Valve problem?

[Doadman]

I have a Marshall DSL401 and I've never had a valve amp before so my apologies if the answer to this question is obvious. I've just switched everything on to practice, selected the clean channel and switched on a Hardwire Metal distortion pedal but the sound I'm getting isn't right. The tone is basically good but it sounds like the volume keeps going up and down a bit and when it goes down, it sounds awfully flat. Is this simply a blown vlave or two or might it be something else?

[Transcend]

i am also on my first valve amp which is a TSL.

From bits and bats that i have read around the internet this can be caused by the bias drifting.

Unsure of the cause of this though.

[Doadman]

I played through my practice session and by the end it seemed to have cured itself. Perhaps it's worse when the amp's still getting up to full temperature. Any insight into what it may be would be most appreciated.

[viking]

I played through my practice session and by the end it seemed to have cured itself. Perhaps it's worse when the amp's still getting up to full temperature. Any insight into what it may be would be most appreciated.

     It looks like a worn out Power Tube.I would check them and get some new ones.Unless nothing happend anymore ...