Category: Technical

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.