Troubles with recent Ampeg tube amps

[Sweetfinger]

I don't like working on modern tube amps made with everything on cheap circuit boards and jumper strips, but two friends are having what appears to be the same problem with their amps.

One is an Ampeg Jet II J12T, which is a 2X EL84 combo, and the other is an Ampeg Reverberocket R 12R with 2X EL 34's. Both are from the late 90's -SLM product.

Both play fine initially, but once they've been on for a bit at stage volume, the sound cuts to a low level with distortion.

Anybody had similar problems with these?

[Enzo]

DO both have effects loops? If so, plug a spare cord from send to return and see if the sound is restored. If so, the return jacks need service.

[Sweetfinger]

The larger amp has a loop, the small one does not.

[52 Bill]

The only "sort-of" common problem with that series of Ampegs that I've seen, is loose or burned out power supply resistors. Check any large 5-10 watt ceramic block resistors in the power supply.

[Amp Kat]

The smaller amp Reverberocket is a sweet sounding little combo. I had one that had some cold solder joints on the tube sockets and were dropping out. Loose socket pins can also cause this in those and just common connection problems in these.

[Sweetfinger]

The big amp ...may..... have been suffering fom bad loop jack connections. Today the amp didn't want to cut out other than intermittently, I cleaned the loop lacks and it didn't go on the fritz after that. Here's hoping one excorcism is behind me.

I was surprised myself by how good the smaller Jet sounded. Has a nice Valco-ish raunch to it, definitely not a tribute to the "classic" Ampeg sound at all. It actually gets too distorted and "splatty" in the upper range of volume settings(blocking distortion?) for my taste, however.

It had REALLY roasted the 84's and anything I put in would start lighting up the screen grids at even a moderate volume, so I upped the screen resistor values which cured that problem and mellowed the amp a bit, which is in line with my personal taste and the owners wishes as well.

It sounds great until it dies!

[_Paulito_]

A friend has an Ampeg Reverberocket 2x12 (reissue) with the same problem: after playing for a few minutes, the volume dies down to a tiny fraction (like a portable AM radio)...but then it returns after a few minutes. This repeats itself.

I reseated the two power tubes (I think they're EL34s) and the problem seems to have gone away (at least, after about 15 minutes of playing I haven't yet reproduced the problem). So perhaps it's the tubes, or their seating.

Another problem: a low, loud hum was being heard when I turned the amp on. Tilting the amp just a few degrees resolved the issue; also, after reseating as described above, the problem hasn't reappeared.

I'm a tube newbie: can anyone comment as to whether these are common problems with tubes/connections?