Making the Original Phaseomatic |
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The 1950s saw the introduction of many new models of guitar amplifer, some of which boasted a vibrato effect. Practically all these amps could only produce tremolo which is the modulation of volume level in time, whereas vibrato is modulation of pitch. This effect has been simulated electro-mechanically by physically rotating the sound source (ie the speaker) and later became known as the Leslie effect. Vibrato is a useful and engaging effect because it adds depth and movement to the sound of the guitar. The only manufacturer to successfully incorporate true vibrato into their amplifer range was Magna Electronics (or Magnatone). The effects pedal market is now flooded with solid-state phaser and vibrato effects pedals, however, tube phasing and vibrato remained a relic of the past. 'effectode' wanted to bring that big sound back to life and a working prototype vacuum tube phaser was completed in the summer of 1998. Originally, the design incorporated an optically controlled, eight stage phase-shifter and all tube L.F.O. based on an interesting triangle wave time-base generator, called a Fruhauf oscillator. This prototype phaseomatic (known then as '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. Four tubes were used for the phase-shifter section, three for the L.F.O., one tube for a gain amplifer & buffered output giving a total of eight tubes. The vibrato generated by this unit was extremely intense and perhaps better described as unuseable. An excellent range of vibrato effects can be obtained with just two stages of phase-shifting.
The present incarnation of the phaseomatic tube phaser features a solid-state L.F.O. and four triode phase-shifter stages in series. This configuation 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 optimised 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. Later on, highly 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. A built-in, low-noise power supply eliminates the need for batteries or an external power adaptor. It consists of separate H.T & L.T. transformers for the tube power supply and the heaters respectively. These two transformers are less bulky than a single larger transformer and this is one of the reasons that the phaseomatic is so compact. The H.T. transformer was custom wound by Danbury electronics and allows the vacuum tubes to operate at 300 Volts D.C. Metal-film resistors are used throughout the circuit for reliability and their low-noise characteristics. Polyester capacitors for stability. Super-linear low frequency oscillator gives a smooth phase sweep across the entire audio spectrum. |
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A huge amount of time was invested in performing audio tests on the phaseomatic
tube phaser. Of paramount importance was to ensure that guitar tone
was in no way impaired with the unit connected in series between the guitar and the amplifier.
'effectrode' stand by their products and guarantee that their effects DO NOT suck-tone and are
simply just great sounding 'tone-machines'. Much of the 'on-the-bench' circuit testing & analysis was performed in the
time-domain using a lovely, tube 15Mhz Dual Trace 
