Marshall JCM900 2101 100 Watt-Volume Problem

[winkofaneye]

I've just acquired this combo,for a knockdown 50GBP.It works,but the volume is very quiet,almost 'talk-over' quiet.Its a 100 watt 1x12 from 1990.I assumed it could be dirty pots,so gave them a good clean-up with some servisol,did the jacks too.No different.I now think it could be the output valves,as the pre-amp gain stage is working fine(just quiet).Any ideas before I send it to my tech?I want to get it working to sell on,as its not really my sort of amp.Thanks in advance.

[SatelliteAmps]

Low volume can be a variety of things, but tubes are always the first thing to check.

[mistercoffee1]

Quickly try plugging a patch cable from the effects loop out to the effects loop in.

[Blue Strat]

Could be one or more tubes, including/especially preamp tubes, or 50 other things.

How do you know the preamp section is working fine?

IMHO, supplying an amp tech with "technical theories" about why your amp is malfunctioning can be useless if not counterproductive. I'm sure he'll figure it out but there's a process which only people with the amp in front of them can follow. The rest of us can only guess wildly.;)