I just had another EHX style (this time "Groove Tube" branded) 7591A tube die on me. This time it was nice and died by putting out far too little bias current (~25mA vs ~47mA) instead of the typical short-and-glow-red death. Life span: about 4 months.
I've counted at least 9-10 boxes for this type of tube sitting around and I've only had my McIntosh MAC 1500 in working order since summer 2007. The repair guy at a local audio shop says 7591s should last through 6-7 years of constant use at a minimum.
In defense of the tubes, the 1500 DOES run the screen out-of-spec (see this post from 2003). A poster recommends lowering the bias current to 30mA to let the tubes run cooler. Unfortunately anything below 45mA results in some oscillation, lower than 40mA results in extreme low frequency oscillation. I always operate the receiver with a fan blowing over the entire unit.
I've discovered that electrically identical NOS 6GM5 tubes can be found for not much more than the price of the EHX 7591As but the 8-pin sockets are riveted to the chassis- replacement is a no go unless I spend $87 on some overpriced adapters.
JJ also makes a 7591 but I've found posts claiming that they're even worse reliability-wise. I've also found some posts claiming that some early prototypes were sold on Ebay so I'm not sure what to believe. I'm not going to spend hundreds on an NOS quad.
Sovtek makes a 7591xyz, which is basically a 6L6GC- the specs show it can handle up to 450v on the screen, which is what I need. But my McIntosh is fixed bias so I'd have to modify the the bias supply. The service manual doesn't make this look easy. Plus they supposedly doesn't sound all that great.
Should I take a leap of faith and get a quad of JJ 7591s? Stick with the EHX tubes I know will keep failing? Something else?
