Peavey XXX head issues

[1WoodHippie]

I've got a Peavey XXX head that the owner says after about 20 mins "the distortion doesn't sound so good". Never looses volume, just doesn't sound as good distortion wise. And he put new EL34 power tubes in it. Never listened to it on the clean channel cause he doesn't use it. I fired it up and played for about an hour. It didn't sound too bad to me but I'm not familiar with what they are supposed to sound like. I did the think that even the clean channel was not all that great.

Started checking voltages and the power section seemed to be good. The only red flag I see is on the power tubes, only one of them have high voltage on the screen. they all have like 2 - 5 Vdc on them except one that has voltage close to the plate. And those screen resistors are those 5W ceramics.

I was going to start with replacing those and see if that returns the high screen voltage seeing as how they all have high voltage on the supply side, just not the tube side. Just looking for a consensus that I'm on the right track and not overlooking something else obvious.

thanks in advance.

[soundguruman]

The screen resistors can arc to other components, and fail.
The 2X 100 ohm filament balance resistors can also arc to other components on the board.

Remove the power amp board and check for burning, carbon traces across the fiberglass.

Move the screen resistors and filament balance resistors to the opposite side of the circuit board, if you see this burning occurring.
This prevents them from arcing and burning the fiberglass.

The octal tube sockets can also accumulate carbon traces across the ceramic...replace them if this is happening.
Get some new sockets from Peavey. These sockets have specialized screw holes, for threaded screws.

Of course, I always suspect that this occurs because the amp is being connected to the wrong impedance speaker....
or the speaker cable is defect and has poorly made plugs on the ends, which are intermittently dropping out, and failing.
Look out for cheap/junk speaker cable plugs with loose rivets.

Also the speaker cabinet...replace the PC mount input jacks.
The jacks oxidize and lose proper continuity.

Anything that causes the amp-to-speaker connection to become unreliable, may cause the output board to arc,
and the tubes to turn red or fail.

Look at R84 and see if the solder connections are overheating the circuit traces.
The board will appear burned or brown, right at the connection point.
Bypass this with buss wire and clean it up and solder it better, so that the connection won't overheat.

Once these issues are corrected, the amp becomes consistent.

[Mick Bailey]

If you only had 2 - 5v on three of the screens then I would expect almost no volume as only a single tube would be working properly. Double check the screen voltages on those tubes. Also, with the amp turned off and discharged, check the resistance of each screen resistor value.

Three blown screen resistors would be unusual - especially 5W WW. If this is the case I'd be looking for a reason why.

[soundguruman]

For some reason this amp eats screen resistors.
I had to change some.
It might be that 100 ohm should be replaced with 1K(?)
It might be that the screens need more current limiting...at least for EL34.
It might be that 100 ohm should be changed to 470 for 6L6(?)...etc...

At least if it were my amp, I might try changing them, and a few other things.
I think this may be a little hard on the screens, the way the factory builds it.
Especially when I am playing the amp wide open. (what's a master volume?)
I wanna torture the output tubes but still, I want it to keep working.
That way I am getting my output tube overdrive, nice and smooth.

And if I change it the more sensitive the plate becomes.
That would be OK w/me too.

[g1]

Typically, screen resistors fail because of failing power tubes.

[Mick Bailey]

Typically, screen resistors fail because of failing power tubes.

Agreed - but three different failing power tubes would be an unlikely scenario. Not impossible, though.

I looked into the reasoning behind SGM's suggestion that they eat screen resistors and found an number of references to potential problems with 100 Ohm resistors. Apparently they were changed in August 2003 to 700 Ohm, mainly to limit screen current with E34L tubes. (which are also rebranded as EL34 by some suppliers).

[1WoodHippie]

double checked screen voltages: 1.2v, 4v, 490v, 1v.

Ohm'ing the resistors while still on the board seemed to jive. All the tubes with the low voltages didn't even tweak the meter, the high voltage ohmed out fine.

And after I got to thinking about it, when I was playing it I had that thing on the overdrive channels almost wide open, it should have been ear bleeding loud like a twin in my small shop but it really wasn't. So maybe only 1 power tube IS really going.

think I'm going to hunt down some 700 ohm WW's and throw them in and see if makes things pop back up.

Agree... Disagree?

[Jazz P Bass]

You should smack the guy that ran EL34's on a 100 ohm screen resistor.

No wonder he was eating tubes.

[g1]

You should smack the guy that ran EL34's on a 100 ohm screen resistor.
(

You mean James Brown the designer?

[Mick Bailey]

think I'm going to hunt down some 700 ohm WW's and throw them in and see if makes things pop back up.Agree... Disagree?

That's what I would do, as well as checking out that no other damage has occurred + check the bias.