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Phaseomatic In-depth

There are more solid-state (VCA/transistor/opamp/modelling) vibrato and phaser effects in heaven and Earth, dear modulation fx enthusiast, than are dreamt of in your philosophy… however, curiously, no tube vibrato pedals, not a single one. Now, the smooth, pulsating, phase-chorused tones of tube vibrato are the stuff of legend—only a handful of guitar amp makers have ever attempted it. Magnatone^® incorporated a patented ‘Cathodyne’ vibrato circuit in their top-of-the-line amps and Leo Fender patented a “Harmonic Tremolo” design, which subtly, although not intentionally, pitch-shifted the signal as it modulated high- and low-pass filters. The musical shimmer Leo’s invention imparts on electric guitar is truly captivating, however Magnatone’s Cathodyne vibrato sounds much richer, deeper, more dimensional—it was a no-brainer to take a shot at resurrecting this electronic relic from a bygone golden age in audio, make a few improvements and then install the tube circuitry in a retro-styled aluminium box.

The Prototype

8-stage tube phase-shifter — All tube 8-stage tube phase-shift circuitry inside Effectrode’s prototype phaser.

The first working prototype vacuum tube phaser was completed in the summer of 1998 [schematic here]. The original design was an optically controlled, 8-stage tube phase-shifter with an all-tube low frequency oscillator (LFO). Some digging was needed to unearth a suitable LFO, which eventually turned up within the time-yellowed pages of a dusty old book containing schematics for dozens of different types of weird and wonderful tube time-base oscillator circuits, including a curious triangle wave time-base generator, called a ‘Fruhauf’ oscillator.

This pentode oscillator circuit was re-engineered using triodes, creating a unique LFO for our all-triode phaser. The Fruhauf’s principle of operation is based on the constant charging/discharging of a capacitor to generate linear rising/falling ramps to synthesise a triangle wave—triangle waves are much better control voltages for a phaser pedal than sine waves as they sweep the phase-shift filters smoothly up and down—there are no dead spots in the cycle.

This prototype (originally named “The Big Green Phasing Machine”) even had an EM80 ‘magic-eye’, which pulsated quietly and eerily giving an indication of the speed and depth of phase shifting or vibrato. The magic eye must surely epitomise “retro-cool” technology—yesterday’s vision of tomorrow. In comparison to the modern equivalent, LEDs (Light Emitting Diodes), it looks like something engineered by the Krell (Forbidden Planet)—a technological marvel, quite beautiful.

Four dual triode 12AT7 tubes were utilised to make eight ‘Cathodyne’ phase inverter sections, each followed by an R-C network, the ‘R’ component of the network being a photocell. Three more tubes were required for the Fruhauf LFO, and another three for a gain amplifier and buffered output giving a grand total of ten glowing bottles—more than most guitar amplifiers! The photograph above shows the electronic guts of the original prototype, all that remains of this once magnificent phaser—the large tube is the magic eye.

The vibrato generated by this electronic lash-up was extremely deep. An 8-stage phaser has a theoretical maximum 1440° phase shift and creates a comb filter with four notches in the frequency spectrum. Some further tinkering confirmed that an excellent range of usable vibrato effects could also be obtained with fewer stages, for instance just two stages of phase-shifting as in Magnatone’s vibrato circuit. But it has to be said: 8-stage tube phasing sounds out of this world!

The Original Model 1A

Cathodyne phase-shift circuit — The 2-stage 'Cathodyne' vibrato circuit patented by Don L. Bonham in 1954 (patent # 3,146,292)

phaseomatic_pm1_advert_320px_pic — Phaseomatic tube phaser model PM-1A

The first commercially available Phaseomatic, the original model 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 at the time, so with the PM-2A we adopted a different approach and designed a circuit that could convert 12VDC up to 300VDC for the H.T.

Efficient power conversion without the use of specially wound transformers was quite a technical achievement, or perhaps voodoo more accurately describes the magic of getting 300V from a 12V wall-wart. It took several months of development (and electric shocks) to work through the design until it was stable and quiet enough for audio. The end-result of our labours was the PM-2A phaser, which operated 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.

