Volume comes and goes

[Rkorn]

hey guys,
its been a long time...

My Mark V lately, when standby is off, and i start to play the volume comes and goes, if it goes completely i pluck a string really hard and the sounds comes back... then, the amp warms up, and the problem stops. The tubes are 1 year and a couple of months old.
I believe power tubes are OK, but pre-amp tubes... which tubes do you think I should switch to end this problem????

Bear in mind that after 5 minutes of playing the problem stops... Then, its tone heaven again...

 

[Rkorn]

Also, i forgot, this happens on all channels!

 

[jb's 52]

My first try would be with V1 since that touches all channels.

 

[primeholy]

Yeah. Sounds preamp. Also try cleaning the effects loop.