chrisc wrote:Sorry I didnt make it clear, the tubes I listed were ones I am thinking of as the power tubes. I just didnt word it very well
Ive seen 6BX7's and 6SH7's used as low output power tubes in hifi before, and the 12au7 in the firefly.
For the preamp tubes I have tons of tubes. From all kinds of 12ax7's, at7's, 12BZ7's etc... to some nice tung-sol 6SL7's, Ef86's and EF184's just to name a few. So I have that covered. Im either doing an Express or a Rocket for the pre, haven't made up my mind yet. I already have two big bottle rockets, so I might go for the express.
I guess my real question would be, how do some of these low power output sections sound? Ive never had any experience with really low wattage amps and dont know what might fit my need best. The 12au7 or 6BX7 in PP or one of the pentodes in SE? Or parallel one of the twin triodes in SE....Self split push put or traditional PI then Power tube(s).
You can see where im getting buried in the options.
I hope that makes more sense.
This opens up a world of possibilities. I think the legitimate limiting factors are selection of the output tube and the output transformer.
I didn't see a good match to the PP tranny you listed. Two of the OT's you listed are for a SE amp. The 7.5K tranny is really to high for most pentodes and too low for common triodes. The Hammond, with the 10K option is about right for the 12AU7 or 6SN7, with the two sections in SE parallel operation.
The 6BX7, which I never hear of before, looks like it is a 10W tube with a very low internal plate impedance. I don't know how to use this tube, but, from reading the spec sheet, I felt it would be more watts than you are seeking. Don't know about the 6HS7 either.
The 6SJ7 has a very high internal plate impedance. I'm certainly no expert, but I'm thinking it won't be a very good match for those three OT's. It does, however, make a fabulous V1, which is why I suggested it there.
A triode power tube doesn't work particularly well with a PI that gives it a wide voltage swing. I suggest the traditional LTP and the paraphase are both too challenging to match to a triode power section. I would have suggested a concertina PI into PP 12AU7 or a self-split 12AU7, but I think your OT selection is not a good match. This brings me back to the 10K Hammond for SE operation.
From what you offered, I think a 12AU7 in parallel SE will be a decent match to the OT. You don't need a PI for SE. I think you'll need some sort of buffer/tone stack recovery tube after a 6SJ7 input. The 6SL7 or 12AX7 are both easy choices. If I used a tube with that much gain, I would probably bias them for relatively low gain. I think you'll get the most tone shaping options with the tone stack and the "middle" tube. BTW, if you have one, I'd choose a 6SN7 over the 12AU7 for the final tube. The operating parameters are nearly identical, but I think it must be the large plates of the 6SN7 that give it something extra.
