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

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...

I purchased an ARC D-115 MKII amp in ‘87. I had it for 20 years before I sent it to ARC to get re-capped. It was good until 2020 when it started blowing the line fuse over and over again. Unfortunately this also was the start of the pandemic.

ARC couldn’t repair it so I gave it to a local hifi dealer to repair. They had it for a year and didn’t make much progress.

I upgraded in 2021 to a REF 75SE and was back in business. Finally I had music again. All in all, 30 years on the D-115 is pretty good. I loved that amp and still have it. One day I’ll send it back to ARC for repair