Gibson GA-6 Guitar Amp

Photos

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These photos are from three different examples of the same model, the "interim" (undocumented) GA-6 with the 12AX7s. In each case I did similar mods, although each versoin built on the previous version. In the first two cases I had no film when the amp was finished, so I can't show the reworked preamps. For the third I had a digital camera and used it extensively.

There's a screw in the middle of the bottom of the chassis; you can see it in the panel view. It attaches the middle of the upper panel to the chassis. I'm guessing this is to prevent vibration noise.

Further inspection showed that at least two pots had been replaced, by cutting off the old pots' kugs with components attached, and soldering them to the new (used) pots! That's RG/174U cable. The center conductor is a bit smaller than I like, but otherwise I like this cable; it's easy to work with.

Panel and guts with top rear cover removed; very much like a tweed Deluxe! I believe the guts are all original except the blue cap and possibly the orange caps. The handle is extremely worn; even looking closely (sorry, no photo) it's hard to believe it's the same as the handle on the unpainted amp below.	View from the back; yes, the cover material (tweed, most likely) has been painted black. Too much Rolling Stones influence? The silver electrolytics may or may not be original; I'm guessing they are. The OT has been moved to the left cabinet wall because of speaker clearance issues.
Original preamp circuit with grid-leak bias (note caps from jacks on right). The wiring looks sloppy, but works just fine. The preamp tubes normally wear tube shields (not shown).	Power supply and power amp; it's mildly crowded around those tube sockets, but much better than a tweed Deluxe, which was clearly designed to be worked on by midgets or six year olds.
Handle and knobs. This handle is in beautiful shape. This is one of my favorite visual details of the old, Gibson amps. Much classier than the tweed style vinyl covering, although they do go together well. The chickenheads are original. The white has faded, and no amount of cleaning will turn those stripes white again. But they are whiter looking than they appear here.
This is a much cleaner example of a GA-6. To start with, the tweed-ish vinyl has not been painted! The output transformer is still in its original place on the chassis as well. The speaker is a 100 watt Carvin; it's huge but doesn't interfere with the OT. The power cord and PS caps are also new. This customer wasn't concerned with keeping the amp pristine; he wanted more headroom, more bass, better tone, and a quiet amp. These amps aren't high dollar collector items like the Fender Deluxes of the era, so they make great mod platforms. Nevertheless, all my mods here are reversible except that an extra hole was drilled in the chassis for the new, larger OT.	And here you can see the difference in the OTs. The smaller transformer at the left is the original; the larger one on the right is the Cerniak replacement. The bigger OT really opens up the amp with better lows and highs and more response. The new transformer doesn't saturate at the levels this amp puts out, either. You can't tell from the photos, but the new tranny is almost 50% bigger, and weighs about twice as much. Yes, the "microphone" jack is out of the amp in this photo. Why the medicine container? I use those to hold small parts such as nuts and bolts, caps, and so forth. Just my version of recycling.
This one is definitely original except for the power supply electrolytics, which were replaced by a shop in Atlanta. They used two 4.7uFs in parallel instead of just a single, 10uF. There was another pair of these from the a ground on the rightmost tube to the common point of the 4 100K (brown-black-yellow) resistors, but at this point I had removed it for access to some other component. The paper/oil caps still look OK, but all have electrical leakage. Since the entire preamp was being rebuilt, anyway, these caps went away. I also ended up replacing every resistor in this amp other than the PS resistors, because of drift. Matching the horribly unbalanced resistors in the phase inverter alone added 30% or so headroom.	The power and output sections appear to be original except (again) for the PS cap and the 3 wire cord. The power switch was unmounted to check the "death cap" (just behind it to the left), which turned out to be shorting to ground, resulting in the amp always being "on". Many of the cloth wires near the rectifier and power tubes have faded from heat and are no longer color-coded. The skinny wires with the electricians tape around them are the output transformer's primary leads. Note the rubber shock mounts for the tube sockets. These really help cut down on microphonics and mechanical feedback. The pilot lamp holder (left of the power switch) is the Fender variety. If you look closely, you can see the plug-in solid state rectifier suspended from the leftmost octal socket.
Your basic faceplate; not too different from an early tweed Deluxe. All Gibsons of this era had the serial number stamped in the faceplate. The odd coloring in the knob area is my reflection. The chorme plating on these holds up well if given half a chance!
This amp is in pretty good condition. The baffle bolts were a bit loose, it has a couple of scuffs, and the the plastic logo is busted, but otherwise it was in good mechanical shape.	Again, fairly Fender-like cabinetry in good shape. The output transformer is mounted on the speaker, presumably a carryover from field coil speaker days. The larger replacement mounted on the chassis with no problems.
Here's the first look at the patient's innards. Other than the tubes, it's mostly stock except for the blue Atom power supply cap, the replacement power cord, and one big, yellow cap, partially hidden behind an Astron[tm] and below the mic pot.	Trouble brewing! The right hand end of this terminal strip has separated from its mount, probably because the other mount twisted while being bolted in, or maybe from warpage and vibration of the strip. In any event, I used a strong piece of wire to tie the strip back to the mount (not shown).
The power section of these amps is the most annoying to work on. Fortunately the only work around the tube sockets on this one was swapping the OT leads. The power cord replacement isn't that hard. Whoever replaced this cord removed the death cap, but use another two wire cord.	Other than the one coupling cap, this looks stock. What you can't see is the flat, ceramic cap underneath the tone control. It's the volume control bypass cap and most likely a replacement.
A properly wired power cord, with the hot leg going first to the fuse end terminal, the other fuse terminal to the power switch, the other ower switch lead going to the power tranny orimary, and the other PT primary lead connected to the neutral.	This one's a bit blurry, but you can see: new plate resistors parallel to the terminal strip), the preamps rebuilt with cathode bias, and the input rewiring. The tan resistors forming a vee are the new mixing resistors from the preamps. The small, brown caps near the upper middle are two silver micas in parallel for the new mic pot bypass cap.
The original resistors tied together to a cap for the grid-leak bias that went to a terminal strip, to a wire to the grid. The caps went away with the cathode bias, and I used shielded cable from the "mic" jack and "instrument" jack resistors to the grids.	Another handle. This is typical of the handles on the black version of these amps, although some had dark filler to match the black covering. No logo here; that came later. I wish I had thought taken this from the other side to get the knobs better.