That's a 'damper tube' used in color televisions, but its basically a rectifier. Since the Sylvania is a perfectly good tube, I wouldn't worry about it.
@atmasphere +1 Those old TV damper tubes make good robust rectifiers! They lack slow warmup though! Tube circuits should have the heater voltages applied first and allowed to warm up before being slammed with high B+ voltages.
Yes, and that's one of the great design elements of an @atmasphere amp
So, I realized in my "box" of tubes, I did have a RCA NOS 6CJ3 hanging around.
I tried it in place of my stock Sylvania and did notice a difference in tone. The RCA was in fact a touch warmer and more melodic.
Interesting for sure.
I have procured a GE made Amperex ( for 10 dollars ) and will give it a try as well.
Thanks for your insights and happy Thanksgiving