I just finished repairing an amp for a local shop a pretty nice 1965 Super Reverb. They were complaining that it was sounding distorted, "All the tubes are good" I was told. I put it on the bench and did some checking, NO bias supply what so ever. It turns out that the diode was bad and not supplying anything. This amp looks like it was run like this FOREVER! The transformers have been hot and I can see where some wax has melted out of them the bottom of the chassis is black/dark brown all around on the bottom near the output tubes.
I repaired the problem, set the bias with a new set of output tubes and gave it a test. The amp sounds just fine, pretty good actually. Just for kicks I put in the old cooked output tubes and they were so bad I couldn't believe it! I've never heard a Sovtek 5881 THAT worn out!
I'm amazed how someone could have an amp that sounds good yet use it while it sounds so horrible never noticing that it's FRYING hot all the time and has no power.
It is a testament to how well built a Fender amp from this vintage really is because this one took a licking..
I don't think I have ever seen more than 1 or two failures on the Russian 5881's they are tough. I never thought that they sounded very good in an old Fender but they do seem to last a very long time and don't break up easily from hauling them around.
These, I should have recorded them sounded like a flat tire!