Gibson GA-9 "Frankenstein"
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You can easily think of this amp as a "Super Champ". With a couple of minor variations, it's a Champ preamp with a tweed Deluxe tone control, and a doubled power output section. The input resistors are slightly larger than those of a tweed Champ, and the feedback circuit returns to the power tube grids rather than the second stage preamp cathode. That's it.The owner called it a Frankenstein; I just think of it as a "typical" transistion model from Gibson. Although transitioning to what I'm not sure; the only available schematic I've found was an older one and I have no idea if they published anything newer. So maybe Frankenstein is a better term!
This amp has a 12AX7, 2x6V6s, came with a 5U4 rectifier, was in a lacquered tweed, GA6 style cab, but has a 10" speaker.
Unfortunately I didn't have a working camera when I had this amp in for work, so I have no photos. The owner just sent the chassis and tubes, so I didn't see the cab, either. Construction was typical Gibson flying lead point to point, rather crowded. The NFB resistor was on a small terminal strip next to the OT (output transformer).
When the owner got the amp, it refused to power up. He knew he would want a larger OT, too, and we went from there. It needed a complete cap job, and new plate resistors. The feedback resistor was orders of magnitude off, and the ground lug on its terminal strip (also the OT ground) was breaking off, which caused further problems. The pots cleaned right up. I replaced the input resistors (I think) and used shielded wire from the jacks to the first preamp grids.
How does it sound?
It plays clean up to about 9 oclock, but with a light touch, there's a good range in there (that's with all pickups going full blast, no volume control backoff on the guitar). After that the distorion comes in gradually, from mild to very thick.
Lots of range on the amp now, too. And very low noise.
My customer loved it.
Schematics
The ".ps" PostScript files will yield the best picture, if you have software to display them. After that, the JPEG is the best - but also the biggest. The larger GIF looks almost as good as the JPEG on most computer screens. The smaller GIF is pretty much just for use on a web page, but lets one see at a glance what the overall structure is.ga9-sm.gif 684x477 45Kb
ga9.gif 1367x954 82Kb
ga9.jpeg 1367x954 285Kb
ga9.ps 25Kb
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Last updated: 20 August 2005Copyright 2003 Miles O'Neal, Austin, TX. All rights reserved. Miles O'Neal <roadkills.r.us@XYZZY.gmail.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514
