Out of boredom, I decided to replace the 6201 tubes in my 600R. I took out my backup pair and looked at the date-2010! Wow, I’ve had that set of 12AT7’s in there for ten years. Of course the new tubes had a little more extension and were cleaner, but still thats pretty long. When Frank says tubes will last for awhile in his hybrid amps, he wasn’t kidding. These were Sylvania gold pin tubes. Everyone stay safe and listen more.
While I am sure that running under the conditions of a 600 hybrid is far more conducive to tube life than a normal type setup, small-signal tubes in any application seem to last and last. My dad's ST70 ran the same 7199s that came with it in 1962 or whenever, and ran an average of several hours a day until it was rebuilt in maybe 2008 or so, and they still test good. It has been running the same GE 6GH8A tubes since then.
But even more so, it is very rare to find a bad small-signal tube even in old tube radios all the way back to the 20's. Gassy, gone to air, yes, corroded pins and loose grid caps, yes, just used too much, almost unheard of, even in AA5s where the filaments are exposed more-or-less directly to all the line variations. It is extremely common to find bad power and rectifier tubes, which is what you would expect, and still seems to be the case today.
Some miniature 1.5 volt/battery radio tubes like the 1L6 are prone to degeneration (usually correctable by running them at slightly higher current) when used at high frequencies, but they barely worked when new, too.
The only small signal tube I have ever had a lifetime problem with are modern-production 12AX7s, particularly the JJ. Even the lowly Sinos seem to be fine in these applications.
It is in no way surprising for a 12AT7 run in the very benign conditions of a Frank amp has lasted for 10 years, I would assume the life to be effectively infinite, probably will last longer than the electrolytic capacitors.
Brett