The Compact Model 2A

The model PM-2A featured one of the most sophisticated LFOs ever devised coupled via photo-cells to four cascaded Cathodyne phase-shifter sections. Four phase shifter sections is pretty much the standard and can be found in many other stompbox 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 a 12AX7 tube for the gain and buffer stages. The phase-shifter stages were optimised to obtain almost the full theoretical maximum 720° of phase-shifting. To achieve this, component values were carefully selected to compensate for non-linearities in the filament bulb response and the characteristic response of the photocells. Further, the reduction in phase-shifter stages and a solid-state LFO meant that only three tubes were needed, with the added advantage that smaller transformers could then be used.

phaseomatic_mopm2_320px — Phaseomatic tube phaser model PM-2B.

Testing, Testing…

A substantial of time was invested in performing subjective audio listening tests on the Phaseomatic’s circuitry to make sure it sounded “good”, that the tone quality of the guitar was in no way compromised with the effect engaged. With many phaser pedals you get the phasing effect plus undesirable artefacts you don’t want. Artefacts such as noise, clipping distortion, pickup loading, loss of ‘top’ and ‘bottom’, the solid-state sound, etc. This is important stuff for Effectrode—to build effects pedals that don’t just preserve the guitar’s tone, but enhance it in some desirable, musical way.

Back in the early days, Mrs E. would would often help out with listening tests. In a way she was perfectly qualified for the job: she’s not an obsessive tube nerd; not a double-dome; nor is she an electronics engineer; she’s just a normal, balanced person; and therefore unbiased. Many tests involved A-B comparisons of the Phaseomatic with other solid-state phaser pedals. I’d play guitar through the different pedals in the front living room, and Mrs E. would listen-in, whilst sewing or reading from the kitchen. Not very scientific I grant you; however, quite amazingly she always preferred the sound of tubes to transistors, and this was over the course of dozens of such tests. So, even whilst listening in another room, whilst distracted by some other task, and having no particular interest in sound quality, she could discern a difference—the sonic differences between tubes and transistors are unmistakable.

Solartron oscilloscope a.k.a. the central heating system for Effectrode's first workshop

Much of the on-the-bench circuit testing and analysis was performed in the time-domain using a lovely, tube 15MHz Dual Trace Solartron CD1400 oscilloscope and Advance—model J14 audio sine wave generator. This outmoded thermionic gear could be a little twitchy at times, but perhaps it helped to keep the workshop a little warmer on those bitterly cold winter days and when working late into the night. The Phaseomatic is an all-tube pedal, designed and engineered using all-tube test gear by a tube nerd; and, being a tube nerd, I often imagined that working with all that archaic old test equipment,somehow helped to infuse more of that vintage vacuum tube ‘vibe’ into Effectrode’s pedals.

…And Materials

Super high quality components are utilised throughout the Phaseomatic’s circuitry. Metal-film resistors are preferred over carbon composition types, which have a tendency to drift with age and generate higher self-noise. The audio signal path of the model PM-2C is built on (expensive!) precision Vishay metal-film resistors. These instrumentation-grade resistors are exceptionally quiet, reliable and stable, and exhibit very low distortion characteristics, in short ensure a beautifully open, natural tone. All coupling capacitors are specified as epoxy encapsulated polyester dielectric types for their high voltage rating, stability and long life. Electrolytic bypass capacitors in the tube bias circuitry have been replaced with modern high capacitance ceramic types. Wherever possible the use of electrolytic caps has been avoided because of their poor figure-of-merit—their initial tolerance is inaccurate and their electrolyte gradually drys out over time, so their capacitance drifts towards zero with age.

Strictly For Nerds

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, debugging, 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.

So that’s all there is to tell of how Effectrode brought vintage tube amp vibrato back to life… well almost. Since writing this article the Phaseomatic has been revised, yet again. The new model PM-3A now has twin LFOs, tap tempo, beat matching capability and a few other additional cool features. The development process really is a never ending adventure, a quest to explore new sonic territory and bring something new, weird and wonderful back, such as a pedal that might just capture the imagination of a musician and inspire them in some small way.

If you’re a Phaseomatic owner you might be interested to know that twenty model PM-1A pedals were built in 2002, fifty PM-2As in 2006, six PM-2Bs in 2008 and one-hundred PM-2Cs in 2009.

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Phaseomatic In-depth