In the middle of a gig, I suddenly noticed my clean wasn't clean anymore.
It wasn't really dirty,  just not as clean as it was before. 
Swapped power tubes and V1, to no avail.
When I got home, I put back the old (JJ) power tubes and measured the bias.
One tube looked normal at about 36mV, but the other tube measured around 70 !
I know the bias was fairly well balanced when I installed the tubes.
I tried the other pair (Mesa) of power tubes again, and found the same imbalance.
I ended up using 1 Mesa and 1 JJ and the bias for 1 is 35 and the other reads 41.
Both sets (JJ & Mesa) were bought as matched pairs, and as I said, when first installed biased correctly.

I built the amp, but I'm not a tech - just a monkey that can follow instructions.
Any ideas or advice ?

Thanks for the (quick) response.   Yes, I think I can manage that.  According to the chart I have, pin 5 should measure -42 V and pin 4 should measure 441 V, is that correct ?

I'll open her up and see what gives.  Seems strange that a supposed matched pair are so grossly unbalanced.  I just found 2 tubes that got a closer balanced reading - but I'm still getting some distortion on the clean channel - though its subtle - not really noticeable at bedroom level, but noticeable at anything higher.
THe OD channel is either unaffected, or the added distortion is too subtle to detect on the OD channel.

Is the voltage on pin 5 dependent on the tube that's in the socket ?
Right now the bias is reading about 41 in V4 and 35 in V5.
I would get different readings if I swapped those tube positions.

So if the bias is not equal, should pin 5 still be equal ? Or within what tolerance ?

I don't think it sounds any better.  Both a matched pair of JJ's and a mached pair of Mesa's were way out of wack. I'm pretty sure the Mesa's have few hours on them.

FWIW, here's a couple of samples of how it sounded in the first set and the second set.

http://www.afgm.ca/JesusJustLeftChicago_Clean.mp3

http://www.afgm.ca/LittleWing_Distorted.mp3

I'd check the screen resistors and make sure they aren't melty or smell burnt. If they don't then I'd check the bias voltage on each tube, preferably with two new matched tubes from a reseller that tests them at the prescribed voltage. If that checks out, check the plate and screen voltage on each tube (pin 3 and the pin 4). The plate and screen voltage should be fairly close on the OTS, if not, something has gone awry. I'm not knowledgable enough to know if you can hurt the OT without destroying it, but if you can, I wouldn't doubt that red plating a tube could cause that. Hopefully one of the more knowledgeable guys will chime in. I will say, be sure to keep one hand in your pocket while testing, and be sure to have a load on the amp, either your speaker cab or a dummy load, and all controls turned down while testing. Also, it wouldn't hurt to change your PI tube. The PI is part of the output half of your amp and is the unsung hardest working tube in your amp. Most times it's the PI we hear breaking down in the power section when we overdrive the power end of the amp, and it handles alot of voltage, so it's a good idea to change it out and re adjust the trimmer when you change your power tubes

Hope this helps

Gregg

As I was preparing to do some tube swapping/testing I disconnected my D-lator from the loop.
Other times I've done this, the sound would fade in and out for a while, then stay on.
This time, instead of fading in/out, the tone went from thin/trebly to normal, then settled on normal.
When I plug a small patch cord through the loop it sounds normal.

So I have a weird problem with those jacks, its seems. The change from in to out or trebly to normal is not instant, it fades over a second or two.

Yeah, could be the screen resisitors, the bias resisitors, or both. I'd check the readings on the bias pins (#4) and see what you get. you are defintely pulling more current on V4 for some reason.

Gregg

More info -
The bias resistors (1 ohm)

The one for V4 reads about 1.3 ohms.

The one for V5 seems dead - infinite ohms - no signal getting through
No visible sign of damage or burn on it.

This would indicate to me the problem is V5, not V4.
Tubes are supposed to get fairly hot,  no ? 
For a power tube to be barely warm so you can leave your hand on it forever doesn't seem right, now that I think about it.

But would that bias resistor affect the tube's performance and temperature ? Or just the bias measurement ?