Standel Studio XV, Model #S15RA, Serial Number 1006-3

Features:
• All Early Germanium Transistors (including output devices!): 2N663 (reverb, SE interstage audio transformer drive), DTG110 (push-pull output)
• Jensen C15N Vintage 15" Guitar Speaker
• Reverb tank, Vibrato Speed/Depth, Bass, Treble, Volume controls
• Rebuilt first preamp stage
• Total labor: 15 hours.

    This vintage Standel Studio XV was given to me by a friend of mine because it didn't work. He decided not to try to fix it because certain key circuit boards are submersed in epoxy to protect the company from having their secret circuitry copied. Of course, one such module was broken, so the amp was not readily serviceable.

My friend had gotten it from someone else who obviously took very good care of it, because the grill cloth on the front is in perfect condition with no strech marks, scuffs or loose threads. The inside of the amplifier chassis is signed and dated 5/20/65, so it is almost 40 years old!! Furthermore, it contains a vintage Jensen C15N, which is likewise impeccable, with no rust, no cone warping, no holes, no rips, no chips on the magnet, etc. It would have been a shame to pull it apart and put some other kind of amp in it, so I did the only sane thing: I sketched out the whole schematic (omitting the epoxied modules, of course), and tried to figure out what went wrong to see if I could fix it.

• After several hours of probing and sketching, I figured out, to my dismay, that indeed one of the key modules had malfunctioned-- in fact it was the very input module, the simplest of the lot ("module #2" by my nomenclature).
• I noticed that the old module #2 had two transistors in it and three capacitors. The resistors and any other small parts were completely obscured by the thick-black, rock-hard epoxy.
• I pulled out my old lecture notes from my junior-year electronics class and started designing a two-transistor circuit, taking note that the three capacitors seemed to indicate that the two stages were capacitively coupled.
• I had my design idea down. I would use a common-collector (CC) configuration on the first stage for high input resistance, and a common-emitter (CE) configuration on the second stage for high gain. I went through the equations and came up with some numbers. I decided to use plain old 2N2222's that I got from Radio Shack because I didn't know if I had a pair of germanium transistors that worked. I plugged in some resistors and tested it out. It didn't work!! At all!
• I started over and decided to do one stage at a time. I got the first stage working fine (it was working before), and started on the second stage.
• I was lucky when I redid the second stage, because I picked base-bias resistor values just right -- one 10-times the value of the other. When I changed one of those resisors by simply doubling or halving it, I got no output whatsoever (completely off the load-line)! I proceeded from there and hooked up the circuit to the amplifier. It worked! But it was noisy. I found out that where you put the ground and power wires were of high importance, as well as shielding the circuitry.
• I got the amplifier working well on the sine-wave generator, so I tried my Gibson SG guitar (with 12 gauge flat wound jazz strings). The gain wasn't high enough, and it got terribly muddy distortion when I played hard. I changed resistors on the second stage until I found one with a higher beta (they frequently vary as much as 50% or more!!). I compensated the charateristics by changing a resistor here and there and decreasing the value of the interstage coupling capacitor. Then the treble was too sharp, so I tried some different caps on the base of the second transistor. Then I tried the vibrato, but it was too shallow, so I settled on a 10k resistor on the output (after some experimentation) and got that to work well. I found a disc capacitor that sounded best, and several other types that didn't sound quite as good.
• I continued to tweak the circuit, resistor by resistor and capacitor by capacitor for 5 hours!! I was pretty happy with how it sounded, although I thought the intermodulation distortion arising from the non-linear transistor behavior was a little too high (it was on a cheap 8" test speaker), but I went ahead and built the circuit anyway on to a circuit board, since I couldn't figure out how to make it more linear without using the tough-to-design cascode configuration (CB-CE combo) or an op-amp.
• I very carefully laid the circuit out, keeping shielding in the forefront of my mind. I edged the board and even the center screw with a grounded wire to keep stray electric fields out, and kept the component leads as short as possible.
• I wired the circuit into the amp and tested in on the 8" speaker. It worked and sounded pretty good, with very low noise.....