half a liverpool problem, need help

[funkmeblue]

first off this amp has worked perfect since 2008 and has been used quite a bit. It is a 2 hole liverpool with a choke and a tube rectifier. I was jamming her out new years eve and she began to cut out. It lasts for only a second and recovers. You can actually see the el84's turn off, not the heaters, but the plates and screens. Voltages at the plates and screens drop significantly when this cut off happens. There is also a post phase inverter master here and anything over half way up causes this problem. It seems when the amp is hit hard, it can't keep up.....output transformer tests good and all voltages are spot on where they have been until cutoff. Maybe a bad filter cap that is loosing charge....used jj can. Thanks in advance for any ideas.

[M Fowler]

Did you change out the power tubes?

[funkmeblue]

yes individually and as a set, They are the russian military tubes from the tube store and only have a few hours on them. All tubes have been switched out with known good ones. I wish it was that simple. It has to be in the power supply I am figuring.

[funkmeblue]

what if the cathode bypass cap on the el84's was shorting out. The cathode voltage changes when the tubes cut out as well....

[selloutrr]

I would test ALL your tubes on a tester with working voltages.
Then recreate the problem.
Hint: if a pair of el84 are shutting off its probably before your output stage. Look at your PI

Meter you filter cap can for leak.

Visually look for a lose wire of cold solder joint. Something shorting that shouldn't be. As well as burn marks.

[Firestorm]

Try cleaning/replacing the MV pot , too.

[funkmeblue]

ok, now with another set of power tubes installed I can't get it to cut out. I thought I had tried this last night with another set but I did have a few beers in me......one of these prefered series from the tube store is shit.Looks like they have a 6 month warranty. Sorry to waste your time, thanks