“6681 was “mobile rated”, otherwise specs were per 12AX7. Mobile rated meant not only rugged and hardy in the face of repeated on-off cycles, just as stated, but also capable of performing under extreme fluctuations of filament voltage as supplied by Jeep or halftrack battery, etc. under field conditions, I believe the spec called for reliable operation w/in range exceeding 5V to 7.6V on the heater! ” Read
” big, upfront, punchy, with lots of bass.”
This is very much like other Sylvania 12ax7 gray plates tubes. Very in your face with great bass, but also very sharp sounding. Not very transparent. Soundstage was better than new production, but can’t really compete with leaders. Good for some applications – like rock or metal, didn’t do anything for jazz or classical. Didn’t like the tube, but can see that is some applications it would be OK.