GT-90

The first effect released by MXR and still a favorite phaser since 1974

 

 

The GT-90 phaser rack effect module compares to the classic MXR Phase 90.  Block and script versions available.

 

Description

The GT-90 provides the classic sounds of the MXR Phase 90, made forever famous with the release of the first Van Halen album.  There have been several versions of the Phase 90 released over the years.  Most players seem to prefer the original script version the best, as compared to the block version that came after it.  The script version is often described as smoother and less aggressive than the block version, and without the slight mids boost of the block version.

For the GT-90 we wanted to offer both versions.  The problem is that there are quite a few differences between them.  It is impractical to try to fit both on the same board.  We decided to focus on the script version, and offer options that would provide about 95% of the block version sound.  There are also some popular mods we wanted to support.  It ended up a little complicated.

First, we lumped the differences between the script and block versions into 3 parts.  The first is the feedback loop.  The script version doesn't have one, the block versions does.  The second is a group of component changes that have a significant impact on the tone in the two different versions.  The third group contains changes that have no audible impact.  We ignored the third category of differences, and just used the script versions of those differences.  For the second group, we provide a Block/Script switch that changes between the script and block values for those components.  For the Feedback loop, we added a switch option to use or not use the feedback loop.  The result is that you can get either the Script or Block config, add the Feedback and Block/Script switches, and switch between the Block and Script versions.  Both versions sound like their originals, even though the circuits are off by a little bit.

 

Configurations

  • GT-90 Script - compares to original script MXR Phase 90

  • GT-90 Block - compares to early block MXR Phase 90

Here is a summary of the differences between the two configurations, emphasizing the differences that will remain if you switch between the two configurations with the Feedback and Block/Script switches.  Your configuration will sound very close to the original pedal of the same configuration.  If you put in the switching options and use them to switch to the other configuration, the switched configuration will sound "close or 90%" like an original pedal of the other configuration

Feature Script Block Note
Feedback Loop no yes use Feedback Switch option to switch it on/off
Phasing Resistors 22K 24K minor difference, not switched with Script/Block switch
Op Amp (6) UA741 (6) TL061 Block normally uses (3) dual op amps but we use the individually packaged version insead, difference betwee UA741 and TL061 is small, not switched with Script/Block switch
Pull Down Resistor 470K 1M minor difference, not switched with Script/Block switch
Internal Power Configuration unimportant unimportant there is an internal power circuit that is configured differently to achieve the same results in the two versions.  It has no audible impact and is not switched with the Script/Block switch
Several small component changes important important These affect the tones of the two versions and are switched with the Script/Block switch.

 

Options

  • Block/Script Style Switch - This switches the components that have a significant difference between the two configurations.  This includes what is sometimes called the "C11 and C12 mod", and a change in the gain structure.  Some have reported that removing the C11 and C12 connections in a block version to get the script tone left them with an increased noise level.  That problem is also addressed with this switch.  Regardless of your base configuration, if you choose this option then the switchable components of the other configuration will be added so that you can switch between them.
  • Feedback Switch - If you have the Block configuration and want the Script configuration, then the first thing to do is remove the feedback loop.  Or if you have the Script configuration and want the Block configuration, then you have to add the feedback loop.  This option does that.  If you start with the script config, this option will add the feedback loop as well as the switch.  This is similar to the "R28 mod", but done better.
  • Resonance Control - In addition to just turning the Feedback on or off, you can also choose to have an adjustable level of feedback.  The midpoint of this control will be the standard block feedback level.  If you have the Script configuration you must choose the Feedback Switch Option to get the Feedback loop added.  The Resonance Control won't do anything if you have the Feedback loop switched off.
  • Bass Mod - Change some components to allow more bass through the circuit.  The difference is subtle for guitar, as most of the bass is already going through the circuit.  Don't expect a big difference.  However, if you find the stock P-90 sound is a little bit thin, this may help a bit.
  • Bass Switch - Same as the Bass Mod, but on a switch so you can turn it on/off.
  • Custom Components - We use metal film resistors and a selection of capacitors that provide good tone and low noise.  The originals used a mixture of carbon film and carbon comp resistors, probably whatever they had on hand.  We can put in carbon comp resistors if you prefer them.

 

Front Panel

  • On/Off indicator LED
  • Rate
  • (optional) Block/Script Style switch
  • (optional) Feedback switch
  • (optional) Resonance control
  • (optional) Bass switch

 

Rear Panel

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

 

Module Width

  • 1.5" standard
  • 2" left wing and 1.5" right wing available for 1-4 control configurations

 

Power Consumption (aprox)

13 - 20mA

 

Base Configurations

Part # Description List Price
MOD-GT90-SCRIPT GT-90 Script module $250
MOD-GT90-BLOCK GT-90 Block module $250

 

Options

Part # Description List Price
MOPT-GT90-OPAMP Block/Script Style Switch $19
MOPT-GT90-TRANS Feedback Switch $19
MOPT-GT90-CLIP Resonance Control $19
MOPT-GT90-SWCLIP Bass Mod $0
MOPT-GT90-GAIN Bass Switch $19
MOPT-GT90-CC Custom Components TBD