Hey Enzo,
thanks for the insight, tried the fx loop thing no joy.  Plugged straight into the power amp, worked fine  so led me too the pre-amp part of the chain. 
Was wiggling each dial/knob/switch on the front and nudged the style select dial near the input jack and it sprung into life. 
turns out just some bad connections between the 5-way selector dial and the board.  So a bit of re-soldering and she should be good as new. 

Ever grateful

=:B

hey Roly,
thanks for the recommendations.
Have had a go at the amp, and the pre-amp valve that is seperate from the others (which i believe is the LM part) was incredibly loose in the socket.  I gave it a wiggle and now there is sound on the lead channel.  Not perfect mind, there is some of the guitar sound in there but it sounds like some kind of weird synthy spaceship.  Is this what a tube that has gone microphonic sounds like?
I will replace the socket as it seems like the pin receptacles are incredibly worn,  and put a new tube in. 
Thanks again.

=:B

sorry typo, 6100.

 
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. 

Ahhhaaaa!
Fantastic I'll give all of these a try.  Dont have the amp with me it is in the studio but will check all of these during the week. 
If this turns out to be the case is it a matter of some contact cleaner or swapping out the jacks then?
Thank you for your reply Ive been searching on line for days and not found a single helpful comment until now. 

 :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.