6SN7 Equivelents

[6A3sUMMER]

kodabmx,

Agreed!
The most often cause of a different sound is the two problems as you stated.
Bad operating conditions or topology
Worn out tube

Or, a bad new tube (seen it too).

 

[kodabmx]

True... I've had some NOS tubes arc over on power up or ring like a bell. Never had low emission though, although I didn't test the tubes that arced over lol