Peavey Classic 30 - preamp hum

[Buddha's Guitar Tech]

Hum gets worse when you turn up the EQ controls.

Everything looks ok inside - i've had it out of the enclosure and saw nothing obvious. i replaced the input jack with a panel-mounted Switchcraft (the original really DIDN'T want to come out!) and got rid of the FX loop jacks, bypassed the loop but it didn't alleviate any would-be ground faults.

i think it's a ground problem but i can't be sure at this time.
After i put it back together i noted a curious thing on the schematic. It looks like there are two 47R resistors separating audio and power ground, which makes sense.

Before i open it up again, i'd like to know if any of you have observed this hum already and can point me in the right direction. i have no reason to suspect a bad filter cap, but i guess stranger things have happened.

Thanks

[selloutrr]

bad preamp tube.

Make sure all your pots, nuts and bolts are secure. also clean the contacts on the jacks.

you need a rework station to successfully work on these amps, to get out the jacks, without over heating other components.

[ampdoc1]

These amps are a bear to work on if you have to remove the boards. But I'd first check all the right angle wires that connect the boards Seen them loosen or break many a time. The PCBs weren't great, but much better than the cheap shit out there today.

That separation of the grounds is used in a lot of Peavey products of the time. I don't really understand why it works,...but it does.

a'doc

[C Moore]

These amps are a bear to work on if you have to remove the boards. But I'd first check all the right angle wires that connect the boards Seen them loosen or break many a time. The PCBs weren't great, but much better than the cheap shit out there today.

That separation of the grounds is used in a lot of Peavey products of the time. I don't really understand why it works,...but it does.

a'doc

Yeah....I hate to say it. These things can be a lesson in frustration. If it is not a preamp tube, or some other "typical" solution.....check all those jumper wires on the tri-folds.
Re-solder/Re-flow all joints you can get to.
Best

[C Moore]

These amps are a bear to work on if you have to remove the boards. But I'd first check all the right angle wires that connect the boards Seen them loosen or break many a time. The PCBs weren't great, but much better than the cheap shit out there today.

That separation of the grounds is used in a lot of Peavey products of the time. I don't really understand why it works,...but it does.

a'doc

Yeah....I hate to say it. These things can be a lesson in frustration. If it is not a preamp tube, or some other "typical" solution.....check all those jumper wires on the tri-folds.
Re-solder/Re-flow all joints you can get to.
Best

[Buddha's Guitar Tech]

Not a preamp valve, that's for sure. Don't think it's a filtering issue. Scoped the caps and after the screens, it's all pretty flat.

Whatever it is, it's happening before the EQ controls. i really want to say it's a ground loop thing. Been scratching my head with it this morning, trying hard not to gut it and make something else.

BTW, if you've ever done any 'star-grounding' and brought every last ground to one point, it can sound pretty dreadful. You need just a fraction of an ohm's difference between audio ground and power ground - makes the whole world a little quieter.
But Peavey have tried this with two 47-ohm resistors separating certain grounds... but in places the grounds are connected... so i'm not sure how it really works. Obviously it must be fine, but not in my amp, of course

When i build, i keep the power ground at the back, as close as i can to the first capacitor ground, and then star the audio ground at the input jack in the front. Works a treat. Buss-bar along (or sound i say underneath) the valve sockets makes such a handy spot to solder cathode bypass caps, grid refs etc., saving space on a turret board or equivalent.

[ampdoc1]

I believe every amp I've built (xcept for kits), I used a single ground from preamp thru the power amp, all connecting to a single chassis ground. I've never had an iota of hum. Maybe I've been lucky, but it works for me.

a'doc

[Buddha's Guitar Tech]

Maybe you are lucky in that case. When i first had the problem, it was a eureka moment when i followed the good advice of Morgan Jones' Building Valve Amplifiers, in which he suggests there is a difference between 0V signal earth and mains earth.

i figured if these were physically apart, interaction between the two would be minimised and it works great for me. Turning the amp 'up all the way' there was so little hum it was mind-blowing...
At the time

[tubeswell]

Check the jumper leads between the boards (hired hand said it earlier). If they flex too much they crack, and you might have bad ground return on one of those leads. Also, check the chassis fastening nuts on the pots and jacks for tightness and possibly re-tension the tube socket pins

[labb]

Buddha's Guitar Tech...? to see if I understand your ground scheme..Do you actually have three ground points? one for the mains, one of the power section(filter caps grounds, center tap for the transformer) and one for the pre amp/PI section, others?

[selloutrr]

I get the feeling this is being over thought.
The amp worked at some point in it's life. With that said I would look at making sure all the ground contacts are clean of corosion and tightly seated. If you have any doubt in the 47ohm resistors splitting ground replace them. If this does not resolve your ground issue look at touching up the solder points on the board where you did modifications or tweaked / bent the board during mods. Next replace any or all electrolytics over 10-15 years old.
Keep in mind even a known good tube can be noisy in a circuit. It all depends on how that tube is implimented in the circuit.

While you are in the amp consider adding an adjustable fixed bias

[Buddha's Guitar Tech]

Labb, i pretty much employ two main points - i do a star-ground as close to the mains input as i can - this is for anything "power" related, like bias, filter caps, TX centre taps, filament refs, etc.

Then there's an earth buss for anything 'audio', like cathode bypass caps, grid references, tone stack, etc and that is connected to the chassis at the input socket. The input socket and mains input are pretty far from each other, so the theory is that the chassis has enough resistance to electrically separate the two grounds.

For the OT secondary (ie the speaker) i might cheat and just ground it at its socket.
i've tried grounding preamp caps at audio earth, like some people have suggested in the past, but i haven't had any moments where the difference is obvious.

Selloutrr, it's never over-thought until someone is threatening to jump out the office window!
In truth, you are right, it was obviously fine at some point. I recently bought it for cheap, broken, with a view to using it as a spare amp.
However, my OCD is kicking in... an amp under my wing, that is a pain to service? i just can't fathom it!

Rip it out, start again... yes, i do it to myself.

[selloutrr]

it's a simple fix.

have you metered your ohms to ground yet?

checked the bleed from the electrolytic caps?

[Buddha's Guitar Tech]

i've done a bunch of metering to ground, yeah. Nothing suspect yet. When you take the whole board out, those 47R Resistors come into play and grounds are 94 ohms apart. When you plug the OT sec back in, it joins them, so those resistances are now finding a way to each other.

But bleed - you mean any voltages on the negative ends of the caps? haven't tried that one yet, good idea. i know that when i take the input jack out of the equation, the hum goes down just a teensy bit.

[selloutrr]

peavey and tubesandmore sell replacement OEM peavey parts.

Also make sure all the clips and jumpers are secure.

Have you tried chopsticking the board while it's on to see if you can make the noise floor go down.

Or tried a grounding stick?


First pic is Stock OEM Peavey Classic 30 combo gut shot.

Second is a pic of a bias mod I installed.

Mounted the "L" bracket in a rivet hole with a bolt so no holes were drilled in the chassis. The original Bias resistor is still installed, the pot bridges the OEM resistor. The wiper goes to a resistor to ground. Giving 2 levels of safety backup in the even of pot failure. The pot is lockable. Total mod 15 min.