This amp is the result of a drawn out 'restoration project'.
It all started when my neighbour asked if I was interested in having his old amp - he'd played bass many moons ago. I asked him what it was and he said 'Oh, an old Fender I think'. Close. It was a Gibson GA80 chassis that had been 'fitted' by a local company into a head. The box was really shabby and poorly made anyway, so out it went. I couldn't see anything particularly wrong with the amp itself, so I cleaned the mouse crap out of it and fired it up. It hummed pretty badly, and the tremolo didn't work, but it made sound, and I was hooked.
About this time, I ordered a copy of Aspen Pittman's 'The Tube Amp Book', hoping the schematic might be in there. It wasn't, but there was a reprint of an early Gibson catalogue showing this amp amongst others. Using the information on dating amps via the pot code, I concluded this one was from ~1963. The catalogue featured cabinet dimensions as well as a small picture showing what it should have looked like originally.
I then contacted Gibson and they mailed me acopy of the schematic and operating manual, free of charge. Tracing out the schematic, it was obvious that the amp was almost totally original - including the 'Made for Gibson by RCA 6L6GC' power tubes. A couple of the preamp tubes were replacements, I think, as was the rectifier. I had to replace the rectifier though, because the one that was in there had a much larger base than the retainer was designed for, and the retainer actually pushed the valve out of the socket. David Crittle (Retrovox) supplied a used Mullard. There is a better scan of the schematic here
I replaced the electrolytic capacitors, but not knowing what I was doing screwed up the grounding, so the hum was still there (albeit for a different reason). After much poking about I finally gave it to a tech to sort out, as well as the tremolo. Unfortunately those classic power tubes were shot, so (not knowing any better at the time), I bought a pair of Sovtek 5881 tubes. I know, I know....
The pointer knobs were no problem, but so far I haven't really come up with a satisfactory substitute for the white knobs that operate the tremolo (those in the photos are actually Strat volume knobs with the lettering scraped off). The push buttons are adapted from knobs that are sold for use on slider pots.
The last remaining pieces of the puzzle were a cabinet - no big drama, now that I had the dimensions - and a speaker. The catalogue touted the speaker to be a Jensen, and the schematic showed the model as a P15P. There was no way in the world I was going to turn up one of these in Australia at any sane price, so eventually I ordered a WeberVST P15N reproduction unit.
Cabinet details were kindly provided by Björn Anger, who has an incredible site dedicated to Gibson amps, and I went to work. The cabinet sides are made from 35 year old hoop pine (Aruacaria Bidwillii), salvaged from kitchen cupboards. Finger jointed, of course (stay tuned for another page describing how to do finger joints with a home made jig that costs about $20). The baffle and back panels were cut from 12mm 5-ply. The tweed covering and handle are from the Mojo catalogue and supplied by Vibroworld, as was the grille cloth.
After all this, I realised that the power transformer, which was the original 115V unit connected via step down, was supplying voltages about 10% too high. Normally not a problem, but in this amp the B+ was *supposed* to be 438V, so +10% put it up near 480V. The capacitors - both the original and replacements - were only good for 450V. For this reason and a couple of others I decided that the best option was to replace the power transformer. I contacted a local firm, Special Transformers at Arncliffe in Sydney, who made a replacement unit for an extremely competitive price. I highly recommend their work to any locals who find themselves in need of a custom wound transformer!
Would I do it again? Well, it doesn't make any financial sense - the finished project is not really a collectors item in any sense - but I sure learned a heck of a lot. And had fun doing it. What more could you want out of a hobby?
(Update: I managed to find someone who had a surplus Gibson logo, so that will be going on soon. I'll post new/better pics after that.)
Oh, and here's the pics:
Front view
Rear view - big and small
Control panel
