Lead boost shutting off

So after about 45 minutes of play, the lead boost shuts itself off and won't turn back on until the amp has cooled off for a while.  The amp runs and plays perfectly in all other aspects.  Running through a 4x12 8ohm cab.  Anyone know what this could possibly be?

Edit: I should have also mentioned that the tubes are in good shape.

Anyone? Can't find a solution to this problem anywhere.

Is it only the Lead Boost or is it Lead+Ultra? The only thing I can see which would easily cause a problem here is the control line to the boost which is activated for Lead and Ultra and incrfeases the Boost level for those two channels.

It's only the lead boost. Once it shuts off the lead and clean channels are still working fine. Tubemeister 18 by the way.

Ahhhh! the 18!  That explains it, ignore what I said then for the moment.

If it's just the Boost which shuts off then the overall stage would seem to be fine as it passes the signal whether the boost is on or off, it just amplifies it more when the Boost is engaged.  I would suspect something wrong in the control circuitry.  This uses the Boost switch, the footswitch socket and a transistor control circuit to switch a JFET on or off in the feedback of the stage.  There is a similar setup in the first valve stage too to increase the gain of that at the same time.  So it would be useful to know whether that valve stage is also not getting its gain increase as that would show whether the fault is in the input stage JFET block or in the circuitry which switches it on and off.

A couple of questions, please answer them both and be as clear as you can.

1) When this problem happens does the Boost LED stay on or does it turn off as the Boost drops out?

2) When this problem happens can you hear any difference at all in the sound between Lead normal and Lead Boost?  Is there any increase in overdrive you can hear, even slight, between the faulty boosted Lead and the unboosted Lead?

Looking in terms of the most likely causes there is a chance that this could be simply down to dirty contacts on the sockets.  The Boost switch on the amp is actually a paralleled pair of switch poles so they would both need to go faulty to cause this which is not too likely.  It is more likely that the footswitch socket is dirty.  The amp Boost switch state is passed to the Footswitch socket and goes across a switch there which is opened when a jack is inserted. This allows it to be controlled by either the amp switch when no footswitch is plugged in or by the footswitch when it is plugged in.  The Boost LED comes after that so it is also controlled directly by those switches which is why I want to know if it works as normal.

If you have a simple TRS jack (3 way - Tip Ring Sleeve) type double footswitch available you could plug that in and try to control the Boost from the footswitch.  If you don't have that then just plug a standard TS jack plug (2 way - Tip Sleeve) into the footswitch socket, maybe a spare guitar lead not plugged into anything else on the other end.  This should turn the Boost on as it is controlled on the Ring of a RTS jack and works by being shorted to the sleeve which is grounded.  Putting in a normal TS jack plug simulates the ring and sleeve being connected together.  If the problem does not occur while you are set up like that then it is a dirty Footswitch socket switch and may just need spraying with contact cleaner.  Even pushing a jack in and out a few times sharply may help it temporarily.

The LED light for the boost turns off as well, and there's absolutely no difference in sound between lead and lead boost once it goes out.

I really appreciate all the advice, Atleast I have something to go off of now. Thank you!

If the LED goes out then your problem is not in any of the circuitry. I think all you need to do is to clean that Footswitch socket and you should be good to go. Try the jack trick to see if it works with a standard guitar jack in there turning the Boost on. That removes the socket switch from the picture.

Let us know what you find, it may help others later.

Absolutely, I'll give it a try after I get out of work today and report back. Thank you again for all your help!

Lo and behold! Plugged a guitar cable into the footswitch jack once the lead boost went out and it set itself to clean with the lead boost on, unplugged the cable and everything seemed to be in working order so I cleaned the foot switch jack and it's been working. We'll see how it goes from here. Thank you so much for helping solve the problem! Kudos.