6SQ7? In a Gibson GA-20...

[Justin Thomas]

Not sure this is the right spot for this, but... ran across an old GA-20 today that was in a damp basement for 30+ years. Cabinet is trashed, probably the original Jensens, too. It's one where the whole chassis is on the bottom of the cab and it has 2 different-sized speakers. Was told it just hummed when plugged in & powered up; I didn't try it myself. Checked the tubes, and had a couple 6SJ7s, 2x6V6, 5Y3, and at least one 6SQ7? 2 channels...

Anybody ever heard of those being used in any amp? Triode&Duo-Diode... the oldest schem I could find asks for a 6SL7, no 6SQ7. Maybe someone put in the wrong tube, broke it, and that's why it ended up in the basement? Either way, the trannies looked okay to me; otherwise, it's just "salvage for parts" to me & I don't want to pay much for it...

I would buy it just to make it sound good, regardless of looks... shop owner basically said "make an offer..."

Edit: just found a schem of a GA-20T with a 6SQ7 for one channel... difference being, that schem had a mix of octal and 9-pin preamp tubes. "My" amp is strictly octal, and most e metal...

Justin

[Jazz P Bass]

This Gibson data may help narrow down what you have.: Superior Music - Vintage Gibson Amp Info

[EFK]

A number of mid to late 50s amps use the 6SQ7. The diode portions are grounded (not used) and the triode section is used as it has comparable gain and response to a 12AX7. Valco used it in one of their greatest (for guitar) circuits, made for an awesome amp. It was also used by Valco and Gibson as well I believe (again with diodes grounded) as a great tremolo oscillator.

[mozz]

Building one now, works on the dummy load and scope, haven't fired it up yet with guitar and speaker.

[dkevin]

Please bear with me, as I have an "(Old Gibson amp") story to tell. I have a GA-30 which uses 2 6SQ7's for phase inversion. It physically resembles a GA-25 (the chassis appearance) but only has one preamp tube (6SJ7) like a GA-25. It also has a tone expander slide switch. When I received the amp (no cabinet sent, only a chassis), I was perplexed that the amp only had one preamp tube/one volume pot and used 2 octal triodes as PI especially in light of the official GA-30 schematic. By tracing out the octal socket heater wiring for the two mystery tubes I was able to identify them as 6SQ7's. So, the amp that I have is stamped as a GA-30 but is more like a GA25 but uses 6SQ7's instead of 6J5's for the phase inverter. Your original post mentioned anyone having an amp using one or two 6SQ7's...check. Are you sure about the GA-20 model number? I have several early GA-20's and they use 2 6SJ7's, a dual triode 6SL7 as PI and the typical 2 ea 6V6's and 5Y3. I have never seen an early GA-20 that uses any other PI. It may be that you have a transitional GA-25 (two speakers/one preamp tube) and the extra 6SJ7 is meant to be a 6SQ7 to match the 2nd 6SQ7 as the PI. Believe me, when it comes to early Gibson amps, stranger things have happened!

[Enzo]

Gibson is the one company where you can never trust schematics. I have one model in my files, somewhere in the GA17 or GA19 or something range, where I have THREE Gibson published schematic versions for the model PLUS THREE hand drawn schematics from models I have had in my shop. All with the same model number on the panel. So six versions that I know of. I don't even consider it remarkable when a Gibson schematic does not match the amp. About the only thing I can count on with the old Gibsons is that for a given model number, the panel features will be consistent.

[Enzo]

Whatever the hell this is, it sure ain't amplifiers.

Reported.

[Justin Thomas]

Whoa, what'd I miss? I didn't even get to read the spam on my own old thread? Phooey...

Jusrin

[Enzo]

It was a huge paragraph written in Malay.