GTM-45

"MOSFET Simulation of Marshall JTM-45 Amp Tone"

 

 

The GTM-45 amp sim rack effect module compares to the ZVEX Box of Rock and JHS Charlie Brown.  In either configuration it is intended to provide tube-y Marshall JTM-45 tone.

 

Description

Nothing but a good tube amp really sounds like a good tube amp, but MOSFET technology can provide tube-y tones from a solid state circuit.  The heart of this effect is three stacked MOSFET gain stages in a row with some tone control added to the end.  Just in the off chance that doesn't provide enough gain, the whole thing is topped off with a switchable MOSFET boost.  Dial it all back, and you have a clean-ish boost.  Crank it all the way up, and you'll almost certainly go into gain overload.  There is a range of Marshall gain tones in between.  While you can certainly crank the gain past usable levels, this circuit sounds better to us set in the range of classic rock gain.  With three gain stages and a boost cranked up, the circuit can get noisy, just like a tube amp does.  We've changed it up to cut back on the noise substantially and added options that include additional tone shaping.  The two configurations we offer have some small differences in tone shaping through the circuit, so you can choose whichever variant you prefer.

With very high gain, there is always the possibility of noise and squealing.  There are noise reduction options, which should be considered, especially with the Boost option.  There is a trimmer on the board which adjusts the maximum gain level.  The default setting is probably on the high side.  If you experience squealing with Gain, Volume, Tone, and/or Boost cranked up, then you've exceeded the max gain.  Just turn something down a bit until you get back to a gain level that works.  If this happens too early on your controls, then you can adjust the trimmer to lower the gain.  Turning the trimmer down won't appear to impact the gain much, but it can dramatically reduce noise and squealing and it affects the Volume.  The trimmer is touchy - most of the good settings are within a small range of adjustment.

 

Configurations

  • GTM-45 BOR - Compares to ZVEX Box Of Rock.  Includes footswitch-controlled Boost.

  • GTM-45 CB - Compares to JHS Charlie Brown.  Boost is not included, but is an option.

 

Option/Feature GTM-45 BOR GTM-45 CB
Standard Controls Volume, Gain, Tone, Presence Volume, Gain, Tone, Presence
Bass/Mids/Treble instead of Tone, Presence option option
Footswitch-controlled Boost included option
Toggle switch Boost option option
No Boost option standard
Remove Noise Filtering option option
Hi Cut, Extreme Hi Cut (no switch) option option
Hi Cut, Extreme Hi Cut (switched) option option
Buffers none none

 

Options

  • Bass / Mids / Treble Tone stack - Replace standard Tone and Presence controls with Bass, Mids, and Treble controls
  • Remove Noise Filtering - By default, extreme treble and noise is filtered out as your signal makes its way through the circuit.  If you do NOT want that done and prefer to be more like the Compares To pedals, we can leave it out.  Any extreme highs and noise that would be filtered out will remain.  If you don't run high-gain, or you have a quiet setup, this may help you retain high-treble sparkle.  If you run high-gain, the default noise filtering will help keep noise and squealing under control.
  • Hi Cut Switch, Extreme Hi Cut Switch - These provide a high-cut at the very end of the circuit to chop out unwanted treble and noise.  They can have a substantial impact on the highs.  Both versions of the high-cut work at the same frequencies, but the Extreme Hi Cut does it twice, reducing the impacted frequencies even more.  If you get the Extreme Hi Cut, there are two cables from the switch to the board.  If you unplug one of the cables, you get the Hi Cut instead.  With our default noise filtering in place, these options won't have as big of an impact, but they do cut more frequencies than the default noise filtering.  If you elect the Increased Noise option (no default noise filtering), these options will have a bigger effect.
  • Hi Cut, Extreme Hi Cut - This are unswitched, always-on, versions of the Hi Cut and Extreme Hi Cut Switch option.
  • Add Boost - Add the footswitch-controlled Boost to the CB configuration
  • Add Toggle Boost - Add the Boost, controlled by a front panel toggle switch
  • Remove Boost - Remove the footswitch-controlled Boost from the BOR configuration
  • Toggle Boost - Change default BOR footswitch-controlled Boost to toggle switch control on front panel.
  • Components - Metal film resistors are used by default to reduce noise.  A mixture of capacitor types is also selected to reduce noise.

 

Front Panel

  • Volume / Level
  • Gain
  • Tone
  • Presence
  • (optional) Bass
  • (optional) Mids
  • (optional) Treble
  • (optional) Boost - Boost level control.  Always included if Boost is included.
  • (optional) Hi Cut, Extreme Hi Cut switch
  • (optional) Boost switch

 

Rear Panel

  • Audio In
  • Audio Out
  • On/Off Footswitch
  • On/Off Override
  • DC Power
  • DC Power LED
  • (optional) Boost Footswitch
  • (optional) Boost Override

 

Module Width

  • 1.5" for some configurations such as standard CB
  • 2" standard for BOR configuration, any configuration with options or Boost footswitch
  • 2" left wing available for standard (4) control, no Boost footswitch configurations
  • 1.5" right wing available for standard (4) control, no Boost footswitch configurations

 

Power Consumption (aprox)

10 - 20mA

 

Base Configurations

Part # Description List Price
MOD-GTM45-BOR GTM-45 BOR module $229
MOD-GTM45-CB GTM-45 CB module $199

 

Options

Part # Description List Price
MOPT-GTM45-BMT Bass, Mids, Treble controls $35
MOPT-GTM45-NNF No Noise Filtering $0
MOPT-GTM45-HCS Hi Cut Switch $19
MOPT-GTM45-EHCS Extreme Hi Cut Switch $19
MOPT-GTM45-HCNS Hi Cut (no switch) $0
MOPT-GTM45-EHCNS Extreme Hi Cut (no switch) $0
MOPT-GTM45-BFS Add Boost (footswitch) $29
MOPT-GTM45-NBFS Remove Boost (footswitch) -$29
MOPT-GTM45-BTS Add Boost (toggle switch)  $19
MOPT-GTM45-BFS2T Change Boost from footswitch to toggle switch -$9
MOPT-GTM45-CC Component Changes TBD

 

Parts

Part # Description List Price
MPT-GTM45-CA Replacement pot, switch, jack, LED cable assembly $19
MPT-GTM45-KNB Replacement control knob $2
MPT-GTM45-PB Replacement power board $29
MPT-GTM45-SB Replacement switching board $29