What are the tubes, please?
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My all time favorite 12AX7 tube is Telefunken smooth plate. For 6922, I have had good luck with Siemens. I think you need to try different brands in your own gear to decide what suits you the best. I would choose a good tube seller. I have had excellent results buying new and used tubes from Brent Jesse Recording. He is very helpful by telephone and has a very user friendly website.
The original stock tubes are supposedly a Chinese Mullard double mylar knockoff T7.
Look up the thread in the Archives-"Tube Rolling for TAD-150". Lots of suggestions there. I just found it recently myself and am looking for suggestions also-mine was the last post. I have a pair of Sylvania 5751s on the way to try. I have a pair of x7 RCA black plates in mine now and they are an improvement over stock. I am also going to order a new pair of Groove tube 12AX7Ms to try.
I own a TAD-150 and have tried various brands of NOS and used vintage 12AX7, 5751, 12AT7 and 12AU7s from the US & Europe. Right now I am using RCA blackplate 12AU7s. These tubes have excellent bass characteristics and do everything else very well. Another bonus is that the volume knob has much more play in it. 12 o'clock is now very loud insted of 9 o'clock. The 5751s would be my second choice. Good luck in your search.
The 12AU7s have less gain and that's why you have more precise volume control which will likely result in better channel tracking. Aside from the TAD thread, you might also check out the detailed descriptions of the various brands of 12A?7s available from Joe's tube lore at Audio Asylum. You will have a bewildering array of choices since there are probably a half dozen variants of each of those basic tubes and usually multiple manufacturers. One thing you should think about is how much is appropriate to spend on NOS tubes for a modest cost unit. For example, I think that the Gold Pin 5751s are now going for well over $100 each!!!
Tube progression, so far....
Stock t7-flat sounding, rolled off top and bottom lack depth and width of soundstage
RCA x7a ribbed gray plate-better than stock
RT u7wa-ribbed gray plate-better than the RCAs but one was a bit noisy (couldn't stand it, wish I could have listened longer)
Brimar u7wa-gray ribbed long plate-better than any above-wider soundstage, increase in bass
Sylvania JHS 5751-wow, a direct hit, improvement across the board-Very nice soundstage width and depth, plenty of detail and prat! Gain match with volume control into my Mc DNA-125 is right on.
Mullard x7-short plates, halo getter-A MAJOR DISAPPOINTMENT-detailed but very flat-lost the soundstage completely (I hope everyone does not read this thread because these will be for sale soon!)
GT ax7m-playing now, just got them today-These are nice, very, very close to the 5751s mentioned above. They may lack some of the fine detail of the 5751s but not much. I can live with either.
I would like to try some nos 50s long plate Mullards or Telefunkens but I got to agree with Swampwalker, it seems a bit crazy to put $100 plus tubes in this pre (or does it?). For $50 a pair or so the 5751s or the GTs would seem hard to beat.
I have never fooled with cryoed tubes so I would be interested to get Stratosaur's impressions...
The Cryoed 12AX7 were not as good as I had hoped. The Cryoed 5814 Sylvanias sounded very, very nice but did not have quite enough gain. So much for the Cryoed ones for now.
I currently have GE 5 Star 6201 in there and they have impressed me. 6201 are military spec 12AT7. They bettered some Raytheon 12AT7WA I had. For want of a better term the 6201 had a more "spacious" sound (larger soundstage and more air around the intruments) then the 12AT7.
The GE 6201 were 20 bucks for a quad on EBay. I have some Sylvania and Mullard 6201 en route from Upscale Audio.
I'd say stay away from 12AX7 as you don't need the gain with the TAD150. Use a 5751 instead (military spec 12AX7 with lower gain, similiar to a 12AT7). Since the pre comes with a 12AT7 it might be best to stay with them (12AT7WA, 6201, 7728, ECC801). If a 12AU7 gives you enough gain you can use them (5814, 6189, 7730, ECC802).
I had asked Paul @ Bizzybee about 5751 and 5814 and he said they would be good choices. He does not believe in leaving the TAD150 on 24/7. I use mine with an Odyssey Stratos Extreme that I leave on all the time but turn the TAD150 off when not in use.
I have gone a bit tube crazier and have brought in several more to try
12ax7 RCA black long plates D getters-good air and soundstage in the can't go wrong with (CGRW) it category
5963 RCA long black plate D getters-also very nice, a little sweeter presentaion than above a tad less detailed
Amperex Bugle Boy 12at7 (made in France w/music clef on the side, big round getters-cream of the crop so far, for my preferences-very fine detail and air but tends towards brightness
Tele 12au7 ribbed plates-very well balanced sound does most things well, very livable
Amperex 12au7 med grey plates world logo-very interesting tube to audition finely detailed nice soundstage but may be brighter than some tastes allow.
I have some 5814 Sylvanias to get into the mix. I still would like to get a pair of tele12ax7 to try
Some of the CGRW others
Sylvania 12au7tw and 7twa
Sylvania 5751 grey plates-Tripple Mica
GT12ax7m gold series
Not making the cut
Mullard 12ax7 short plates
Mullard 12au7 short plates(better than the above Mullard)