Pro Reverb questions:

[mnexsen]

I have an early silverface Pro Reverb. I had the bias circuit changed to bias adjust from the stock bias balance. I also had filter caps, coupling caps and the bias cap replaced. Otherwise it's stock.
Recently the amp has lost most of its punch and clean headroom. I replaced the power tubes with some that I knew were OK. I measured bias on the new tubes using a cathode current bias probe: They started out around 22 ma, gradually climbing to 28 after 1 hour. Then I played a little and the bias current jumped up to 35 ma and seemed to stay there, even with the amp idling. I tried to raise the idle current, but ran out of room on the bias pot.
Questions: Is the gradually rising bias current normal? If I wanted to increase the bias current, I "assume" I could replace the fixed resistor soldered to the bias pot with a higher value, increasing the resistance to ground. Is that the right way to fix this problem?
I basically need lots of clean headroom and can get my drive from pedals. I'm just looking to get that big, meaty tone back.
I've ordered some new tubes and will check the amp after a few hours of burn-in to see if they help. I've also ordered another rectifier tube just to have on hand.
More questions: The amp has a bunch of big, ugly brownish-orange caps that didn't get changed. Do these contribute to tone? Do they need to be changed? If so, what would I use to replace them? Finally, the preamp is a mix of 7025's and 12AX7's. Are there any specific recommendations for replacing these?

[Alex R]

It's not unusual for current to rise a little as you describe though to go from 22 to 35mA is quite a way. I think I might be tempted to put new caps on the bias circuit just to be sure they're ok. Yes you could change the bias adjust range by changing the resistor to ground, but I would make sure the rest of the circuit is ok, first. Anyhow, whether it's 22mA or 35mA won't affect headroom as much as you describe, in fact hardly at all, so I would say the bias current in itself is unlikely to be the cause of your problem. And 35mA is high enough, I wouldn't want to raise it any more.

Maybe a good first step would be to watch the B+ like you've been watching the bias current. Does it hold up over time? Headroom issues might be power supply related, and the bias current changes just another symptom.

Personally I like the bias balance circuit in sf amps but I'm funny that way.

Do caps contribute to tone? Hmmm. Plenty of discussion on that one elsewhere. I would say, for the purposes of actual fault diagnosis such as we are engaged in here, no they don't. The differences cap types might make are very small; we are talking about a faulty amp here.

7025 and 12AX7 are likewise best regarded as identical and replaceable with any 12AX7/ECC83 type, for the purposes of fault diagnosis. Best not to mix up the business of fixing a broken amp with the strange world of tonal improvement, imo.

[txstrat]

Did the amp lose it's punch in both channels?
If not you might check the preamp tubes. Faulty preamp tubes can cause low volume as well. If you don't have new/other preamp tubes handy you could interchange them with each other.
What Alex R said 7025 and 12AX7 are interchangable.