aikido 12b4 input 6sn7 output???

[cbutterworth]

OK, I am sourcing parts for my Aikido. I really want a low gain, as my power-amp seems pretty efficient with my speakers. So, I was thinking maybe of using 12b4 for the input (gain of around 10db) with 6sn7 outputs (gain of -0.5db) this would give an overall gain of about 9.5db.

What considerations are there regarding input tubes versus output tubes. Should such a low gain tube (as the 12b4) be placed as the input, with the 6sn7 as the output???

At some point, I'll have learned sufficient that I'll quit the (dumb?) questions, and maybe begin answering.

Thanks,
Charlie

[SY]

The gain you'll get from the Aikido is about 1/2 the mu. So first stage tube is an easy choice- whatever gain you need, twice that will be the required mu.

The output tube should be chosen for a high transconductance; the source impedance is proportional to 1/gm. 6SN7 is OK, not great in that respect. A look at the datasheet would suggest that you'd want to run it rich, 8-10mA.

I don't know any high gm octals off the top of my head, but maybe someone else could suggest one. For 9-pins, ECC88 is an easy choice. It's not the very best, but it's in that league and more than good enough.