by Phil Taylor
I’ve modified and repaired and several different tremolo circuits in my time, from DIY pedal kits to those installed in tube amplifiers such as the Fender Blackface Twin and Tremolux. The tweed-covered Tremolux was introduced by Fender back in 1955 and first tremolo-equipped amp. There’s no doubt in my mind that these classic American tube amp circuits produce a beautiful, deep and warm tremolo, that still remains unsurpassed by the current offerings of transistor and opamp based effects.
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.
The Shape Control
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.
The Delta-Trem ships with a standard LFO configuration where the pedal replicates a range of vintage tube amp tremolo and custom waves developed by Effectrode. With the “Shape” knob fully anticlockwise the LFO generates a unique “crossover” wave. This can be used to add a subtle shimmer to chord-work. As the knob is rotated clockwise the wave gradually morphs into a triangle, through to a sinewave at the 12 o’clock position to recreate the buttery tremolo of early Fender amps. Further rotation results in the wave sides becoming steeper until a smoothed squarewave is produced. This intensifies the tremolo throb so that it becomes more reminiscent of the old Valco-designed amps.
The Delta-Trem pedal can be configured to generate variable rising and falling sawtooth waves where the “Shape” knob sets the ramp rate. This adds lopsidedness to the throb of the tremolo creating “Leslie” and unique amplitude modulation effects, where the sound level gradually increases (or decreases) then suddenly jumps back. In the anticlockwise position a rising sawtooth is produced. Rotating the “Shape” knob clockwise gradually alters the characteristic until a triangle wave at 12 o’clock. As the knob is further rotated the wave changes to a falling sawtooth.
On Bias Modulation Tremolo
The Delta-Trem is a photo-optical tremolo, not bias modulation tremolo. Signal modulation tremolo based on photcell circuitry is more costly to implement than bias modulation designs because it requires extra tubes and photocells, however I prefer it for the following reasons. Bias Modulation tremolos work by altering the cathode bias to the power tubes in the output section and forcing the tube into “cut-off”. This introduces “crossover distortion”, which is a nasty, odd-harmonic distortion caused by the fact that the signal is not being accurately reproduced. This inherently less costly, less complex circuitry is usually reserved for lower-power, inexpensive amps, such as the ’64 “Fender Princeton”.
The Delta-Trem emulate a bias tremolo. One of my customers has spent some time on this and provided some settings: Mode: LeslieShape: 11 o’clock Depth: FullSpeed: 3 o’clockish
The key to the bias trem effect is the asymmetrical wave coupled with smoothed peaks. Obviously speed and depth are to taste but this is a good demonstration of the full on effect.
The Delta-Trem is a sophisticated pedal with many possibilities for tweaking the L.F.O. to generate different tremolo characterstic throbs. With this pedal design, I was aiming to get as much flexibility in a compact unit whilst keeping the look of the pedal conservative/traditional.
The way “Setup Mode” operates is as follows. When the “Speed” knob is rotated fully anticlockwise the LFO will modulate at a predefined rate and the “Shape” knob functions as mode selector allowing one of 4 modes to be selected:
- Shape knob controls amp waveforms – ping-pong mode
- Shape knob controls Leslie waveforms – ping-pong mode
- Shape knob controls Amp waveforms – indepndent LFO mode where the Depth knob acts as a control for the second LFO
- Shape knob controls Leslie waveforms – indepndent LFO mode where the Depth knob acts as a control for the second LFO
Imagine the Shape knob is like a clock face where mode one is roughly 6 to 9 o’clock, mode two is 9 to 12 o’clock, mode three is 12 to 3 o’clock and mode four is 3 to 6 o’clock. Once the mode has been selected with the shape knob, rotate the speed knob out of the fully anticlockwise position, say 12 o’clock and you’ll be back into normal mode.
There is a lot of functions in the Delta-Trem pedal to allow for a great deal of flexibility and options, however this comes at the price of some extra complexity in operation.
- When using mode III or IV the ‘DEPTH’ knob becomes a ‘SPEED’ control for the second LFO. The depth setting for both left and right channels will be the current ‘DEPTH’ knob setting and if it was set to zero there will be no tremolo effect. Turning the pedal on/off does not reset this. You must go into mode state (‘SPEED’ knob fully anticlockwise) and turn the ‘DEPTH’ knob clockwise, then switch to mode III or IV.
- Switching from mode III or IV to mode I or II can shift the LFO sync. In mode I or II, the LFOs are normally synced to produce a stereo panned or ping-pong pulse, however this sync can be shifted when switching from modes III or IV to modes I or II. (While in mode state, switch back a forth between mode III and II and monitor the sync between the orange and amber LFO lights). The shifted LFO sync may produce a dual mono pulse or any variation between that and the normal ping pong pulse. This may be a desirable effect though difficult to reproduce. Simply bypassing the effect will reset the LFO sync to normal operation.
Note: when I get the opportunity I’ll make an animation to demonstrate visually how this works.