Phase inverter

[Johan Potgieter]

Nothing really wrong but ....

The 12DW7 is half 12AX7 and half 12AU7 (i.e. a "good" 12AU7). Depending on what goes where ....... Very high mu and quite low mu. Of the lot I would prefer the 6FQ7, but then really a rather higher mu tube like (again) 12AT7 or ECC88. 12AX7 can do duty in pre-amps (low microphonics), but again one must consider h.f. response; proper design.

Ray, I think you meant 6CG7 (same as 6FQ7)? The 6SG7 is a pentode, old type, more for r.f. work.

[ray_moth]

Johan, you're right, I meant 6CG7. Considering the availability of good alternatives in 9-pin, I am amazed that so many amp designs use the 12AU7.

[ThSpeakerDude88]

Johan: So basically the 12DW7 is close or like a 12au7 in one half, only with better linearity and distortion?


By the way, are there any other dissimilar triodes I might try, such as 12DT8?

I am however liking the idea of using a 6FQ7, however will it only be able to be used in one stage or two? Its too bad I'm not using 6V6 output stages, an all bottlehead amp would be pretty sweet looking. I just might have to do that sometime... 6v6's and 6sn7's.

[Dave Cigna]

I think you have a lot of choices. If you want to copy the ST35 front end exactly you could use discrete 12AX7 and 12AU7, 1/2 of each for each channel. Of course, if you go that route then you have a mountain of alternatives to the 'AX7 and 'AU7. Maybe 5751 gain stage and 12BH7 splitter, just as an example.

If you want to stick to a single bottle per channel there are still a lot of choices. The EL84 is stupid easy to drive and the 6V6 only a little less so. Almost anything with mu greater than 15 or so would give enough voltage gain with an EL84, though you might want to double that (or more) if you intend to use negative feedback. With a 6V6 you might want to double that again. 12AT7 would work. I like 5751 better. Millions of good sounding EL83 amps have been made with a single 12AX7 for input + splitter.

If you'd like to try dissimilar triodes in a single bottle there are a few like the 6DR7. 1/2 is similar to 6SL7/5751 with mu=68. The other 1/2 is a small power triode similar to a half-size 2A3. Definitely overkill for a concertina phase splitter, but sometimes overkill is fun! Should have no trouble driving PP 6V6.

-- Dave

[Johan Potgieter]

Originally posted by ThSpeakerDude88
Johan: So basically the 12DW7 is close or like a 12au7 in one half, only with better linearity and distortion?

Yes.

By the way, are there any other dissimilar triodes I might try, such as 12DT8?

My tube manual gives the 12DT8 as an 12AT7 equivalent - not dissimilar triodes. I would still prefer that, respecting what Dave said above but 12AX7 not my choice. The 12AX7 could give stability problems as an input stage with an unknown G1 impedance (Miller effect) - not getting too technical.

If of any interest I prefer (in a one-tube format) to use a pentode-triode here (6BL8, 6U8, etc.). I simply get a better open-loop set-up, but the choice may be personal. (Pentodes are sometimes shunned as input stages because of "higher noise". That is theoretically true, but it is still inaudible unless you poke your head right into the loudspeaker.)

Apart from my small difference, good advice from Dave C.

[Cycline3]

Poor 12AU7.... Poor 12AU7. Always getting the rundown... I feel I have to say a little here. If I had to dog a 9 pin, it would be the 12ax7. Outside of my guitar amps, they have no place in my gear. But back to the 12au7. Somewhere down the road I came across a bunch of old stock 12au7a and 5963 tubes in like new condition. As I had lots of them, I've used them and made some great sounding pieces with them. In fact, what I am listening to right now uses them as the pre-amp and driver tubes. Check it here:

Go have a look here...

And that setup sounds very nice. It will do a Borat "crush you" on normal hifi gear out there. Ive listened to a lot of expensive systems that dont sound as swell as my little set there... Now my main amp uses 6sn7s - and I prefer them (that's another one!) but I am in the process of rebuilding it now... but to sum up - almost any tube can sound really fine... it comes down to creating the circuit for that tube that makes it work and sing. I think many are put off by the low gain, but used properly... those tubes can make some magic sounds too.

[Johan Potgieter]

Cycline3,

Yes, the gain is uncomfortably low for many applications, and the linearity is worse than for the almost equivalent, 6SN7. Measured facts, so other alternatives simply give better designs.

Sound ... if you like it, it is your money. But as was said before, sound is subjective; some hate the same things others like. For whose taste must I as a designer design? Can't go that way - at least going by measurements I can motivate my choices. Simple as that.

[ThSpeakerDude88]

I found a cleartop version of the 6FQ7 by RCA. Do these sound good? Not that being a cleartops has anything to do with it, but they sure would look sexy on the amp.