6J6A 7-pin tube amp 7W or so?

[David Root]

I was on the related thread about PT ratings that eventually turned into a review of a 6-6AQ5 30W design, when I was reminded that a while back I had contemplated a small amp using nothing but the 6J6A, which is I believe the only 7-pin twin triode. Never did anything about it at the time, although I did build a tweed Harvard circuit with all 7-pins, which turned out very well. 6J6As can be had NOS for less than $2 each.

Well, why not? It is rated 300 Vp, Pa is 1.5w, mu is only 38 but gm is 5300 mmhos and rp is a low 7100 ohms.

Preamp and PI seems easy enough, but what about the power section? Two pairs of P-P triodes? Never tried that before, it might sound real good in the right circuit don't you think?

It does have one downside, although it's a twin triode it only has ONE CATHODE. So may have to use it in parallel for linestage or output. No problem with the PI, though.

6GK5 (6FQ5A) has mu 78, gm 15000 mmhos and only 5400 rp, might be better even though it's a single triode.

I'll put it into TubeCad and see what comes up.

[lastwinj]

i have used a 6J6 b4. it was fine.

germ

[David Root]

Well, it looks fine on Tubecad, in grounded cathode, and for a longtail PI too, as low as 200V B+, but in TCJ P-P I had problems with it as a PP output triode. Two tubes, i.e. two triode pairs, gave way high unrealistic Po. I think I need to try again at lower B+ and maybe 5 to 6k OT primary. I'll try printing out the plate curves and doing it manually.

[David Root]

No luck as a power tube. Its Pa is just too low. 0.75 W Po P-P from one tube w/8K Rpp, just makes 3W Pa at 200V Vp. Back to the 6AQ5A!