Not a Sphinx, but FWIW when I had a Rogue RP7 (driving a ST100 I still have), my favorite 12AU7 type in the primary position was the venerable Amperex 7316. It was the most dynamic, holographic, with just the right amount of "sparkle". The Amperex 7316 is difficult to find these days and commands quite the price. For a "fatter" sound, but still plenty of top end air and a wide bandwidth, RCA black plates did very well. These tubes were sourced from Brent Jesse. I tried many 12AU7 combinations and spent entirely too much money doing so, but I learned a lot and it was fun. Honorable mentions go to the Ken Rad 12AU7, which is a very unique and beautiful, almost chimey (in a good way) tube. I did not like, and TBH have never liked Mullards in my equipment.
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$200 is a very good budget. Brent is a good guy, so that is great if you buy from him.
I recommend the RCA 3 MICA Black plate 5814A tubes from the 1950's. Nice warmth and bloom - good transparency and dynamics...maybe $77 a pair. Also, the CBS 5814A - less warmth, better extension on top and bottom, great transparency. Raytheon is also excellent and similar to the CBSA.
@mesch if you can, try to take listening notes first 20hrs, 100hrs, 200hrs. Often times people don’t give it enough time with the stock tubes -or- internal caps before replacing them prematurely. Me included on a few occasions. Good lessons learned.
While I’m over here making cool upgrades on mine, friends chose to "leave it alone" keeping the stock tubes and caps in their units for 200-300hrs. Their new vs. well burned in amps and stock tubes, caps, sounded better, given some patience and extra time.
Then say at 200hrs if/when you still decide to swap in other vintage tubes, you’ll know what changes the changed tubes actually make. For your consideration.
@decooney That is exactly what I intend to do. I may have 30 hours on the unit now. Running a computer as a server into it as I work on the new house. Will get to the 200 hour point within the next month. After that time I will consider all the suggestions acquired herein and purchase a pair of NOS tubes for trial.
It's all fun & games with no one getting hurt :-)
@paul_lindemann Thanks for the recommendation.
@thecarpathian Please let me know if you put your tube up for sale. Thanks.
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