"Classic 70's phaser with improved control options"




The GT-100 phaser rack effect module compares to the MXR Phase 100, a great 70's phaser that was recently reissued.  Keith used one on Some Girls, one of our favorite Stones albums.



The GT-100 module compares to the MXR Phase 100 phaser pedal.  The original pedal was available in the early 70's and a good-sounding re-issue is currently available again. 

The re-issue adds an LED, so it is easy to tell them from the older vintage versions.  The re-issue version of the pedal has a number of changes.  The LFO section that controls the depth and speed is different.  The original circuit has 10 phaser stages (so does the reissue), but it also had input and output stages that do some tone-shaping.  The re-issue doesn't appear to have both the input and output stages, only one of them, and that one is implemented differently.  There are also some part differences in the reissue circuit.  The new ones have plastic jacks and speed and intensity controls, unlike the metal ones of the originals.  All in all, it may still sound good, very similar to the original, but cost-cutting changes are enough that the originals will probably still be in use when many of the reissues have to be tossed out due to broken plastic parts, which are soldered directly to the board.  The metal box itself is very heavy, one of the best I've seen in current pedals, so the cheap plastic parts seem extra disappointing.  Anyway, due to all the changes, our module will still be comparable in sound to the reissue to the extent that the reissue sounds like the original.  Our module compares much more closely to the originals.  The rest of our discussion about this circuit concerns the original script and block versions, not the reissue.  We do not offer the reissue circuit.

This phaser has a whopping 10 stages, as opposed to the more common 4 or 6 stage phasers, and a few 8 stage phasers.  Four of the stages are fixed, while six are variable.  The controls have always been the same.  There is a simple Speed control that adjusts the rate of the phaser.  An Intensity control allows selection of one of 4 different pre-set modes of depth/intensity of the phasing.

With its 10 stages, this phaser produces excellent phasing.  It was used by Keith on the Some Girls album, perhaps notably for Beast of Burden.  The original was undoubtedly used by many artists in the 70's and 80's, and the pedal remains popular today.  Gary Hoey has used the re-issue to get his killer metal/surf tone.  If you aren't familiar with his playing, take a break from reading this and go find him on YouTube right now.  He destroys Wipe Out!  Go on now, we'll wait...

Now for the confusing part.  The vintage pedals were released first in "Script" and then later "Block" versions, named after the type of font used in the pedal graphics.  But those designations do not reliably identify the version of the circuit inside.  The circuit had only minor changes over time.  Some changes are different brand/model of op amps, types of capacitors and resistors, and values of a few resistors.  The different op amps are all very similar.  It is doubtful that one could hear the difference between the different capacitor and resistor types.  And the resistor value differences are so small that they can be within each other's spec tolerances, meaning that in reality they are more or less the same value range in randomly selected resistors of either value.  There were also some minor variations in parts that are not in the audio path - those really don't matter.  The resulting pedals sometime do sound very slightly different.  Those differences don't match up nicely by "script" vs "block", production year, board version, etc.  So if you want our module to sound like a particular vintage Phase 100, you'll have to look inside that pedal to see exactly what's in there.  It's easy to do if you know what to look for, since the parts are easy to see and access.

We offer two default configurations, which we call "script" and "block", just 'cause.  While the circuits had these very minor variations, our two configs are typical of the earliest script version and the latest block version, which would also represent the biggest overall differences between the various versions.  Clear as mud?  Unless you are really into trying to match a specific old Phase 100, it's not really worth worrying about the differences.  Crudely, they are probably 95%+ the same, regardless of the minor differences.  But if you know you generally prefer the early script sound or the later block sound, then you can choose your config based on that.  Otherwise, just eeney, meeney, miney, moe it.  Or choose options that give you both configs!

