Gibson GA-6 Interim Model
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Gibson was known for changing their amps fairly often. The amp in question was produced before the Lancer, yet had a tube complement like that of a Lancer. I expected the circuit to be that of a Lancer, but...
I first got the chance to work on one of these for an Austin guitar teacher and all around cool guy, Andy Bullington. The amp already had its original 12" speaker replaced with a Weber VST version of a Jensen, and sounded OK but not great. As I worked up a schematic (there were none available) it became clear this amp was extremely close to being a Fender 5C3 tweed Deluxe, which can be a real tone monster! And just like the Deluxe, it broke up very early. Since Andy plays a variety of musical styles, including jazz, he wanted the breakup to not occur so early. He also wanted the amp to be generally as good as it could be.
Along with the usual three wire cord change, cap check, addition of hum reduction via DC bias, this amp got a new output transformer (built to original tweed Deluxe specs!), the instrument channel was converted from grid-leak bias to cathode bias (with shielded Teflon wire from jack to tube), and two new plate resistors for the PI (the old ones had 100K color codes but actual values of 8M and 19M). I also spent some time playing with different tubes; using a 12AY7 in the preamp, as the 5D3 Deluxe did, helped a lot. Andy played the amp for a week or so, and decided he wanted the mic channel converted to cathode bias as well; this got rid of practically the only hum left in the amp, and added the top end missing on that channel.
The transformer alone made a phenomenal difference in the sound. Much better low end, more clarity across the board. The old tranny was fairly small; it was like putting a 55MPH governor on a Ferrari.
This amp was a minor pain to work on. It's point to point with no component board, but the components are all condensed into two areas by the tubes. The rectifier/output tube area is a regular snakepit of thick, cloth insulated wire, and half the components in both the preamp and power amp areas are well hidden behind others with very short leads. But it was worth the effort.
I also worked on a GA-6 owned by Steve Kunkel, who shipped it here from Atlanta (about half the amps I work on are shipped from other cities and states). Steve's was slightly different (primarily power supply resistor values). A shop in Atlanta had given it a cap job, dropped in a solid state rectifier, added a three wire cord, and installed a Carvin 100 watt speaker.
Steve wanted the amp as quiet as possible, and wanted the tone opened up. I ended up replacing almost all the components in the amp, between the preamp conversion, setting the PS caps up for a tube rectifier (a 5V4 let the amp run safely without a standby switch), and replacing the substantially drifted PI and output circuit resistors.
The "mic" input now has no grid resistor. This results in the fullest tone imaginable, and is now the main guitar input. The other inputs (labeled "Instrument" 1, 2 and 3) each have a different series resistor, so that each input jack provides a different attenuation and rolloff. In other words, more tonal possibilities!
More recently I revamped a GA-6 for Dan Chewning. He wanted more clean headroom with a "jazzier" sound. The amp also had a buzz at some frequencies. Someone had already changed out the first PS cap and a couple of pots. The instrument volume bypass cap looked like a replacement as well, and one of the preamp plate caps was new. A newer, 2 wire line cord had been installed and the "death cap" was gone.
I converted the amp to a three wire cord, wired properly (wall to fuse to switch to PT to wall). I rebuilt the inputs along the lines of Steve Kunkel's amp, but with a grid resistor on each channel, and no cathode resistor bypass caps (maximum clean tone); cathode resistors are carbon film but grid resistors (on the grid terminals) are metal film. I added a volume bypass cap to the instrument channel. I added resistors after each preamp's plate cap, forming a passive mixer so the volume controls and bypass caps wouldn't interfere with each other much, and left the tone control on the instrument channel. The preamp and phase splitter plate resistors were noisy and one had drifted badly; I replaced these with carbon films. A new OT hand-built for an 18 watt Marshall clone added more headroom all around, and as usual opened the amp up quite a bit.
Someone had apparently clipped or broken the leads off the mic pot, and soldered them back on. Maybe they swapped the pots that way. They definitely swapped the instrument volume pot by clipping the leads off the old one (with other components still attached) and soldering those on to the new pot's leads. Weird, but it worked. I cleaned all that up as much as I could. I replaced the mic pot terminals with wire (heavy cap leads) simply because I could get a good mechanical connection that way and not depend solely on the solder to make the connection and hold them in place.
The instrument channel stays very clean up to 9 o'clock, and is only mildly dirty by 10. By 12 it's back in serious tweed Deluxe growl, and going around to max it just gets crunchier and crunchier. The mic channel starts getting dirty by 8:30, and screams more and more like a siren in ecstasy toward max. Rebuilt and tweaked, these amps can do almost anything, though you'd need to mic them with a band (of for serious clean work, like country or jazz). I think this is my favorite version of the GA6 so far.
SchematicsThe ".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.
Original Schematic (Andy's amp)ga6-sm.gif 597x414 35Kb
ga6.gif 1194x828 85Kb
ga6.jpeg 2654x1841 630Kb
Triple R (modified) Schematic (Andy's amp)ga6-r3-sm.gif 597x414 40Kb
ga6-r3.gif 1194x828 90Kb
ga6-r3.jpeg 2654x1841 660Kb
Triple R (more modified) Schematic (Steve's amp)ga6-sk-r3-sm.gif 605x436 38Kb
ga6-sk-r3.gif 1210x873 62Kb
ga6-sk-r3.jpeg 1210x873 224Kb
Triple R (even more modified) Schematic (Dan's amp)ga6-dc-r3-sm.gif 605x468 37Kb
ga6-dc-r3.gif 1210x935 90Kb
ga6-dc-r3.jpeg 1210x935 142Kb
Yes, the value of C9 is unknown. It was the one component I could not see to get any useful information, even with the most convoluted use of a dental mirror and lighting. The GA-14 Titan used a similar cap, and since that amp is almost a 5D3 tweed Deluxe, the cap in this amp could well be .0001 as well.
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Last updated: 13 November 2005
Copyright Y2K 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