marshall 20/20 el84 problem :-(

[eggle]

hi all, my marshall 20/20 stereo power amp has gone weak as hell on one side(left channel).I have replaced all valves and no difference.I changed my preamp to eliminate if it wasnt my ada preamp but still the same. i also tried another power amp to make sure the speakers hadnt failed but it worked fine with another power amp.
when i just run the left side in mono mode its weak as hell and distorting at low volume, and when i use the right side its loud and clean.there is no burn marks on the valve casing or on any of the circuit board.all fuses are fine...any ideas please? jas

[Amp Kat]

Do you have a multimeter ?

[eggle]

i do indeed

[Amp Kat]

Ok I did a search but couldn't find a schemo so lets do some simple checks first and see what we get. There are some lethal voltages here so do be extremely careful. First without any power and the tubes out put your meter on ohms and check from pin 9 of one socket to pin 9 of the other socket and tell me what you read ? Am also assuming this amp doesn't have loop jacks right since it's a power amp only ?