Overall, this is a great phaser.  It is as quiet as you'll likely get in a phaser.  It doesn't color the tone too much.  And the full ranges of all the controls produce "useful" phasing tones, as opposed to video game sound effects from the 80's.  Well, OK, if you get the Faster Max Speed option, you'll probably have some arcade sounds at those extra fast speeds, but other than that, everything is very reasonable.  The differences between the early and later model feedback and phasing resistors isn't huge, but definitely there.  With Feedback and Depth both cranked, you can get some pretty deep and semi-spacey warble going.  Mostly, this is a nice, quiet, well-behaved phaser that should stack well with other effects.

The default GT-100 controls are:

  • Speed control
  • 2 Mode switches.  The four combinations of settings of these two switches provide the same four modes available on the original pedal:
    • Mode 1 - Feedback Switch Off, Depth Switch On
    • Mode 2 - Feedback Switch On, Depth Switch On
    • Mode 3 - Feedback Switch Off, Depth Switch Off
    • Mode 4 - Feedback Switch On, Depth Switch Off

If you want to consider some options without getting things to complicated, consider the following config.  This config fits into a 1.5" module and gives you considerable flexibility:

  • Speed control
  • Depth control
  • 3-way Feedback Switch
  • Switched Phasing Resistors
  • External Center Frequency control

The Phase 100 circuit uses 3 double vactrols to control 6 of the phasing stages.  A vactrol is an optical device that consists of and LED with an LDR (light dependent resistor), or in the case of a double vactrol, two LDRs, sealed in a dark case.  The resistance varies with the level of light.  For some reason, the price of these parts has jumped considerably in the past couple of years, even though they are still in limited production.  Complicating this is the fact that the vactrols must be matched to get good phasing.  Vactrols have a wide range of values, so it takes many vactrols to find a set of 3 matched double vactrols.  It requires matching all 6 of the LDRs to within a very small percentage of difference.  It might take 50-100 vactrols to find one or two sets of 6 reasonably matched parts.  At about $10-15 for each vactrol, plus the considerable amount of time it takes to match them, the vactrols contribute a significant amount to the cost of this circuit.  The vactrols are a key part of what makes this circuit sound the way it does, so there isn't a good alternative that would help lower the cost.  



  • Script - compares to the earliest "script" versions of the original MXR Phase 100
  • Block - compares to the later "block" versions of the original MXR Phase 100







Several of these are technical options for those that want very specific versions of the parts that varied over the years.  If you are looking at these, we think you'll understand from the names what parts we are refering to.  If not, just drop us a line.

