Having had Bobs ST-70 for a while now I have experimented quite a bit with different 12at7's. I started out with the stock JJs that came with the amp. The sounded kinda thin to me. I then bought some NOS military Mullards from the 80s which were being sold at some tube outlets. Nice sounding tubes. Maybe not as defined as I had hoped. Then I got for free with some other tubes I purchased some 60s GE 6201s. They were ok but nothing special. I also got a set of Sylvania Black Plate 12at7wa's from the early 60s and they were some decent tubes. Very clear sounding tubes. I also got some Knight branded Mullards from the early 60s. They are nice but have that very tubey Mullard sound too. I got some Valvos recently from a guy in Germany. I had them in all three positions and they sounded kinda boomy but still nice. What I did is replace the middle tube, the voltage amp) with a 60s Knight Mullard and left the Valvos in the left and right positions and I think I have finally hit on a winner.

Looks like the JAN tubes are what came with my kit, Bob.

Saved me from ordering a set from you.

Don't hesitate, by the way, to let us... or me know when you have another combination to recommend trying out.

-Brenton

Any recomendation on the center tube if you are using the VTA low gain board with 12AU7s?Thanks,
Bugs

NOS RCA 12BH7. Better yet, get three and roll a trio. I preferred them to NOS Amperex 12AU7s in back-to-back comparisons.

BTW, I was also using the SDS power supply upgrade kit and a solid-state rectifier. Both are also recommended.

"Solid State"...?

What kinda modern-era trash talkin' blashemy is that?

I would be really cautious replacing the tube rectifiers in Mark II and Mark III's if you are using any can capacitor and an original power transformer. Scott was getting 525V with a tube rectifier, a stock transformer, and a 120V AC supply. This is already edgy even with a 550V filter capacitor. I sure wouldn't want to add 30-40V more with a solid state rectifier. Some of the add-on power supply boards have a 630V or higher rating and would be a requirement in my view.

I do like the sound of a modern power supply, but it comes with some new issues.

Tube Nube wrote:"Solid State"...?

What kinda modern-era trash talkin' blashemy is that?

When I was running the stock board the Weber Copper Cap was a definite improvement. The amp certainly sounded less "winded". Adding the SDS power supply upgrade raised the bar even further, giving it a bit more low-end sustain and less audible sag.

When I improved the driver, the differences between the SS and tube rectification became less apparent and I often found myself alternating between the two trying to see if I had a true preference anymore. The differences at this point were similar to the Triode vs. Ultralinear subtleties.

The K&K Audio board I now use has a switch mode power supply (totally solid state) but uses the tube rectifier as a "soft-start". The influence of an absolutely unflappable power supply is shocking. It's really outstanding to hear these ST-70 output transformers maintain their poise when used in conjunction with such a modern design.

The VTA driver has a different, more diplomatic presentation and is a lot more affordable than the K&K audio driver. I wound encourage builders of ST-70s to start with a VTA driver in mind. An ST-70 with the VTA driver is quite possibly the best tube amp deal out there.

Hmm, I use the Webber WZ34's in my Mk IV amps with the stock 525 volt quad caps, the left an original. I have just changed those quads to the 80-40-30-20 550V version. Does the WX68 offer better characteristics in my application?
Oh, back on topic: I also found the JJ 12at7 tubes to sound thin in my Audio by Van Alstine Trancsendence Eight preamp. Next I tried RCA black plates which were much more listenable. Then I lucked into a pair of Telefunken ECC801s which help the preamp disappear.


You know with the preamp gone and the speakers disappearing it's amazing there's any sound at all.

I discovered a couple of older private labeled Daystrom Mullard 12AU7s in some old Heathkit equipment I had kicking around. I plugged them into the outboard sockets of my VTA70 board with a JJ 12AU7 in the center socket. It made a difference in sound, especially on the bass end. I guess I'd have to say that the bass became more transparent if not a lot less boomy. Right now I have one of the Mullards plugged into the middle socket with two RCA clear top 12AU7s running in the outboard sockets and I find the sound very pleasing. I’m not a real fan of systems that generate boomy and unclear bass responses so that may tip you off to my listening preferences.