How far over the limit can I go

One thing you absolutely need to keep in mind is to replace the 6550 power rectifier tube at 2000 regardless of whether you remove the 8 power tubes. If you don’t and that tube fails (you won’t hear anything in terms of sound because it’s not in the signal pathway) you can fry the board. And that is a very expensive repair......

@co93

let’s not confuse matters, ok?

the arc ref110 does not run a 6550c as a rectifier -- it uses solid state rectification

https://www.arcdb.ws/model/REF110 -- see ’tube complement’ under specifications

Another silly post with a simple obvious answer.
If you can’t hear any difference why change the tubes???
You gain nothing.
But by all means keep a spare set waiting for the day.

yes this is generally sound logic with most tube amps but arc tube amps run the risk of needing a repair to bias resistors and/or the circuit board area near it, if a power tube has a ’hard fail’ - despite the wonderful sound of these amps, this is one of the issues arc amp users face, and thus, typically users replace tubes before they are close to being totally shot

I would contact somebody like Upscale Audio or Brent Jesse let them know what type of system you have and the tubes you are currently using and let them recommend some NOS tubes not only will you be replacing your tubes but you will also experience a much better sound.

nos power tubes are not a practical option with modern arc tube amps - we are talking modern day kt120 or kt150’s which the amps are tuned for driving, for maximum power and transparency

but kevin deal or jim mcshane will sell you properly screened and burned in sets of these for the ref110 upon request - there were shortages of these through last year when covid hit but as of late in 2020 supplies started catching up

to @smrex13’s point

if you do swap in a fresh set of power tubes to hear them, BE SURE to turn down the bias current substantially BEFORE installing the fresh tubes, then, over 30-60 min playing music, gradually increase the bias to get to the 65 ma reading across the test resistor for each side holding steady... if your present set of well used tubes are properly biased now, that bias setting will be WAY TOO HIGH for the new set

detailed instructions on biasing (using a simple multi meter with hook probes) are in your manual

you don’t want to fry your new set or age them prematurely by doing this swap incorrectly

2000 hrs use recommendation is conservative by audio research

you can extend a good 20-25% past that if you are careful don’t listen loud or have had to drive speakers

the problem with arc amps of your ref110’s lineage is that there is no tube failure protection, so a failing power tube can (and often does) take out a bias resistor and maybe scorch a little section of the pcb, needing a minor repair

given the size and heft of the amp, even a minor repair becomes a pain for handle and ship

so most play it safe, replace tubes sooner than later, follow their recommendation

you can always monitor how close to being used up your tubes are by how much the bias current needs to be turned up from when they were new, i track that...