Bugera 333XL half dead

[daniel_pt]

I have a Bugera 333XL that the preamp stopped working as it should.
The Clean channel almost has no volume and both Crunch and Lead sound clean, ie, no distortion at all. Plus the EQ pots seems to not do much at all either.

I say that the preamp stopped working, because if I plug my guitar in a HeadRush MX-5 directly into the fx return input and the amp sound as loud as ever. So, I'd say the power tubes/stage is working as it should.

I tried already with different preamp tubes and there's no change in the amplifier behaviour.
Also checked for broken connections inside the amp and cold solder joints... unfortunately also no luck and tbh, didn't see anything bad with the wiring and solder joints...

I'd be much appreciated if anyone who's familiar with these type of amplifiers could give some pointers to me start with. What voltages should I have at the preamp tubes or any other particular part of the amplifier? What signals should I follow and have?

Thank you!

[The Dude]

Here's the schematic. I would inject a signal and follow it through the amp looking for the point where the signal level drops significantly. Use a scope if you have one. If not use a DVM set to AC volts.