  • Feedback Resistor - 15K or 18K.  The 15K resistor allows slightly more feedback than the 18K.  Script Config default is 15K.  Or choose some other value for more/less feedback.  Block Config default is 18K.
  • Switchable Feedback Resistor - We can replace the mode selector Feedback Switch (normally on/off) with a 3-way switch that will give you off, 15K, or 18K.  Regardless of Script or Block configuration, in some modes there is no feedback.  It is either on (at Script or Block value) or off.
  • Feedback Control - The Feedback Switch turns feedback on and off in the four Intensity modes.  In modes 2 and 4, the feedback is on and the level is determined by the feedback resistor (15K or 18K).  In modes 1 and 3, there is no feedback connection and the resistor is unused.  We can instead replace the resistor with a control that lets you adjust the level of feedback when the Feedback Switch is on.  The levels of feedback that would be provided by both the 15K and 18K feedback resistors are in the range of this new control, so you don't lose anything.  You just gain a range of additional feedback levels.  The range would be between about 5K and 55K feedback resistor values.  The settings between 5K and 15K/18K provide higher levels of feedback, and settings from 15K/18K to 55K provide lower levels of feedback.
  • Phasing Resistors - In 4 of the 10 phasing stages, the phasing resistors were either 20K or 22K.  The very earliest Script versions seem to be the only ones that used 22K.  Later Script and Block versions used 20K.  Script Config default is 22K.  Block config default is 20K.  
  • Switchable Phasing Resistors - We can add a switch to change between the 22K early script phasing resistors and the 20K later script/block resistors.  Lots of switching going on here, resulting in a bit higher price than most switching options.  You may or may not be able to hear a difference between the two settings.
  • Custom Op Amp - Have seen 4558C, TL022 variants, and quad TL044 variants.  TL022 seems to be much more common and is used by default
  • Carbon Comp Resistors - Some of the early Script versions used some carbon comp resistors.  These give a little "good" noise and old-school mojo that some people like.  But in a phaser you may not want that.  We use quiet metal film resistors by default.  Note that carbon comp resistors have a 10% spec tolerance.  This could nullify the results of the switchable phasing resistors option - just like in the original units.  You may or may not be able to tell any difference between the resistor values settings with carbon comp resistors.  Our default metal resistors have 1% tolerance, so the difference in resistor values would be there, although you still may or may not be able to hear the difference.
  • Faster Max Speed - We can replace one resistor which will increase the max speed modestly.  It also increases the minimum speed, but not enough to be noticeable.
  • Depth Control - Replace the Depth switch with a control.  The two extreme settings of the control give you the two original switch settings.  You get a range of "in between" depths you can use.  Not sure why the originals didn't do this - simplicity, perhaps.
  • Mix Control (50-50) - Vary the mix of wet and dry signal.  Center of the control gives max notch depth.  Good for dialing down the effect for bass guitars.
  • Mix Control (Wet Only) - Another style of mix where the zero position of the control is the normal 50-50 mix.  Adjusting the control reduces only the wet path signal.
  • Noise Cap - Adds a small cap at the end of the wet path to tame high-frequency noise.  This is a pretty quiet phaser, so this probably won't be needed unless you have a noise problem already.
  • Switched Noise Cap - Add noise cap but with a switch to turn it on/off.  Probably not needed - see above.
  • External Center Control - Move Center Frequency trim pot to external control.  Sort of a Tone control for the phasing.

Note that with the Switchable Feedback Resistor (or Feedback Control) and the Switchable Phasing Resistors, you can have about 98-99% of both the Script and Block configs in one module.


Front Panel

  • Speed
  • Feedback Switch
  • Depth Switch
  • (optional) Feedback Resistor Switch
  • (optional) Feedback control
  • (optional) Phasing Resistors Switch
  • (optional) Depth control
  • (optional) Mix control
  • (optional) Noise Cap Switch
  • (optional) External Center control


Rear Panel

  • Audio In
  • Audio Out
  • On/Off Footswitch
  • On/Off Override
  • DC Power
  • DC Power LED


Module Width

  • 1.5" standard 
  • 1.5" or 2" with options
  • 2" left wing and 1.5" right wing available for some configs


Power Consumption (aprox)

10 - 15mA


Base Configurations

Part # Description List Price
MOD-GT100-SCRIPT GT-100 Module, Script configuration $259
MOD-GT100-BLOCK GT-100 Module, Block configuration $259



Part # Description List Price
MOPT-GT100-FDBK Feedback Resistor $0
MOPT-GT100-FDBKSW Switchable Feedback Resistors $0
MOPT-GT100-FDBKPOT Feedback Control $0
MOPT-GT100-PHRES Phasing Resistors $0
MOPT-GT100-PHRESSW Switchable Phasing Resistors $29
MOPT-GT100-OPAMP Custom Op Amp $0 - TBD
MOPT-GT100-CCRES Carbon Comp Resistors $15
MOPT-GT100-FASTER Faster Max Speed $0
MOPT-GT100-DEPTH Depth Control $19
MOPT-GT100-MIX5050 Mix Control (50-50) $19
MOPT-GT100-MIXWET Mix Control (Wet Only) $19
MOPT-GT100-NOISE Noise Cap $0
MOPT-GT100-NOISESW Switched Noise Cap $19
MOPT-GT100-CENTER External Center Control $19



Part # Description List Price
MPT-GT100-CA Replacement pot, switch, jack, LED cable assembly $19
MPT-GT100-KNB Replacement control knob $2
MPT-GT100-PB Replacement power board $29
MPT-GT100-SB Replacement switching board $29