Problem in a McIntosh MC60 amp

[CB Amp]

This is about a hifi amp, but perhaps some of you know about it...

Here is the schematic :
http://www.kbapps.com/audio/schematics/ ... /mc60.html

One of the power tubes glows (not dangerously), so I checked the voltages. The plate voltage falls slowly when I turn up the amp from 450VDC to 330VDC (then I turn off).
I changed C9 and C10, but it did not solve the problem (without the 6550, the voltages are good of course).

McIntosh uses "unity coupled transformers", the power tubes are biased with a winding of the output trans.
Does anyone have an idea about what happens ?

[Structo]

Have you tried new power tubes?

It is not unusual for a tube to get out of balance when it is worn down.

Unlikely that the OT is bad, unless it was ran without a load.

First suspect in a tube amp is always the tubes.

So check the power tubes and rectifiers.

[CB Amp]

Yes, I forgot to precise that I tried some new tubes (power tubes and rectifiers). Always the same side of the PP glows.

[billyz]

Did you check R 26,27, 24, 25 and c14 ? what is the bias voltage on each tube.

[Andy Le Blanc]

its a fixed non adjustable bias, check the bias filter cap (c14)
it could also be a bad coupling cap on the side of the circuit that shows the symptom.