Peavey Delta Blues Question

[JMMP]

A friend is having a problem with this amp, looses power after about an hour of playing time. Has a new quad of power tubes, had the speaker replaced several years ago, preamp tubes may be original to the amp. Had him plug a short cord cord into the send/return when he experienced this, said that power issue got worse. Looking at the schematic I'm thinking I'll swap out V3, probably all the pre's when I get this amp in a few days. Gonna be checking voltages as well. Anyone have any insight as to what else to look for here? Also, why do only two of the power tubes, V4 and V7, have screen resistors (according to the print)? Seems odd to me.

[The Dude]

A schematic would help.

[JMMP]

Schematic here: Prowess Amplifiers - Misc - Schematics - Peavey Delta Blues - Peavey Delta Blues

[JMMP]

Schematic here

Prowess Amplifiers - Misc - Schematics - Peavey Delta Blues - Peavey Delta Blues

[The Dude]

Yes, do try swapping preamp tubes first. Given the length of time before the amp malfunctions, it's a likely candidate- a tube that croaks after it gets hotter. As far as the screen resistors, many of that era Peaveys are designed like this. The screen voltage is already dropped by R58 at the screen supply node. The additional resistors (R43&53) are just for isolation/decoupling.

[Leo_Gnardo]

Had a similar problem recently in a similar amp: Classic 30. In this series of amps tube filaments are series wired. In the Classic it turned out one of the preamp tubes had a filament that opened up after 15 minutes or so warmup. Took 3 tries swapping in a new 12AX7 until the problem ceased. So there's something to look for, do the filaments in pre or power tubes quit at the same time the sound does? If that's the case you know what to do next.

[JMMP]

Thanks guys, will post follow-up when the amp gets here.

[JMMP]

So I got the amp here in my shop, ran it for 2-3 hours with signal, got it very warm. I noticed a slight volume drop with time (power tubes are new, pre's probably 5-6 years old), nothing really significant tho. Anyhow I replaced the pre's with a fresh set and it seemed the volume came back to what it was shortly after initial fireup. Ran it another 1-1.5 hrs with consistent performance, gave it back to the owner. He reported back that after playing it about 2 hours he again noticed a vol drop. Started at 5, had to turn it to 9 (!?) to keep up. I'm wondering if the OT is heating up affecting output. Amp was running pretty warm after 2 or so hours here in the shop, but ran fine after replacing the pre's. Ideas, thanks in advance.

[JMMP]

So I got the amp here in my shop, ran it for 2-3 hours with signal, got it very warm. I noticed a slight volume drop with time (power tubes are new, pre's probably 5-6 years old), nothing really significant tho. Anyhow I replaced the pre's with a fresh set and it seemed the volume came back to what it was shortly after initial fireup. Ran it another 1-1.5 hrs with consistent performance, gave it back to the owner. He reported back that after playing it about 2 hours he again noticed a vol drop. Started at 5, had to turn it to 9 (!?) to keep up. I'm wondering if the OT is heating up affecting output. Amp was running pretty warm after 2 or so hours here in the shop, but ran fine (still very warm but no vol drop) after replacing the pre's. Ideas? thanks in advance.

[Enzo]

OT? That is just wire wrapped around an iron core.

Have you cleaned the return jack cutout contact? Measure it with an ohm meter - not just continuity test - if it measures more than an ohm, clean it. I expect close to zero ohms. A contact that measures say 10 ohms will work, but that non-zero resistance means the contacts are dirty, and can cause issues. Of course this is only one possibility.

Use those jacks to isolate the problem. But first, he had a significant drop in volume, not a subtle one. SO does he use pedals or other effects, or does he plug straight into the amp? To the amp, plug a signal into the FX return and let it play, any symptom? Run FX send to some other amp, is that diminished?

[JMMP]

Cleaned the contacts but didn't measure them. I'm assuming that is done across the jack with nothing inserted? Enzo, I'll do what you suggest when the amp comes back to me, hope the anomaly occurs. It didn't for me after changing the preamp tubes, I ran it for at least an hour with no appreciable drop in volume.

[Enzo]

Yes, when the jack is not in use, those contacts close.

Measuring the contact resistance is one of the first things I do when I encounter a seemingly OK amp that does something the jacks could cause. Like switching channels by itself (FS jacks) or volume dropping (FX loop jacks).