Marshall JTM45 with problems: help

[flatart]

I'm trying to repair a marshall JTM45 of a friend of mine.
If I play this amp very low, less than 2 (on 10), the amp sounds apparently good. If I play it louder it sounds very bad, a very ugly distorted sound, it seems like we have a broken speaker.
Consider that I have already tested all tubes changing one at a time and they are ok.
I have measured all tube pins voltages (without playing) and they are ok, so I guess that it might be the output tranny.
what do you think?
Consider that my friend admitted that the problem appear after he tried to play for several minutes with a speaker cable broken! So I guess he tried to play without any speaker load...

[billyz]

Could very well be an output transformer. But check the bias current, blown screen grid resistors, and bad/ arcing tube socket first.

[mhuss]

One failed output tube will sound much like you describe.

--mark

[swarty]

+1, always tubes first.

[flatart]

Bias current it's about 40mA on both output tubes (KT66) so it's perfect.
Moreover I tried with another pair of svetlana 6L6GC and I have the same problem so it's not a matter of tubes. I tried also changing one at a time all preamp tubes and rectifier tube and always the same problem.
That's why I was thinking of the OT, but I dont know how to test it and I haven't anoter one of the same type to do a fast test

[dobbhill]

Coupling/tone stack cap leaking?
D

[swarty]

Try measuring resistance from CT to each plate. They should be within 10% or so.

[KT66]

It does sound like no load might have fried the OT. You should tell your friend that no sound = turn it off and check everything. I see a lot of threads like this about problems that could be troubleshot quickly with a signal tracer. I made mine out of an old boom box I got at Goodwill for $10. Just connect a probe in series with a high voltage coupling cap to the aux. input, connect the amp chassis to the ground connection of the aux. input, and use the probe to listen to the amp starting at the plate of the first gain stage and follow the signal path until you find point where it gets bad. If it is the OT though, this will only help you verify that the problem is not in the preamp or PI - don't use this method on the power tube plates.