Stick w/Mullard.Anything NOS,ESPECIALLY 1950's Long Rib Plate..Holy Grail would be to find the pretty much non existent Western Electric,also from the 50's...
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May I suggest that you look at 5751 tubes. Slightly lower gain but much cleaner.
They are a direct replacement.
Google "Joe's Tube Lore" for a nice review of many different brands.
While your and my results will vary from those of Joe's, it is a good general starting point.
You could buy several pairs and try them to find what best fits your system and then sell those that don't float your boat.
A very good one with a bold presentation and rather sweet treble is a Raytheon 5751 with the windmill getter.
Good luck and Have Fun with it.
(RCA "clear tops" generally refer to their 12AU7's, 6201's are a substitute for 12AT7's) ...since the Thor TA-1000 has plenty of gain, I use black plate 5751's with their lower gain in mine. Telefunkens are generally thought to be "brighter" and better for older vintage equipment.
- As an aside, a knowledgeable friend compared black plate vs. grey plated tubes and could not hear much of a difference.
Yeah, I was a bit surprised by "no difference between black plate vs. grey plate", too. I hear very clear differences between Sylvania black plates and grey plates; same thing for GE black plates and grey plates.
For the preamp adjustment the OP is addressing, I would think 5751 RCA Black Plate triple micas would be a nice fit. I find them to be richer, but still have good enough detail and extension.
I own a Thor TA1000 Mk II and its one of the best preamps I ever owned. I did have to do some tweaking to add some warmth but at no time was it ever bright or harsh. It's very detailed and with excellent separation and air but to add some warmth I am using Brimar 12AT7s and RCA 5751 Triple micra Black Plates. also add a good power cord, I am using Cardas Golden Reference. Also Amperex 12AX7 Bugle Boys are a good choice. Also a good choice for the 12AT7s.
Remove the Telefunkens.
You sure cannot lump all 5751's together. Try some GE grey plates and you will get a leaner presentation. Try some RCA black plates and you will get warmer. And there are a whole lot of variations in between. I do think "Joe's Tube Lore" does a pretty fair assessment of the options available, and is worth your reading (it's in the FAQ for Tubes in audio asylum).
I compared RCA 5751 triple mica black plates to Mullard CV4004 tubes (12AX7 equivalent) from Upscale Audio in the input section of an EAR 890 and I found the Mullards to be too closed in on top, too warm, and a bit "fuzzy" in focus for my tastes (the EAR 890 is a warmer sounding amp to begin with IME). If you live in Minneapolis area I could loan you a bunch of different 5751's to roll - - it's a lot of fun!