Here’s some good information, from a tube vendor: http://www.vacuumtubes.com/12au7.html As is mentioned, no one can tell you, exactly how a tube will sound, to you, in your equipment(generalities, maybe / specifics, no).
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The question is what type of sonics are you looking for from your amp?
I was very pleased with the Brimar CV4003/12AU7 from Upscale... provided warmth with extension. When I changed my power tubes, I went with NOS Tung Sol 12AU7’s which provided transparency and realism. Fantastic tubes and affordable.
Have also tried RCA 5814, but they were better suited for an input gain stage due to their wonderful timbre.
In a power amp the tube or tubes in the gain stage have the most influence on sonic signature. I've been told that the driver/phase inverters have little effect on sonics; eg, timbre, dynamics.
Not so, IME. The tubes in the driver stage have an audible influence on the sound (depending on the design of the amp).
I'm in agreement with all the comments thus far to choose a good NOS tube for this stage even if it's purpose is simply to drive the power tubes.
I tried the Mullard 4003 tubes years ago sourced from Kevin Deal. They sounded fine (a bit rich) but became noisy faster than expected from experience with other tube brands.
I have been using Ron Sheldon's Cryoset Gold Lion 12AU7's but since Ron's unfortunate illness caused him to shut down his business, I just purchased a matched pair of new production Tung-Sols from Tube Depot. When the time comes we'll see how they sound.
I may be a bit fortunate in that my tube gear doesn't seem that sensitive to the tubes used. More a question of longevity rather than tonality. I suspect from the experiences of others, that Power Tubes have a greater impact on tonality and dynamics.
What sound are you looking for and position really matters driver vs input etc. Anyway you have a nice long list to choose from at reasonable prices. My favorites are Amperex yes esp 7316 overall detail and excellent tonal balance very quiet and for a strong clean signal use a 5814 mostly RCA. these are also consensus picks good luck and roll what you don’t like. Want a mellower richer tone go for a Mullard or Brimar!
The M125 monoblocks...
There are indeed only 2 driver tubes on each monoblock. If I’ve read this correctly, the driver stage generates higher than typical voltage, so there isn’t a separate gain stage.
The sonic effect of the driver tubes will be significant.
Have you decided on power tubes?
My experience is that they can make a bigger difference than the smaller tubes. You can fine-tune your sound quality by mixing different qualities in the power tubes and the driver tubes.
FULL DISCLOSURE: I occasionally sell tubes from my personal collection. I can post a listing here if I have something that interests you.
I have several power tube options available, again depending upon your budget.
The premium European 12AU7s will likely be above your $30-$40 each budget. The US options should be well below that.
I have New issue Gold Lion KT-88’s that I bought for my Jolida amp. They have may 100 hrs on them. My understanding was also the driver tubes voice the amp the most. I was under the impression the output tubes have minimal sonic effect. Once I get them built, I may hit you up. I won’t receive them until Thursday. Thank you for sharing.
The Gold Lions are nice.
I have quads of GE, RCA, and Ken-Rad domestic and 2 different styles of RFT Germany 12AU7s that are under your budget.
I have some Sylvania and Raytheon pairs and trios, but no matched quads that would also be well below your budget.
My Amperex/Valvo/Mullard/Siemens/Tungsram varieties aren't going anywhere anytime soon. ;~)
You should check out Brent Jessee Recording, great source of NOS and he has some real sleepers that won't break the bank. I got some really good early 60s RCA 12ax7 and Philips Miniwatt 6922 and they were everything he said they would be . His website is a great source of historical info on NOS.
Jim McShane is a good source too, nothing but positive comments about his restoration services and tubes on every audio forum. I haven' t used au7s in a long time but stay away from the later production sylvania nos... mazda cifte 12au7 that I tried other than modern production and they were great.
There are more parts than tubes in an amp that affect sound. I have replaced noisy metal film resistors in $12,000 amps that had gone high resistance and very noisy.
Replaced them with wirewound resistors for better than original sound.
Of course coupling caps have been a target for replacement and should be explored. Power supply caps also has an effect.
The emission and gain characteristics from tubes is very voltage dependant. You could tweak the filament voltage a bit up or down, maybe 10 or 20% to see what effect it will have.
Jim McShane is my source . He tests and tests and is very picky. He will not sell you tubes without knowing what amp they are going into.
@ Broussard that must be some private collection ! I consider mine large but don't have inventory like that. Well not as organized anyway.Yes Gl tubes can transform an amp I know, also the GL -KT-88s made a huge difference. Good Luck
@mechans, my personal collection could easily be called obsession or even addiction. From 2008 to 2010, I acquired over 1,500 tubes.
Fortunately, I regained control and am down to somewhere in the 275 range. Still, entirely too many.
The adrenaline rush of buying or winning an auction is far out-weighed by the real work of prepping, testing, taking pictures, posting listings, answering silly questions, ignoring low-ballers, packing, and shipping involved in selling.
Buying is fun; selling is work.
@BBrousardI hate selling. The perfectionist buyers who think they are buying from a store with unreasonable expectations . I don't even want to think of the bath I'm going to take on some of my tubes. I never really thought about selling, and therefore keeping track of what I spent. I would leave it to my heirs but they will probably end up giving them away.I like my tubes and really don't want to sell. If I did I would buy more to replace them. What I am going to do with all my 6SN7s I'm not sure, I have a couple of dozen of Ws alone. Some great some not so great. I am too honest to be a good salesman . Telling people AS IS, AS Found, ETC. when I know better.