Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: davohilts on April 09, 2012, 12:13:58 am

Title: Gibson GA20 w/9 pin preamp tubes
Post by: davohilts on April 09, 2012, 12:13:58 am

I'm looking for a schematic and am hoping someone might have one. I'm looking for a schematic of a ga20 with 9 pin preamp tubes. I've got one of these I'm working on that has low gain on the mic channel but i'm flying blind. My amp has 3-12AX7 preamp tubes with the grids of the first two tubes feeding the 4 inputs, it has grid resistors 1M on V1P2 and 470k on the other three. The plates of V3 are tied together with a 100pf cap. Does this sound like your schematic? If yes could you send me a copy?
Thanks!
Dave

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: John on April 09, 2012, 05:43:25 am

http://www.el34world.com/charts/Schematics/files/gibson/GA-20T.pdf (http://www.el34world.com/charts/Schematics/files/gibson/GA-20T.pdf)

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: bruno on April 12, 2012, 11:26:18 am

Fixed one exactly like those a few years ago (1959 GA-20 with 3 12ax7 preamp tubes), and posted the schematic which I drew here in the forum, here you go:

http://www.el34world.com/Forum/index.php?topic=605.msg12767#msg12767 (http://www.el34world.com/Forum/index.php?topic=605.msg12767#msg12767)

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: sluckey on April 12, 2012, 12:23:46 pm

I don't see any 9 pin tubes in that schematic???

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: John on April 12, 2012, 08:49:54 pm

Yep, they are.  I think Sluckey is referring to Bruno's schematic, which is showing the 6SL7 and 6SJ7 preamp tubes. The one I linked to shows 12AY7s.

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: bruno on April 13, 2012, 03:07:16 am

Sorry, I really thought that was the one with 12ax7s, I don't even remember doing the one with 6SL7. I'm 100% positive that I do have the schematic with 12ax7, as I drew it directly from the amp. I'll find it and post it this weekend, it might be helpful.

Title: Re: Gibson GA20 w/9 pin preamp tubes
Post by: Shrapnel on April 13, 2012, 04:38:53 am

See if the GA20RVT is close enough