None of the regulars have joined in, so your first response will be from outside the 6AS7/6080 tube rolling canon. That canon can be found in the non-obvious cable section of head-fi:
https://www.head-fi.org/threads/for-6as7g-tube-rollers-here.410326/ - starts in 2009. Just don't look at the prices back then if you want to avoid tears.
Could just go there and save yourself my personal output tube rant.
Finding a 6AS7/6080 tube I can enjoy daily has been my Goldilocks dilemma in the Eufonika H5 (6SN7/6AS7 amp). After spending hundreds and hundreds on all the cool or rather hot output tubes, I've discovered that this AC regulator family turned audiophile grail isn't my cup of current.
In my (jaded) opinion the 6AS7 - from RCA to Chatham are a bit murky sounding. Either too dark or veiled compared to many 6080s. To be fair, I haven't listened to the GEC 6AS7, regarded as the pinnacle. Of pricing. Refuse to pay $500+ for a pretty light bulb.
However, I don't enjoy the 6080 types very much either. Find them generally too upfront, pushy and grainy for my delicate tastes. Have 6080s from Tungsol, RCA, Sylvania, Raytheon, GEC and soon will try the 'semi-holy grail' Bendix 6080WB graphite plates. Maybe they will bridge the gap between lush and dull and annoying and grainy. Maybe. Could be another expensive disappointment in waiting.
Have also used the uber-rated 5998 and 7236. The 5998 might be the closer to what I want in an output tube - excellent soundstage, but still not that enjoyable. Especially at the absurd prices 5998 now command. I recall when they were a good solid $50 tube. At $200+ each, not so much. The 7236 tube - shares some 5998 characteristics, but I find it a bit cold and clinical even tho it runs hot. But is much cheaper than 5998s. It's today's solid $50 output option.
If you've read this far, thanks for reading... and I'm happy to report I found my balanced bowl of porridge... er... output tubes. First the still cheap and plentifully available. The 6BX7/6BL7 family are a notch up on the utilitarian ladder - designed for video use rather than plebian regulation duties.
One can find NOS 6BX7/6BL7s for under $20 each. However, the caveat is that a single 6BX7 might be a bit anemic depending on the amp and the phones. It can't deliver the phone-melting 120mA per section current that a 6AS7/6080 can. 4-6mA is sufficient to push a pair of HD650s into the threshold of pain for me.
6BX7 has a slightly higher output impedance than a 6AS7, which isn't ideal. Many use them in pairs - which halves the output impedance - to deliver a better damping factor. Which may or may not be important. However, each 6BX7/6BL7 requires 1.5A of heater current. Depends if the amp can safely deliver 3A of heater current. I prefer the presentation of the 6BX7 to most any 6AS7/6080.
My fav all round output tube is the GEC CV4079. It's a proper one-triode, one-tube regulator tube, so two are required to replace a single 6AS7. Easily solved with a
dual adapter. They are well-balanced tonally, smooth, yet detailed with a wide deep soundstage - what I imagine a GEC 6AS7 might sound like if I could find one for $35.
A pair of CV4079s offer a much lower output impedance than 6AS7/6080s while consuming only 800mA each. And best of all, they are still quite low cost. Or were, until idjits like me blab about them on the internet. Langrex still has a decent price on
GEC CV4079s.
Good luck on your journey to find the most satisfying bowl of porridge!
