Originally Posted by
Blackdog
The only difference between a 12SN7 and a 6 is filament voltage. If it is working with the 12, does it take an inordinately long time to warm up? Do the 6's glow overly brightly. Next question, is this a brand new piece or bought used?
What I'm getting at is, it shouldn't work properly with the 12's. Tube emissions will be too low,. On the other hand, if designed for the 12SN7, then you will burn the 6's very quickly.
You need to check the filament voltage with a meter.