Metal Beast... thanks for reading my article.
In case anyone is curious... it is a misty rainy morning in Chicago!
I'm not sure which tube you are calling the middle one. If you mean the one between the two in the front and the 4 power tubes towards the rear, that would be the phase inverter/driver tube. If you mean the second tube from the left in the front, that would be the second preamp stage.
The first rather obvious rule of thumb, anytime you suspect a tube problem, replace the tube. If you use the amp a lot, and the preamp tubes haven't been replaced in say... 3-5 years, replace them all.
Also make sure you are using the metal shields on every tube. Get some if you aren't and they are not that expensive. But they go a long way in cutting out the tendency for the amp to oscillate (which can happen above a frequency that your speakers can reproduce) causing really lousy sound. They also shield the tube from any stray RF or electrical interference - important for a low level high gain stage.
Dirty or worn out pots, used for the controls, can cause a lot of the symptoms you describe - from a channel cutting out to the scratchy sounding master volume. If you can get a channel to "come back" by jiggling the volume gontrol for the channel that seems to have "gone away", then this is definitely that problem.
Clean the controls with pot cleaner - remember you need to get the cleaner INTO the control and you don't want to get the outside and the rest of the chassis all wet with the stuff. Do each control, move the control quickly through a few turns several times, then do them all again just for luck. It's best to remove the chassis from the cabinent for this. If that doesn't do the trick, the controls are worn and should be replaced.
I did quite a lot of writing about amp hums buried in the last page of the article, so after the above you might want to read through it and adjust the hum balance control as I explained there. It is possible that you may need to replace the electrolytic caps.
Replacing caps and controls is not rocket surgery. If you know how to solder you can do it yourself, just take what's there out and put the new one back in exactly the way you took it out. If you are not confident in your ability to do this... or just don't want to mess with it, then I would take the chassis out of the head, removing the tubes putting them in a safe place, and take the chassis to any radio/TV repair place in the area. If you buy the controls/caps yourself it will even be cheaper, and it shouldn't cost you more than a couple/three hours of labor.
You can certainly replace the preamp tubes and unless you have money to throw away, I wouldn't pay someone for that.
