power loss and static

[Tubes rule]

I'm new to this forum, hoping to get some answers :/ I have a vht 12/20 special and it has two el34s and three 12ax7s and just lately after playing for a couple hours and it warms up real good, all of a sudden it'll cut to less than half output and I can here what sounds like TV static, then when I shut it down it'll increase the static and then fade out........anybody have suggestions please......THANK YOU

[The Dude]

Welcome to the place!

It's a good idea to post a schematic when you start a new thread. The first thing to try is tube substitution. It could easily be a tube that fails after it warms up.

[Tubes rule]

Welcome to the place!

It's a good idea to post a schematic when you start a new thread. The first thing to try is tube substitution. It could easily be a tube that fails after it warms up.

I actually put the old tubes back in it, and unfortunately same results, I even moved them around to other places, the 12/20 is self biasing, with a switch to go el84 6v6, el34 or 6l6s

[The Dude]

http://music-electronics-forum.com/a...210-1220rt.jpg

Are you comfortable opening up the amp? What do you have for test equipment?

[Tubes rule]

I'm not equipped with any thing to test, my amp is the special it doesn't have reverb, guess I was hoping on some kind of easy fix lol, if I put it in trode mode the static goes away and seems to work as it should, if that enlightens anything

[The Dude]

That does narrow it down, but you'll need to get inside of the amp and test some things to figure out the exact problem. If you're not comfortable with that, it's time to take it to a tech. Do tell them about the triode/pentode thing if you take it in. It will save them some troubleshooting time. It may be the triode/pentode switch itself, but you'd need to take the amp apart to clean it properly. Try running the switch back and forth several times with the amp off. It's not likely a permanent fix, but might tell you if that is the problem.

[Tubes rule]

Awesome yea I'm pretty good with cleaning and what have you, I'm gonna go threw it and post an update, thank you for your time

[The Dude]

Oh, and FWIW, I generally find the VHT amps with similar model numbers are mostly the same- just with or without reverb. That is to say, the rest of the amp is probably the same as the schematic- just without the reverb circuit.

[pdf64]

I would (temporarily) take the switch out of circuit, hardwire it to pentode mode, and see if that got rid of the fizzle.
Putting a regular 250V switch on to a power tube plate seems a bad idea anyway, though I guess that mains spikes may have magnitudes even greater than the back emf spikes resulting from a overdriven tube amp feeding an inductive load.
Whatever, triode mode generally sounds naff.

[Tubes rule]

Awesome that'll be my next move, did I mention after it warms up and starts fizzleing and I power it off, the fizzle becomes louder til it shuts down