MusicMan HD150 reverb head

[retrorod]

Hey all, just got in for repair a 1980 MM HD150 head. The complaint is total distortion and no clean tone. The owner had re-tubed with a brand? of tube that I am not familiar with. They seem very light compared to good 6L6's There is a symbol and under that what looks to be 6n3C and 8809...? I asked him to supply the replaced tubes....they are Sovtek 6L6B's ...even though the amp calls for 6L6GC's.
I see no scorched parts, however the transformers show some hot operation. All components look to be original including the 'massive sized' Mallory 100@450vdc under the doghouse. An extra-long 3-prong power cord has been installed.
I have not tested anything yet.....just done a physical inspection.
Can anyone clue me in to the 'cheap' ouput tubes described above? As far as I can see, they are not even labeled 6L6....but are 'about' the same size.
Thanks, Rod

[g1]

The russian 6n3C is similar to a 6L6, but from what I understand, may not like high plate voltages. The 6n3CE is supposed to be more robust. Musicman amps tend to run high plate voltages so those tubes may be problematic. However, the problem could also be completely unrelated to the power tubes.

[Jazz P Bass]

The 6n3C(e) are Sovtek tubes.
The same as the 6L6GC.
Check the power supply nodes for excess Vac ripple.
Get the amp on a scope & find out where it's distorting.

[Enzo]

Isolate the problem, it is either the preamp, the power amp, or the power supply. Are all supply voltages up and clean? Are all the power tube voltages close to spec?