enoch0)))]

Its a local Mom and Pop shop, and they were swamped as heck. I implied that I was capable of replacing the part, if they could just diagnose what the problem was. In essence, they went beyond that by getting it back to working order. If it acts up again, I'll just replace it myself.