Attention readers: I have recently revisited the Fender optical "vibrato" circuit and have more detailed results. Please click here for my closer look at Fender optical tremolo. The importance of photocell recovery speed that is discussed below is confirmed in that more recent article, but the "all-or-none" neon lamp output implied here is demonstated to be incorrect (light pulses are actually smoother).
(My corrections and comments to this original page are added in red.)
I have found a subtle yet important flaw in a replacement optical coupler (OC) for Fender vibrato amps. The OC of a 1972 Fender Deluxe Reverb-Amp had been replaced by someone else before the amp came to my shop. The amp's owner complained that, ever since the coupler was replaced, turning up the vibrato intensity control reduced the overall volume as it increased the relative effect level. Thus, when he stomped the amp's vibrato switch, a reduction in volume was heard. This made the effect useless for most of his music.
After a little bench-testing I found that the replacement optical coupler's photocell was responding too slowly to work in Leo Fender's classic design. Specifically, the OC's photocell did not recover a high enough resistance during the dark period between neon lamp pulses. I confirmed this conclusion by replacing the photocell with a faster unit (see below), resulting in a tremolo free of perceived volume reduction when engaged. This is basically how a Fender optical "vibrato" circuit works:
The adjustable low-frequency oscillator (LFO) uses one triode of a 12AX7 tube, and is enabled by closing a switch attached to the "vibrato pedal" jack. The ouput of this LFO controls current flow in the other triode of the 12AX7, which effectively turns the optoisolator's neon lamp on and off (actually produces rounded light pulses; see new article) at a frequency determined by the vibrato speed control.
Modulating the audio level to make the "vibrato" effect uses the photocell's characteristic light-dependent drop in resistance. In a network with the vibrato intensity control, the photocell forms part of a voltage divider which decreases the audio level when the coupled neon lamp is active. In other words, when the photocell is illuminated, its lowered resistance lets more signal flow toward ground and less along the amplifier's signal path. (A more detailed circuit description is included in the new article.)
The next drawing summarizes what I observed using my oscilloscope:
Neon lamp pulses represented along the top ("a") are time-aligned with their effect on audio levels ("b"-"d"). (This sketch is generally correct except for neon lamp pulse shape; see new article.) Waveform "b" represents a 1 kHz test tone; it is the analog output of the vibrato circuit when disengaged and/or when the vibrato intensity is set fully counter-clockwise. Waveforms "c" and "d" show the modulated signal with vibrato engaged and the intensity set at maximum--"c" is with the slow-recovering photocell in the replacement OC assembly, and "d" is my results after changing the photocell to a faster one. It's easy to visualize how a sluggish photocell caused the volume reduction.
I reasoned that photocell dynamics must have played an important role in Leo Fender's design process. If the photocell lags too much, volume reduction occurs, as we have seen. On the other hand, if the OC's photocell responds too fast (like a mechanical switch, say), the effect would be too choppy. Luckily for our analog world, photocells can't switch that fast (unlike eg. phototransistors). This characteristic of photocells is crucial here, because the coupled neon lamp cycles on/off according to voltage thresholds (i.e., all versus none is the neon lamp's predominant mode, from the photocell's point of view [not really; see new article]). I imagine Mr. Fender tried different photocells, picked a relatively fast one, and tweeked values of other components in his vibrato circuit experimentally, with pleasing musical results the criterion. (According to the Fender amp chapter in Ritchie Flieger and Jon F. Eiche's 1993 book Amps! The Other Half of Rock 'N' Roll [Hal Leonard Corp.], Mr Fender didn't have a particularly "good ear" for music himself, but had a knack for translating the comments of musicians who used his prototypes into musically pleasing results as he revised his designs.)
Whatever Leo Fender's design process, clearly photocell response time cannot be ignored when making replacement OC's for Fender amps. I suspect the necessary attention to detail was not invested by whomever made the replacement OC that I encountered. Since I was not the one who performed this replacement, I can't say where this part originated, or even whether the technician fabricated his/her own unit from spare parts. Nor do I know how many different folks might manufacture such replacements, or how widespread this problem is.
My suggestions? If OC failure is with the neon bulb, just cut the OC's shrink wrap open, replace the NE-2 bulb (only) and re-shrink with the original photocell. To maximize neon bulb life, keep the "vibrato" effect switched off when not in use (via pedal), instead of merely turning down the intensity control.
How I obtained a fast photocell. After learning about photocell characteristics at Perkin-Elmer's web site, and looking at photocells in catalogs (which don't usually specify photocell speed), I decided to perform major surgery on a Vactrol; I had a few Perkin-Elmer VTL5C1 units on hand, and got out the hack saw. These are hollow opaque plastic packages with a red LED at one end and a photocell at the other. Cutting one in half was not difficult. I carefully filed down the plastic housing around the photocell (don't be tempted to hold the unit by its leads during any of this). In re-building the OC, I had to use larger diameter shrink wrap than in a standard Fender OC or replacement, but I stuffed the ends with little wads of black electrical tape to insure isolation from ambient light.
The photocell in the VTL5C1 has one of the fastest recovery times rated in the Perkin Elmers VTL series of Vactrols. After a standard illumination, it recovers to 100 K ohms in just 35 msec. Other devices in this series range up to 1.5 seconds. You can find all the details at perkinelmer.com.
FUN FACT: The effect called "vibrato" in the Fender tradition is actually tremolo, musically speaking. Amplitude modulation causes tremolo, while frequency modulation is the defining characteristic of true vibrato.
More recent article: A Closer Look at Fender Optical "Vibrato"
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