1961 Gibson Discoverer, rather rearranged, opened up
There's not much left that's original except the circuit design, cabinet and the chassis metal. There were so many problems on the original that I just removed it all. The project got shelved for several years, then reanimated when I received a meticulously laid-out eyelet board, courtesy of Mr Jim Seavall, who introduced me to the difference between this model and all the rest of Gibson's amp line, and Mr Jim Foote who laid it out and fabricated it. The point-to-point wiring was abandoned and replaced by the board. I did all the wiring and spent some time reading up on proper grounding for audio. I put what I learned to use on a modified star-ground scheme for this amp and it doesn't hum at all. If you aren't playing, you can barely tell it's on. The input wires are shielded and the one preamp tube is a JJ 12AX7 with a spiral filament which made a huge difference. The original speaker had a magnet the size of a walnut and was completely dead. I wanted the loudest, strongest speaker I could fit into the box without modifying the box. I landed on the Jensen Neo 12 for its small, powerful magnet, and still had to cut off two cast iron cooling fins on the back to clear the power tube in the middle. Don't try this at home; I got iron filings in the magnet gap and had to send the speaker out for reconing. The idea for the 4-8-16 impedance switch is borrowed from Southbay Ampworks "Scumbag" amp, which started out in life as a modified Gibsonette/Discoverer. The Hammond output transformer has several taps, so it's possible to hook up an external speaker and change the setting to match. Big rubber feet will clear an amp handle when I stick this on top of another amp. The 15-foot power cord is the greatest thing since sliced bread; I never need an extension cord for this guy.
The impedance switch was tricky. The best part for this is a NKK type HS13Y which can be had at Mouser. I couldn't wait for the backorder, so I got mine from Doug Hoffman instead. The knob I picked up at a surplus joint years ago and just stashed. My Fender Electric XII uses the same one so I was thinking ahead. I went to a trophy shop and had a plaque engraved with 4-8-16 strictly for looks, for once I wanted to do better than a Sharpie scrawl. The box is just a standard aluminum Hammond stompbox. A short piece of 1x4 lumber shims the box up so it will clear the lip and also gives me something to screw into the side of the cabinet.
The control panel was an adventure. The original looked like this:
After all that work I thought a fresh control panel would be nice. How to do it? You can't buy one. So I took all the knobs off and scanned it, figured it would take an hour or two to clean it up. In Photoshop, the more I worked on it, the worse it looked, I tried every trick in the book. Finally I faced facts, it was going to have to be a hand-traced vector image. So I imported the scan into CorelDraw, blew it up to about 3200% and went over every line, curve and angle by hand, minimizing the number of curves. Lots of Bezier work. Took two weeks of spare evening time. When it was done what should I do with the image? I went through ideas about engraving, etching, silkscreening. The trophy shop worked with me on it and picked some shiny metal they had around and used one of their many magic processes to apply the image, and the result is a brand-new-looking replica of a 1960 control plate. I drilled the necessary holes, and the controls and other hardware hold it in position.
In this photo you can see that I added a choke to the power supply that wasn't in the original, I just picked a 5 henry job that looked about right, no science to it at all except I made sure it could handle the current. I think this is part of why the amp is so quiet at idle.
Output transformer
Here's one reason it sounds better than stock, the original, blown tranny is sitting on top of its replacement to show the difference. If there's a bottleneck in this circuit, it isn't the O.T.
At this point, the weak link is the power transformer which I salvaged from an old chassis from eBay. It works OK but it is serving up about 250 volts. To really shine, this baby needs closer to 350. The next step will be to swap that out and finally, play with the EQ parts to open it up a little.
How I like 'em
This is the way I like my amps. Electrically and sonically superior, with high grade parts, minimum signal path, and minimal changes to stock, and all battle scars and history showing. I like to glue down flaps and patch holes if possible, but I do like them to look their age, and perform at the top of their game. Yes, I know, I replaced every single part in this one, but this is an exception, it was a basket case. How does it sound? Well at the time of this writing, it hasn't been voiced yet, but it screams, sounds fat, squawky and "woody"...like a cello on steroids. I like it.