Hi Chris (Waltersalas),
Regarding 6SN7 upgrades, a point to be aware of is that depending on the specific design of the amplifier in which they are used vintage 6SN7GT tubes (as opposed to GTA’s or GTB’s) may or may not be suitable. If you were to consider a vintage 6SN7GT I would suggest asking the distributor if it would be suitable for use in the 508ia before making a purchase.
Specifically, the vintage 6SN7GT is rated for a maximum plate voltage of 300, while the GTA and GTB are rated to be able to handle 450 volts. Also, the maximum plate power rating of the GT is 5 watts per tube and 3.5 watts per section, while the GTA and GTB ratings are 7.5 watts per tube and 5 watts per section.
Some and perhaps most recent production tubes that do not have any of those suffixes conform to GTA/GTB specs, so depending on the specific design a vintage 6SN7GT might not be a suitable substitute for a recent production tube identified only as a 6SN7.
Also, I believe the VT-231 that was suggested is equivalent to a 6SN7GT, and does not meet GTA/GTB specs.
Finally, FWIW I had used a matched quartet of 1950’s Sylvania 6SN7GTB’s in my VAC Renaissance 70/70 for a couple of years, with great results. One of them eventually developed a microphonics problem, and I replaced it and its counterpart in the other channel with a matched pair of 1950’s General Electric 6SN7GTB’s, which I have found to be excellent performers despite their low price. Along the way I’ve also tried Raytheons (pretty good), CBSs (yuk), and initially a new set of VAC-supplied Chinese tubes (one abruptly shorted out in a rather spectacular manner after two hours of use, and I haven’t let any of the others get anywhere near my system ever since).
Best regards,
-- Al