I picked up the above-depicted Pioneer SR-101 all-tube Stereo Reverb unit for a few dollars at the final flea of ’12. It worked after some minor repairs and I am happy to report that it’s actually a pretty fine lil box. I made a few modifications and added some hardware to adapt it to studio use. I’ll describe the whole fandango here in case any of y’all are thinking of going down the hardware-analog-reverb path. There are plenty of these things on eBay, often closing in the $50 – $200 range. Even if you have to spend a lil time or money on some repairs, it could still be a lot cheaper than the roughly comparable Orban 111B or the Sound Workshop 242, both of which we also have + love at Gold Coast Recorders.
Above: the ‘pickup,’ AKA ‘output’ side of the twin tanks. Unlike the Fisher Space Expander (which I also just picked up… deets on that one soon…), the Pioneer is a true stereo machine. Each input feeds its own physical reverb tank. This is a big, big benefit over the mono-summing of the Fisher. My SR101 unit was passing direct signal, but not reverb, on one side; the culprit was actual just the output lead of the tank (above), which was over-heated during manufacture and had a signal-leak-to-ground on the coaxial cable. A quick snip-n-solder and we’ve got SOUND.
Because this is 60′s piece, the AC mains are not grounded. So I hacked up a nice long IEC cable and added that. Above: I connected the ground (green) wire to the common lug of the multi-cap cap. Seemed to be the most convenient option… The only other repair was of a more mechanical nature. The tanks are suspended from steel risers via small springs, with foam rubber pressed between the tanks+chassis. 45 years of tiiiiiiiiiiiime marching-on had turned much of the foam suspension into sticky goo; I replaced the rotted foam with some generic foam road-case-material.
Above: the schematic of the SR-101, courtesy of this handy web forum. Notice the two red wires: the fellow who originally posted this schem was kind enough to highlight them. Here’s why. When I originally got the unit, it was a little tricky to troubleshoot; the left input came out of the left dry output, but the left channel reverb emerged from the right out. WTF? Turns out that this was a gimmick that Pioneer used in order to ‘widen’ the stereo effect. And it does work, but that would just be confusing as hell in the studio. So I re-reversed (versed?) the direct-signal wires and then reversed the leads going to the RCA output jacks.
While I was at it, I drilled a hole in the front panel and added a DPDT on-on switch that cuts the direct signal fully out-of the signal path. So now the left channel input and its associated reverb both emerge from the left output, as one would expect, and vice-versa for the right channel. PLUS, now I can flick the switch up and get reverb-only in the outputs. Easy enough…
Above: the rear of the rack-case. That lil silver box on the right is a bi-directional stereo balancing amp designed to interface consumer audio gear with studio (or broadcast) audio systems. Basically, it takes a stereo balanced +4 input signal and drops it to -10 unbalanced output, and simultaneously takes a -10 stereo input signal and boosts it to a +4 balanced output. I own many of these sorta things, but the unit above is notable in that it is really, really, really fukkin cheap. These things are generally in the $70 – $200 price range, but my fav purveyor of dirt-cheap electronic crap MCM electronics has em now for $39. There are often sales too; I think I paid $35 for this one and $30 for the last one I bought. Both worked fine BTW. Anyway, I wouldn’t recommend that you mix a record thru the thing, but I can’t imagine it doing any harm to the signal coming from a 45-year-old box of tubes and springs and carbon-comp resistors.
Above: the front of the balancing amp as seen from front of the rack-case. The knobs set the send and return levels to and from the SR-101. This is super-handy in terms of setting the right nominal level to ensure a good signal-to-noise ratio without creaming the tanks too hard (wow that sounds gross). Unlike the reverb tank in a fender guitar amp, for instance, the SR-101 hits the tanks with power amp tubes (around 2 watts, as opposed to maybe 100 milliwatts in a fender). So it is possible to get a pretty good signal level out of them without too much objectionable noise in the tank return circuit, provided that you hit the tank input hard enough. I might be repeating myself now, sorry, it’s late…
And above: the sole audio control on the unit, charmingly labeled ‘REVERBE TIME’ Yes Reverbe. Love it. As the schematic reveals, this is simply a passive gain control in the tank pickup amps. So yeah it’s a one-sound box. But it’s a glorious sound. This dusty gem just got put in GCR today, so once I get a chance to try it on a mix I’ll post the results.
Wow, I used to dismantle stuff like this for parts to fix guitar amps up with. That was when it was like pulling teeth to buy the “Correct” parts for Fenders. There were no Mojo, WeberVST, etc, etc back then. It had only been a couple of years since the aftermarket hot rod guitar parts industry even started making parts you could use to build or fix a guitar with except through authorized dealers at ruinous markups.
We’d put output transformers from Fisher, Scott, Pioneer or Sansui receivers in small guitar amps: power transformers you could get from Stancor or Thordarson, except they never had the bias tap so you had to cobble up a bias supply with a backward surplus filament transformer.
That ended pretty much when Fender was bought out from CBS and the new distributor was a lot easier to deal with, plus Mesa Boogie and others would sell these parts too. We have it really good now, by comparison, especially if you have a state business license and tax ID. It was fun to gut out all that old junk, except now it’s valuable and in demand.
Interesting to note that that schematic looks just like the ones in MJ to this day. Surely the old draftsman still isn’t working there? Or maybe he is.
the interior construction also has a somewhat MJ vibe… at least those cool recessed tube sockets, which seem pretty uniquely japanese.. Cheap as this thing prolly was at the time, ya kinda can’t fault Pioneer; it’s a 45 year old piece that needed no caps or resistors changed. that’s pretty good quality in my book….
Would love to modify mine the way you did (especially for the dry out part) would you post where you did the mod on the schematics? thanks best K