I've always wanted a Uni-Vibe. I REALLY wanted a Uni-Vibe! So badly that I went ahead and built one for myself. Now the problem with most modern Uni-Vibe clones is that there's no foot controller with which to control the speed, you have to bring along a bulky expression pedal. Now, believe me, controlling the Uni-Vibe's speed while playing is half the fun. My solution was to convert a broken down Morley echo pedal, you know, the huge one with the rotating drum. It was big enough to house the entire Uni-Vibe circuit and already had an AC cord for the power supply. The original Uni-Vibe footpedal uses a stereo potentiometer in order to control the speed. This was easily replaced by the LDRs that were originally used to control the echo speed.
The Uni-Vibe is a four-stage phase shifter similar to an MXR Phase 90. The sweeping effect is controlled by four LDRs and a lightbulb, and the value of these components is vital to the overall sound of the effect. Unfortunately, the original LDRs are discontinued, the particular lightbulb was never identified on the schematic diagram, and today no one really has the same combination. This is why all the clones sounds different, even though they use the original circuit and even copy the original circuit board. The distance between the lightbulbs and the LDRs also varied from unit to unit and no to Uni-Vibe sounded the same either, although the difference between them was rather subtle!
After trying a few dozen combinations of LDRs and lightbulbs I finally settled on something that I liked. The Uni-Vibe has a reflective metal cover over the LDRs which I made out of cardboard and lined with aluminum muffler tape. I was able to use the existing power transformer, and built the whole thing on Vero composition board. Because of the wide range of movement of the Morley pedal mechanism and the greater control offered by the Morley opto-electrical system over the original foot controller, I was able to get a much larger variation in speed, something that is hard to do with the Uni-Vibe.
I added an external center-frequency control in order to vary the sweep effect. This makes for some rather dramatic changes. The Morley Echo pedal conveniently provided 2 DPDT foot switches, so I used one as an effect bypass and the other as the chorus to vibrato selector. All that was left was to add a few LEDs
The original Uni-Vibe did not have a true bypass and colored your sound even when the effect was not on. When the foot controller was raised to its highest position the oscillator would switch off but the sound still was routed through all all the pedal circuitry. You get an overall brightness boost and, if the volume is up to maximum, a volume boost as well. Listen to Hendrix's tone at Woodstock at the Fillmore East concerts, for example. The impedance is fairly low and will load up your guitar signal if it's not buffered. On the other hand, this is the part of the overall sound and I found that increasing the input impedance of the Uni-Vibe can make it sound overtly bright and brittle while removing some of the warmth of the effect.
In order to avoid any coloration I chose to go the full bypass route. This way, I could buffer the pedal or not depending on the situation. Oh, I should point out at that this point that it is possible to wire up a second output jack to the Uni-Vibe and to use it with 2 amplifiers. Send the dry signal to one amp (main output jack) and send delay-line output of the phase shift network to the other amp (second output jack). This gives a really nice spacious effect. Don't forget to mix the two signals back together again if you want use the pedal in mono.
All Uni-Vibe clones sounds different and mine is no exception, but, it compares very favorably to the original and seems to be a lot closer than some of the other clones I've tried.