Tube replacment? CONFUSED!

Preamp tubes don't need to be matched in any way.

I'd be more suspicious of the power tubes with volume drop off and unwanted distortion. A preamp tube crapping out would certainly account for the distortion, but you should still be able to get full volume unless it's just about dead. Besides, preamp tubes last longer, usually outlasting several sets of power tubes.

Preamp tubes don't need to be matched in any way.

One of those 12AX7 tubes is probably the phase inverter that feeds the power amp stage. I was under the impression that you wanted that one balanced.

In a hi-fi amp.

Actually, many if not most guitar amps are built so that the phase inverter is somewhat imbalanced with a perfectly balanced tube. Sounds better unbalanced. Creates some even harmonics, which are totally cancelled in a balanced push-pull stage.

Personally, I never pay (and never will) for balanced output tubes, either.

Personally, I never pay (and never will) for balanced output tubes, either.

What about matched pairs output tubes especially in a fixed bias amp?

I still won't do it. If one tube of the pair is running too hot, you've either got a tube that's plumb out of spec for the tube type or you're running biased way too hot anyway. Bias it down till the hotter tube's OK.

Just my opinion. I think Class AB amps are supposed to be Class B amps slacked off enough on the bias just enough to not have excessive crossover distortion when not pushed hard. That means a "cold" bias. Biasing them up towards Class A as many are doing now decreases the original benefit of higher power output for a pair of tubes of a given type and is abusive to the tubes. It brings on the transition from "clean" to the crossover distortion of overdriven push-pull tubes earlier, with a more gradual and smoother transition (while giving up output power and increasing the waste heat on the tube plates), which is why many players prefer it. An amp set up for this sort of operation should be run with lower voltage on the screen grids and plates, closer to ratings for Class A operation. Or just go to Class A push-pull, which once was popular.

So now I know that I don't have to get the pre-amp tubes matched like I would with a set of power-amp tubes. And that the volume drop off could be caused by a failing power-amp tube. What I would like to find out is which pre-amp tube is which....There are three in total and they are all 12AX7EH's (Electro Harmonix) but is one of them for the clean channel, one for the Lead channel and the other for the Reverb? or am I barking up the wrong tree here? Cause the problem I am having seems to only be with the clean channel and the Lead still sounds full and powerful. The Reverb also seems to be working as it always has with no problem when I tilt it to the maximum. Is there a way of telling or testing which valve does what?

So now I know that I don't have to get the pre-amp tubes matched like I would with a set of power-amp tubes. And that the volume drop off could be caused by a failing power-amp tube. What I would like to find out is which pre-amp tube is which....There are three in total and they are all 12AX7EH's (Electro Harmonix) but is one of them for the clean channel, one for the Lead channel and the other for the Reverb?

Do you have a schematic or is there something in the manual? One of them is for sure for the phase inverter - inverts the phase of the signal to feed to the power tubes. In a modern amp it's unlikely that you have a tube powered reverb. So, those other two tubes are your pre-amp. Maybe it only uses one for clean and adds in the second for your dirty channel? If it does that then the amplification on the second pre-amp tube might mask any decreased output on the first tube when using the dirty channel. Do you have to turn up the second (dirty) channel to get the same volume as you used to?

The tubes might be:
V1 - clean and first tube in dirty channel
V2 - second tube in dirty channel
V3 - phase inverter

They will probably be laid out in something like this pattern too going from the input jack towards the output though it might not be straight. On my amp it goes in an L pattern.

Buying two pre-amp tubes won't break the bank (~$8 / piece) so if you order two (or even three) and only need one then you've got yourself some spares for the future (they won't go bad lying around).

Here's the owner's manual for it: http://www.peavey.com/support/searchmanuals/list/valveking.cfm

Thanks guys, you have been more then helpful in my time of Tube amp dispair...
...The tubes seem to be laid out in an L shape with the first pre-amp tube nearest the input jack, the other next to it and the third behind that at the back of the amp next to the power tubes. I am going to get a set of three pre-amp tubes so I got some spares and swap them out until I can tell which tube is failing.

Thanks again.

Replacement with a known good tube is actually the best way to see if a tube's causing the problem. It's always a good idea to have spares on hand.

Sometimes simple oxidation or dirt in the tube socket contacts causes problems that get blamed on tubes. Pulling the tube out and replacing it can clean up the contacts enough to make it work. It's a good idea to have something like De-Oxit, tuner cleaner spray, or even WD-40 (don't laugh, it's been used as contact cleaner in house at places like Hewlitt-Packard) to spray in the contacts and work the pins in and out a few times before diagnosing one as bad.

Oh I'll try that out. I've got some WD40 here, so that wont damage anything then if I spray a little in to the contact itself. Maybe it would be a good idea to give them all a bit of a clean out because I bought the amp second hand from a guy who lives a few hours away from me. He might not have been much of a caring owner. The amp is just under a year old but I suppose it wont hurt to try cleaning it before I put the new valves in. If its dirt causing the problem then I will still have three brand new valves on hand for future problems.

On another note. I am pretty handy with a screwdriver and can pick things up very quickly. Can you guys suggest a good book that would teach me the basics of maintaining and servicing a tube amp? And maby some background on how a tube amp works etc.

I wouldn't spray anything into the tube sockets. You could lightly spray the pins on the tube, wipe off excess and insert and pull the tube a few times. But liquids and electronics do not mix. :D

Actually that's standard recommended practice. You don't want to make a big mess with it, of course. A very small amount is all you need. Putting it on the tube pins and inserting them is a good way to limit the application so you can't spray too much in, but if you're careful it's OK.

I haven't read any of the modern books on amps, so can't personally recommend them. This one's gotten a number of recommendations as a basic book aimed at musician amp users: http://www.amazon.com/How-Service-Your-Own-Tube/dp/0962817007

(For theory I recommend electronics textbooks from the early 1950s.)

There are lots of websites. Google the words "tube amp maintenance."

Do not use WD40 on tube socket pins.