Phaseomatic In-depth

by Phil Taylor

Audio Archeology

Inside the prototype 8-stage Tube Phaser

There are many, many variations of solid-state phaser and vibrato effects pedal available today, however, tube phasing and vibrato has remained a relic of the past. I wanted to resurrect the vintage designs that Magna Electronics had begun so long ago, improve on their work and bring that rich and lush sound back to life. I completed my first working prototype vacuum tube phaser in the summer of 1998. Originally, the design incorporated an optically controlled, eight stage phase-shifter and all tube low frequency oscillator (L.F.O.) based on an esoteric triangle wave time-base generator, known as a "Fruhauf" oscillator. I did quite a lot of "digging" before I eventually discovered an old book containing different variations of tube timebase oscillator circuit, including a pentode implementation for the "Fruhauf". The principle of operation is based on the constant charging/discharging of a capacitor to generate linear rising/falling ramps to create a triangle wave. I then set about experimenting with this circuit to modify it to operate with triodes, creating my own unique version of the "Fruhauf" oscillator for my 100% vacuum tube phaser.

The first prototype Phaseomatic (originally named "The Big Green Phasing Machine") even had a "magic-eye" (EM80) which pulsated to give an indication of the speed and depth of phase shifting or vibrato. Magic eyes must surely represent the pinnacle of cool retro technology - in comparison to the modern equilvalent, LEDs, they look like they were developed by the Krell. Four tubes were used for the phase-shifter section, three for the L.F.O., three additional tubes for a gain amplifer & buffered output giving a complement of ten tubes in total - more than most guitar amplifiers! The picture above shows the electronic "guts" of the original prototype, all that remains of this magnificent phaser - the large tube is the magic eye. The vibrato generated by this unit was extremely intense - perhaps better described as unuseable for typical guitar applications. More development work confirmed that an excellent range of vibrato effects can be obtained with just two stages of phase-shifting, however 8-stage tube phasing is delicious!

Golden Tone

The original Effectrode workhsop in Dedham, Essex (1996)

I invested a great deal of time in performing audio tests on the Phaseomatic circuitry. It was important to ensure that the quality of tone was not compromised with the pedal connected in series between the guitar and the amplifier, i.e. the effect is engaged. This is still a key philosophy for Effectrode - to build effects pedals that keep guitar tone pristine and intact. With this approach the true-bypass fad that has developed over the last decade or so becomes a non-issue - we're designing and building real "tone-machines" here! Much of the on-the-bench circuit testing & analysis was performed in the time-domain using a lovely, tube 15MHz Dual Trace Solartron CD1400 oscilloscope and Advance - model J14 audio sinewave generator. This older gear could be a little "twitchy" at times, but it did help to keep my first workshop warm on cold winter days. So, the Phaseomatic is an all-tube pedal, designed and engineered using all-tube test gear by a tube nut! I like to imagine I've captured the essence and spirit of all-tube tone in the process.

The current model of the Phaseomatic (PM-2C) tube phaser features one of the most sophisticated solid-state L.F.O.s ever developed coupled via photo-cells to four cascaded triode phase-shifter stages. The four stage topology can be found in many other commercial phasers, such as the "MXR phase 90" and "Uni-Vibe", however these manufacturers implemented their designs in silicon. The Phaseomatic tube phaser uses 12AT7 tubes in the phase-shifter circuitry and 12AX7 tube for the gain and buffer stages. The phase-shifter stages have been optimized to give almost the full 720 degrees of phase-shifting. To achieve this, component values were chosen to compensate for non-linearities in the filament bulb response and the characteristic response of the photo-resistors. The reduction in phase-shifter stages and a solid-state L.F.O. meant that only three tubes were required, with the added advantage that smaller transformers could then be used.

The Science

Later on, more accurate BSPICE models were created for the tubes used in the Phaseomatic. Circuit behaviour in the frequency domain could then be modelled extremely easily, allowing the phase-shifter sweep across the audio spectrum to be tailored precisely. All modelling was performed using "BSPICE" within "MultiSIM" from Electronics Workbench, which is also an excellent schematic capture package as well. Simulating the circuit in this way was easier and faster than constructing prototypes, powering up, de-bugging, testing, changing a component value here or there and then iterating until it worked as expected. It also allowed many more possibilities to be explored because development time was not such a constraint and not so mind numbingly tedious.

Construction

Vaccum Tube Phaser - Model PM-1A

The original Phaseomatic (PM-1A) was designed with a built-in, low-noise power supply consisting of separate H.T. and L.T. transformers for the tube high voltage supply and the heaters respectively. These two transformers were less bulky than a single larger transformer making the pedal relatively compact. The H.T. transformer allowed the vacuum tubes to operate at 300 Volts D.C. and was custom wound by Danbury Electronics, UK. Custom wound transformers are expensive to produce in the small batches that Effectrode was using, so for the PM-2A I adopted a different approach and designed a circuit that could convert 12VDC upto 300VDC for the H.T. Efficient power conversion without the use of specially wound transformers was a big technical achievment for me taking several months of development (and electric shocks!) to work through the design until I was completely happy that it was stable and quiet enough for audio. The result was the PM-2A phaser, which operates from an external wall-wart power supply. An additional advantage of this approach was that the tube heaters were operating on DC power for reduced hum and a lower noise floor.

High quality components are used throughout the circuit. Metal-film resistors are prefered over carbon compostion, which have a tendancy to drift with age and produce much higher self-noise. The PM-2C utlizes precision Vishay metal-film resistors which are instrumentation-grade, meaning they are extremely quiet, reliable and stable making for an exceptionally transparent and natural sound. All coupling capacitors are polyester dielectric for their high voltage rating, stability and long life. Electroytic bypass capacitors in tube bias circuitry have been replaced with modern ceramic capacitors. If it's possible I avoid using electrolytic caps because of their poor figure-of-merit - their inital tolerance is poor and they have a tendancy to dry out, so that their capactiance gradually decreases with age.