#1
Hello there,

my amp sometimes seems to 'kick in' and it becomes more powerful and has a lot more reverb and gain to the sound, but 5 minutes later it will go back to a more plain sound. It's a Marshall DSL 401 amp and i've had it for about a year and 9 months.

I opened up the back and all the valve (both powertubes and preamp) lights are still on, although 2 of the preamps on the right are dimmer than the two on the left.

I've had this problem for about 3 months, I have a spare preamp tube, but it looks like the preamps i've got are fine. So what could my problem be?
#2
Just based on what you said, I would guess it is the power tubes going bad. Buy a matched pair of new valves - I'd go with JJs.

www.dougstubes.com
www.thevalvequeen.com
www.tubedepot.com
www.hotroxuk.com (if in europe)
www.watfordvalves.com (if in europe)
#3
^+1
I'd get a set of preamp tubes if it has been a while since they were done also
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate