How often do you send your tube pream/amp to the manufacturer for service and calibration?

Yeah, amps are not tape decks, still I thought it might not hurt to do it every five/seven years or so.

I would probably do it at twenty years... but with the exception of my first great preamp... an Audio Research SP4 (?... not tube... but that would not change my behavior) I’ve never had one more than about ten years.

Never have sent a tube amp or preamp to a manufacturer.  The only thing I ever sent to the manufacturer was a CD player.

Never have sent them anywhere, but I bias output tubes regularly, change tubes, and replaced caps, and resistors as needed.

Never...I've had ARC and BAT preamps...