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PostPosted: Wed Jun 09, 2021 8:06 pm 
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I recently plunked down the cash to try a wide selection of A(almost)NOS 12AX7 tubes and just got around to trying some in V1 and V2 of my Triwatt build. While genuine never-used NOS tubes have become ridiculously expensive, it's still possible (easy, even) to find fully functional, great-sounding slightly used versions of the same tubes at prices mere mortals can afford, provided you're willing to spend more than the cost of a new production tube. I wanted to first ask you knowledgeable circuit analyzers a couple questions about V2 and V3, then share my impressions of the tubes I tried in V1 and V2 of the Triwatt.

My question: What do V2 and V3 do in the Triwatt? My familiarity with other circuits is much greater and I have a pretty good idea of which tube positions will do what to the tone, but V2 and V3 of the Triwatt have me guessing a bit. Here are my guesses so far:
-V2: V2A appears to be the overdrive channel? So if the overdrive control isn't engaged, this gain stage wouldn't be utilized. V2B appears to be the 3rd gain stage, after V1A and B? My impressions of the tubes I tried in V2 would seem to confirm these guesses.
-V3: My only guess on this one is cathode follower based on the schematic, but I'm having trouble understanding the function it serves, since it appears to be after the tone stack and the cathode followers I'm normally used to are buffering the tone stack. If any one can educate me, I'd greatly appreciate it!

V1 Tube Auditions:
- Amperex Holland (Bugle Boy equivalent): Sounds good, but my Triwatt has KT66s and an FC12 speaker (and their attendant big bottom end) and I was surprised at an unexpectedly light amount of treble with this one compared to other amps I've tried it in. Sounded good, but just not enough cut for my ears with the Triwatt.
-Tungsram: I really love these tubes--if you've tried any (A)NOS tubes and haven't tried a Tungsram, you're missing out--it's the classic case of a component that enhances all of the frequency ranges that are what makes the amp great--really vibrant high end and mids and a well controlled but punchy low end. It didn't let me down in the Triwatt and provided a more assertive punch, enhancing the mid "juiciness" of the FC12. Compared with the next tube, it had a little more mid "honk," which sounded better in, for example, my TC-15 and my Princeton Reverb.
-Telefunken smooth plate: I've tried this one in most of my amps and didn't really like it--the high end was there but more clinical and not especially euphonic, and everything else seemed kind of soft and tentative. Somehow in the Triwatt, it's my favorite. I think it might be the amp/speaker's lack of piercing high end focus, but this tube brought out a lot of articulation and seemed to compress the big low end in a way that really suits the amp. I'm happy to find a use for this one in the Triwatt!
-I started with a Ruby 12AX7AC7HG+, now a rare tube with the supply interruption from the Shuguang factory, which sounded good in the Triwatt but was definitely outclassed by the NOS tubes--Shuguang's tubes sound good and modern with lots of everything, but the high end is noticeably harsher when set against the older tubes, especially when the Overdrive control is engaged--this is a big place where the differences between the tubes really shone through.

V2 Tube Auditions:
-GE JAN 12AX7WA: This tube sounded great and rounded out the overdrive tone quite a bit from the Ruby 12AX7AC5HG that was in there previously--I had hoped this would be the case when I was rolling the V1 tubes and some of them got a little shrill when the distortion hit, and I deliberately tried this one because they are generally fuller than suits my tastes for V1, although they have a wonderful rich tone, and it didn't let me down.
-Tung-Sol 1960's: This one edged out the GE JAN tube by a hair--much rounder and less fizzy than the Ruby, but similarly to the Tungsram in V1, it slightly emphasized the upper mids and added a bit more punch and definition.


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