Best modern 6550 tube for ARC per-amp

Wow, it's a regulator in the power supply, helps keep the B+ at a constant voltage. Music doesn't pass through the power supply at all. That happens at the output stage.
If the music is modulating the power supply than the power supply is sagging due to poor design, good in a guitar amp, not in a hifi amp. And certainly won't be happening in an ARC amp.

BillWojo

op

you do realize that the audio research ref 6 linestage uses its single 6550 tube as a voltage regulator in the power supply section, feeding the 4x 6h30p controlled gain section, right? this has been the case since the ref 2a model... where the power supply/regulation circuit incorporated a single 6550

as such, you do not/will not ’hear’ the 6550 in that unit as it does not directly participate in the amplification circuit of the audio signal... unlike 99% of other amps using that tube as the power tube pushing/pulling the actual musical signal...

given this, i suggest you simply follow arc’s factory guidance on what 6550 tube to use... longevity and adherence to performance spec as designed is more important than any perceived sound quality difference  -- or email greg christensen at arc service/support and ask him what they recommend as an upgraded 6550 for your ref 6 (they may say there is none)

Brother. Just so you know, by this "logic" the diodes, power supply caps, fuses, transformer, power cord, AC power conditioner- none of that matters because "it does not directly participate in the amplification circuit of the audio signal". The reason they call it "parroting", the bird too is just repeating sounds it knows not the meaning of. And yet, at least it doesn't get too snooty about it. Score one for the bird.

Meanwhile, back in the real world (or at least planet WeCanHear) where tubes do make a difference we can hear: JJ. These are the tubes you are looking for.

well there you have it, op - the experts and truly knowledgeable have spoken... yes, everything matters... of course it does!... no concern over matters of degree, or relative importance, sensitivity of inputs on the ultimate output/objective function ...

your $15,000 p-o-s minnesota made linestage has a sucky, marginal PS circuit that requires a gold-plated, absolutely best sounding tube (when used in a totally different application) to sound its best!!! yes -- be sure to get your money’s worth now... remember, we can’t take it with we when we expire, right?

now excuse me, need to join my fellow parrots as we listen to some birdland  :)