Anybody who has spent even an hour or two in the pointy-end of an airplane as a student pilot becomes aware of the Hobbs Meter, which is a small digital-readout instrument that logs the time between "switch on" and "switch off" on the engine(s). The Hobbs is useful for logbook entries, and it's what the mechanics use to determine the number of hours on the engines to enable them to schedule oil changes, overhauls, and total engine and airframe time.
I'm wondering if some DIY'er out there has come up with something similar for our tube gear. One gets a pretty good idea of how long-lived their particular tube complement is. Since they decay gradually, might it not be handy to know when they're past their prime? I'm not going to keep a written logbook for my M-125s and preamps, but it would be nice to know if they're hitting the 5,000 hour mark.
Just curious if there's something like this out there, or if one could be practically and inexpensively built.