Ampeg SVT-4 Pro. Had the same problem I often see in these, damaged solder connections on the rear of the main PCB. Bass heads get lots of mechanical stress because they’re always sitting on top of a box full of speakers. Solder joints on flexible and poorly-supported PCBs go intermittent, making the amp cranky/possessed/unresponsive/etc.
This amp in particular wouldn’t turn on (no blown breaker) because the solder under the IEC AC mains jack had crusted and flaked away. I reinforced it with buss wire and epoxy, and resoldered every pin on every jack along the back. This is a needlessly expensive repair, because of how time-consuming it is to get the whole board out, for such a silly problem.
B&K Model 607 emission tester. Strange anachronism, a fully solid state tube tester. This one was designed for light weight and portability. It’s basically a clone of the Sencore TC162, in a class of testers used for basic on-site go/no-go testing, as opposed to the big mutual conductance testers you’d find in audio repair shops. Probably lived in a van, and would be at home in a touring tech’s toolbox. It has a shorts test with 90VAC applied and 1M sensitivity (as opposed to most others at 200-300k), awesome gas test (100M grid leakage sensitivity), and basic emission testing. The tweaks vs. the TC162 are good, but I think I’d prefer the TC162 if I were going to use one or the other consistently — this one doesn’t calibrate emission results, has no life test or meter zero, and has fewer sockets.
This was a full rehab. All board components replaced (resistors all 1%), meter cleaned out and adjusted, switches cleaned, grounded power cord added with RFI shunt network, and .66R/10W resistance added to the filament circuit to compensate for higher line voltage than when the unit was produced. It’s much more accurate now than it was when it was purchased, though “accuracy” is pretty meaningless with emission testers. That said, this tester seems to have emission results that consistently make more sense, relative to the actual in-circuit performance of a given tube, than my Sencore MU150. New tubes actually test “100” most of the time, instead of 110 or 120 or off the scale. Old bad tubes hit 50, and I don’t need to drop the filament voltage (Sencore’s “life test”) to detect an impending failure. Strange, since this tester’s emission can’t be calibrated, and is affected by line voltage and the leakage calibration, while my MU150 has been carefully calibrated with an oscilloscope and uses a regulator.
If you happen to work with one of these or a similar unit, be aware that all 6550s and some KT66s/KT88s connect pin 1 to the metal base. If you’re in shorts test mode and grab the tube, you’ll get a nasty shock. Use lockout switch #1 with such tubes to avoid this (and make a note on the setup charts).
Epifani PS600 bass amp. Notice the integrated switchmode supply and Class-D amp in the center. Very difficult to work on if the issue is in there. Lots of SMD and modular stuff. There’s about 1Vpp of ultrasonic noise (190kHz) on the output of these amps at all times — not sure whether this is a carrier residual from the Class-D power amp or SMPS noise. In most cases it’s a non-issue because speakers won’t reproduce it (and you couldn’t hear it if they did). Nice thing is that it does 550W into 4 ohms and only weighs about 10 pounds.
Sencore PR57 Powerite. This is a variable autotransformer and an isolation transformer with voltage/current/leakage metering. Great unit if the meter is working, mostly useless if not. Many on the used market have no leakage probe, but they’re easy to build — just a normal banana-terminated probe with a 135k resistor in series. A momentary switch shorts out the resistor to get the true leakage current.
Manual is at BAMA.
Silvertone 1464 head. This is a “100W” solid state head (more like 55W in reality, an overstatement common to a lot of vintage solid state), which tucks into a 2x12 cab that it’s hardwired to.
This amp got a grounded power cord, polarity switching and ground caps removed, trem and reverb repaired, hardwired speaker cable removed, and speaker jacks added to both the head and the cab. The big black cylinder in the picture is the reverb tank. Rather than a metal pan like Fender used, this is made of some sort of acoustically dead fiber material, and is spring-mounted. The transducers on the springs are piezo elements, which is why the reverb has a broadband sound, rather than a typical Fender response. It also feeds back quite a bit, despite the mounting. The trem oscillator is more of a triangle-wave modulation rather than the sine-wave you’d get with a Fender tube circuit.