Peavey Classic 20 distortion when warm

[lmolter]

I bought a Peavey Classic 20 from a local musician who used it on small gigs. To me, it seemed to work fine. However, after giving it to a family member as a gift, the complaint was that distortion creeps in after the amp has been on for an hour or so. I witnessed it myself, and no fiddling with the channel volume, the master volume, or the instrument volume could get rid of the mild overdrive. Mind you, it's not offensive, but my brother-in-law likes to play blues and folk without any harmonic coloring. And the amp will produce those clean tones when it is cold.

The amp itself had been re-tubed with Electro-Harmonix tubes before I bought it. And a small 6v muffin fan was added to blow across the EL84s for some reason. This to me is a curiosity point. Someone must have thought they got too hot? I'm not sure. Anyway, after I got the amp, I put in on the bench for a couple of hours in the dark to see if the tubes were glowing in places they shouldn't be. Nothing unusual here. I then started measuring voltages and biases. This is not so easy because the amp has a printed circuit board that is cut and folded into a 3-sided box (with jumpers connecting the cut sides), and some parts are not accessible. But I did manage to get the plate, screen, B+, and B++ voltages as well as the cathode bias of the power tubes, and the first 3 preamp stages. Can't get to the phase inverter too easily.

Here's what I have so far: Plate = 316, screen = 309, B++ = 298, B = 282. These all creep downward a little over time. Is this a/the problem?

The first two preamps are 12AX7's with a cathode bias resistor of 1.5K, a plate load of 150K, and a resulting cathode bias of -1.1V, and B+ of 282V. The 3rd stage preamp (which drives the tone stack), is also a 12AX7, with a cathode resistor of 1K, a plate load of 100K, and B++ of 309V. This puts the grid at -.5v, which doesn't seem quite right. The resulting plate current is about 3ma, and the plate dissipation is well within norm, but there's no chart for a grid bias of -.5 with a plate voltage of 300V. The chart shows a plate voltage of 200V when used with this grid voltage. (All tube technical data was taken from the RCA Receiving Tube Manual, 1973). But, since I'm kind of a newcomer to tube electronics, maybe this is ok. But something is introducing distortion, and I thought it might be the biasing of one of the preamps. Oh, BTW, the EL84's are also cathode-biased, and their grid voltage is -10V, almost right on spec.

My next plan of attack is to connect my sine-wave generator to it and follow the signal though with a scope. Might be a good learning experience for me anyway. Just for reference, what is the nominal output of a good guitar pickup. I'll set my signal source to that value.

Any thoughts out there?

Larry

[Enzo]

Before we run off screaming into the night after taking thousands of voltage, current, and resistance readings, did we try different tubes?

I don't care how old they are or are not, tubes can fail at any time. A new pair of EL84 - or even a used pair that we know work OK - and different preamp tubes. This amp doesn't have many tubes.

Alternatively, pull the chassis and run it on the bench. Let it run an hour or whatever, run wires from it to a speaker somewhere so it has a load. With teh chassis open like this, heat can't buld up inside. SO this will tell us if there is a thermal problem in there. SInce you had it out running, did the problem show up under those conditions?

[lmolter]

Ohhhh, running and screaming into the night is the fun part<g>. Seriously, I don't have a supply of replacement parts handy, so replacing any of them is not possible yet. You're right in that I should have done a few more tests with the chassis out to see if the 'problem' reappears. I say 'problem' because I have to do a complete re-evaluation. I have to listen very carefully when the amp is cold for any harmonics that don't belong. Then, after leaving the unit on the bench for an hour or two, listen again for any distortion.

Now for the newbie question: Can the tubes themselves have thermal problems, or are we looking for a passive component (or transformer)?

And while we're on the newbie thing, here is something not to do after just turning off the power: Make sure you put the tube into the correct socket when replacing it. The EL84s and the 12AX7s are both 9-pin. When you put the 12AX7 into the EL84 socket (and the filter caps haven't bled off yet) sparks appear inside the tube! Happy 4th of July!

[Enzo]

Sure tubes can have thermal issues. They are things made of metal, and their parts are joined togehter inside. As these metal things heat, they inevitably expand, which can potentially cause something unwanted to occur.

[lmolter]

Well, I played through the amp when it was cool and no noticable distortion was heard, even at moderate volume. I tried it again after an hour and still no noticable distortion was heard. Ok, maybe a little. But I think the real culprit is my brother-in-law and his not knowing how to the various volume controls interact. He has trouble with the fact that there is a channel volume and a master volume when the amp only has one channel. Good question, I guess, and if you have the channel volume up too high, it will overdrive the 2nd preamp and PI. I think that's the idea. Oh, well... I'll buy it back from him so I have a small practice amp of my own. Then when I get the Bassman finished he'll want that. No refunds.

L

[Steve A.]

Oh, well... I'll buy it back from him so I have a small practice amp of my own. Then when I get the Bassman finished he'll want that. No refunds.

L

"Mind you, it's not offensive, but my brother-in-law likes to play blues and folk without any harmonic coloring." Ummm... you better not sell him the Bassman. Are you sure that he wouldn't be happier with a nice clean solid state amp?

Steve Ahola

[lmolter]

Actually, he's got a Fender Princeton solid-state, and an old Randall of mine. He likes them both (prefers the Randall). But that little Peavey had a richer tone that made the other sound flat.

Hey, do you think if I swap the first 12AX7 for a 12AY7, he won't overdrive it as much? I've got one on order for another project so maybe I'll try it. And what about the PI? Could I swap the tub there, too?

Sorry. This is a newbie speaking.

L

[tubby]

pre-amp tube swapping is no problem and absolutely harmless to guitar preamp designs. there's gonna be a little less gain with the 12ay7 in v1 position. the difference is not gonna be as articulate in the PI position. you might want to experiment with a 12au7 or 12at7 for even more headroom.

[jrfrond]

...Now for the newbie question: Can the tubes themselves have thermal problems?...

The entire operating principle of tubes is thermal, therefore thermal problems can and do exist, even right out of the box. The Eastern Bloc is good at manufacturing tubes, but their consistency and QC is not yet even close to what the USA was yet, so you must be on your guard at all times. Swap tubes first, period.

[lmolter]

Well, I bought it back from my brother-in-law, swapped the first AX7 with an AY7 (then swapped it back) and it's a great little amp. I like the smooth distortion. Nothing 'wrong' with it.

L

[felixq78]

Your brother in law.

Ah,..the classic brother in law.
Usually they're the ones who tell you, after you've just scored a cheap Bassman or an incredibly cheap '63 Telecaster, that you should have gone to them because they could have got it cheaper.
In your case the goose doesn't know the difference between the "Master" and the "Volume".
I guess he can be forgiven, most amps have them labeled Volume & Gain or in my Marshall's case Volume, Master Volume and Boost.
If he's so in love with that dry soulless tone you should get him to play through a PA amp, absolutely no harmonic colouring there!