My peavey valveking 112 keeps frying the 12ax7 tube. its the one behind the input jacks.once its gone it gets distorted sound on clean side.when you turn it back to standby static noise back through speaker and again when turning off. can any one help.don;t want to scrap it just yet.

SOURCE: I have a Peavey Valveking 112 . The standby switch

SOMETIMES the screws holding the handle go down into the amp. Sometimes there are screws from the sides also.

The LAST problem is the Tolex covering often bonds and interferes with removing AND installing the chassis...

It is best to NOT put too much force till you can tell it is free... they still can be nasty to slide out...

SOURCE: I have a Peavey ValveKing

Buy ONE new 12AX7 and try it in location V1 and if it doesn't help, try it in V2. This tube has TWO heaters in it and if one is intermittent that could cause your problem. The vibration MIGHT be "fixing" a briken heater after some time.
There are MANY components that could cause this, so you might have to take it in for repair if it isn't a simple tube problem. Since it works on the clean channel, V3 should be OK.

SOURCE: my peavey valveking vk 112 is making a whistling

A high frequency feedback can be very hard to isolate without test equipment... you MIGHT have a "microphonic" tube that picks up from the speaker and feeds back... Tap the small tubes with a pencil and see if any of them "ring"... gentle taps of course... If one seems sensitive, swap like numbered ones around to see if you can find a happy place for them. Electrically there could be a bad ground inside causing these.
I notice you have 4 tubes in the output... REMEMBER when you used matched pairs... the members of the pairs do NOT go side by side all the way down the string...
Put each tube of a matched pair at the ends of the string of four, and the other pair as the middle two...

SOURCE: my peavy vk 2x12 have

* ALWAYS UNPLUG YOUR AMPLIFIER BEFORE REMOVING TUBES *

If you can see that a tube is definitely not lighting up then it may be blown. If this amp has seen regular use then it may be time for a new set of tubes. Try replacing the 12AX7s first before committing to a new set of 6L6s. Though, if you have the cash available, a new set of power tubes isn't a bad idea either. I would advise you to do the preamp tubes first and play through the amp for a few days first before changing the power tubes.

Keep in mind... if a tube (or tubes) has blown, there could be another issue in the circuit. Swapping in a new tubes is a quick way to determine that. Then it would be time to take it to your local repair shop.

Good luck and keep rockin'!