I have a Marshall 6100 LM 30th anniversary head where just the lead channel has no output. 
Clean and crunch channels work fine.
All the pre-amp tubes glow/get hot.  ( i dont have a tube tester some cheap good recommendations would be welcomed).  Also for future reference which tubes are for which channel?

the tone controls for the channel are working, When I put the master and channel volume up and twiddle bass, mid and treb the tone changes.  But the gain knob for the channel doesnt have any effect, and feels a little scratchy to turn. 
Is this simply the gain pot needs changing?  Or is this a dead tube?
Will tubes still get hot and glow if they are not functioning correctly? 
Also if anyone has a schematic for this amp please link or send I have had no luck finding one online. 

 :trouble
I have an Ampeg SVT1000 head that needs repairing.
Suddenly the amp has almost inaudible output. 
Cab is fine have tried with another head. 
I am adept with a soldering iron and have no problem replacing which ever part may be at fault.
I have a good multimeter and voltmeter and am a qualified electrician.  Electronics is more of a hobby.     
have looked inside the amp and no apparent signs of overheated components or bad solder joints. 
Whats stumping me is that there is some volume when all pots on max.  about the same as if playing the bass unplugged. 
any suggestions on how to diagnose or if anyone has experienced this fault with any ampeg gear or anything solid state. 
Relatively sure I should be looking in the output stage.  any suggestions or experience muchly appreciated.