6SJ7

[michaelj]

Has anyone any experience in employing a 6SJ7 [octal, sharp cutoff pentode] in a topology that would normally call for an EF86 (as in matchless)? I ask because I have several NOS RCAs [metal can]. One of those things that make you go HUMMMMM>

Mikej

[stokes]

The Gibson GA-20 uses 6SJ7's,but it will require an octal socket,the EF86 is a noval.The 6SJ7 has a much lower amplification factor than the EF86,meaning much lower gain.

[6267]

It's a great sounding tube, but pentode design was still being developed at the time (read: they expected the tube to be in a heavy piece of furniture, not a guitar amp) and so they sometimes exhibit microphonics. If you can find a spring mounted octal socket or improvise one, that would be neato. They do have a really good sound, maybe a little less bright than an EF86 (high inter-electrode capacitance?) in somewhat similar circuits. Got plenty of gain in most circuits, unless you're trying to drive a power tube directly or something.

[michaelj]

Thanks for the replies. I did find the schematic for the Gibson GA20 and that looks interesting. I have a good supply of NOS 6SL7 and 6SN7 as well so I think I'll work in that direction.
Mikej

[soundmasterg]

On those Gibsons, they mounted those pentodes in isolated sockets so they are generally not microphonic in those amps. Makes it hard to plug the tubes in but well advised.

Greg

[michaelj]

I was planning to build this a a "head" version. Do you think that would obviate the need to come up with some sort of isolated sockets to deal with potential microphonics?

Mikej

[6267]

It may alleviate that need when there isn't a pervasively high SPL going on around it. I figure some heads may as well be combos since they're either sitting on the speaker stack or in a room/stage/whatever with tons of watts getting pushed all around. For smaller jams or more limited environments I'm sure it'll be fine. My 6sj7 was a stand up radio, with the 6sj7 feeding a paraphrase (6sq7/6j5) preamp and PI with 6k6 outputs. Sounded damn nice until you hit an open D string (not E!) and the whole thing would start feeding back microphonically. You could start it by flicking the tube too. At least it was in tune I guess .

[GibsonLover]

Those come in Glass Tube version too. I built a head using the 6SJ7 preamp circuit of the Gibson GA50 since my 1948 model sounded so good. I've tried a half dozen metal and glass tubes in there with no trouble. I think they are less noisy than the EF86, and franky guys - the gain is quite hi. I annotated my schematic and forget exactly, but it had a bit more gain than a 12AX7. Don't take the data sheets as your Bible - who has EVER gotten 40 dB (100x) out of a 12AX7? No one - that's who!