I wanted to recreate the buttery pulse of this classic amp trem and eliminate some of the shortcomings as well. Firstly, the LFO (low frequency oscillator) in some of these these older amp tremolos were notorious for generating an obtrusive audible “ticking” noise. Culprits include many on the older Fender amps, such as the Deluxe Reverb, Twin Reverb and Vibro-King. This noise is generated by the amplitude modulator (AM), which is based on a light dependent resistor and neon lamp (cold cathode tube) arangement. The well-known fix is to install a small capacitor in parallel with the neon lamp, however this is not always an effective solution and additional adjustments often need to be made to lead dress and layout to prevent capacitive coupling of the noise signal. So eliminating this ticking issue can be a challenge, even a “black art”, to say the least.
The noise is caused when the neon lamp strikes (lights up). From zero up to about 90 volts, the lamp is dark and non-conducting. When the LFO reaches the 90V threshold, the neon gas in the lamp ionizes, conducts electricity and the lamp emits is characteristic orange glow. The sudden change in voltage across (or current through) the lamp that’s responsible for the ticking.
To eliminate this noise problem, I developed a custom “Raysistor” based on a cadmium sulphide photo-resistor and filament lamp assembly. Raysistors were manufactured during the mid 1960s, however can no longer be obtained, which is why Effectrode developed their own device. To my knowledge this approach is unique to the Delta-Trem as vintage tube amps utlize neon or bias tremolo and pedals are based on V.C.A. (voltage controlled amplifier) designs. Replacing the neon with a filament lamp means there is no sudden increase from dark to light at a threshold as illumination intensity varies continuously for smooth and quiet amplitude modulation.
These properties of the neon present another limitation in that there is a sudden jump down from maximum modulation amplitude when the neon suddenly strikes. After this point the light intensity varies relatively linearly with the LFO control voltage. There is no way to adjust this, it is intrinsic to the design and totally dependent on the physics of the neon lamp. So, even though the phase-shift oscillator configuration in these older amps generates a sinewave control voltage, the photo-resistor does not see this because the transfer characterstic of the neon lamp transforms this into a resistance change that is more pulse-like than sinewave shaped. Subjectively this gives nice deep tremolo sound, however there is no control over the shape of the wave to produce smoother more shimmery tones.
This phase-shift LFO also has another shortcoming – it looses gain at lower modulation rates. The result is a weaker intensity of effect (less depth to the tremolo) at slower settings. The original Uni-Vibe has a similar phase-shift LFO configuration (implemented in silicon) and therefore also exhibits this drop-off of intensity as the oscillator begins to collapse. In hindsight it is easy to criticize these tremolo circuits, however it should be recognized that some of the designs being discussed here are over half a century old. In their time they were the state-of-the-art, cutting edge innovation.
Today, in the light of the accumulated body of knowledge and wisdom on electronics laid down by engineers at RCA, Leo Fender and others I saw an opportunity to build a better tremolo. The idea behind designing the Delta-Trem was to engineer-out the shortcomings of vintage tube amp tremolo units, whilst retaining the benefits of an all-tube signal path to create a more musical tremolo pedal. Additionally, I wanted to add more flexibility on the shape of the modulation waveform. The Delta-Trem features a totally unique wave shaping circuit which allows continuous control of the LFO waveform and can generate sine, triangle, square, rising and falling ramps, along with a whole lot of other waves in between. All the hard edges on the waves are smoothed to produce a more organic and musical tremolo tone.