Roadstar wrote:
Firstly I've never used a 12AT7 in anything (and don't own any yet) and I undertand that a 12AX7 can be used in it's place so other than increased gain are there any other considerations? If I do go with a 12AT7, can I get some recommendations as to what brand to use?
I'd always been more of a Marshall guy myself until I started designing the Triwatt, so I'd never used a 12AT7 either.

Basically any NOS 12AT7 should be good. Apart from the increased gain, there's no benefit to using a 12AX7 in the PI. In the case of the TriWatt, with the Biacrown-era Hiwatt Overdrive stage, there's absolutely no need to resort to using a 12AX7 in the PI.
Roadstar wrote:
In a vintage Marshall. V1 is always the critical tone tube. What about the Triwatt?
Same applies for vintage Hiwatts - and the Triwatt is entirely vintage Hiwatt design. NOS Mullard is highly recommended in V1, or possibly Brimar. Your Amperex may get you close to the Mullard vibe, since both are actually Philips, and sometimes were even made in the same factory.
Roadstar wrote:
Quite happy to pick up a nice RCA 12AT7 at A1 yesterday for $15. looking forward to giving it a try.
A good score! I've used RCA's in that position as well.
coco wrote:
RI Mullards or Tung Sol for AX7s, NOS AT7s or your RCA are great. The JJ 6V6 or Tung Sol 6V6 if you don't have some nice old 6V6 around.
With 6V6 power tubes, I usually prefer the more balanced tones of the Mullard-branded Russian 12AX7s. They *don't* sound the same as real Mullards BTW, and can't really be called a RI, but they still have a great sweet, warm tone. With the darker tones of KT66 power tubes, I prefer the brighter Russian Tung-Sol-branded 12AX7s. Those NOS Sylvania 6V6s should sound great in the Triwatt BTW.