Randall MTS Ultra Module


When I got the module (new) it was clear it had some sort of issue.  It would get loud and quiet by itself and generally mis-behaved.  After some exploration and diagnosis, I tracked the issue down to a bad Bass pot.  Replacing this pot (it's an Alpha A1M) fixed the problem.  Evidently, these modules did suffer from cracked pots on occasion.  Pot replacement requires pretty much the complete disassembly of the module down the PCB as follows:

  • remove module from unit;
  • remove two screws that hold on tube shield and remove shield;
  • remove all knobs;
  • remove all nuts and washers that hold the pots to the face plate;
  • remove remaining 4 screws that hold PCB to faceplate/PCB support;
  • un-solder broken pot - there is solder on both sides of the PCB holding the pot in place, so its not easy to unsolder without a good soldering iron and solder sucker.  Take your time and don't force the issue.


Disclaimer: I'm playing this module through my RM20B combo which has a 12" Vintage Celestion Greenback.  I also pay the combo through a Mesa Road King 2x12 horizontal cabinet which lets me choose between a closed back Vintage 30 Celestion or Mesa's own Celestion Custom 90 in the open back side (or both together).

The Ultra gives a great high-gain sound.  It's got a tight, controlled bottom end, palm mutes are powerful without flubber and more complicated chord shapes retain their musicality.


Selection of 12AX7 tubes for tone experimentation, including Mesa, Sovtek, Tung-Sol, JJ and Ei

I've managed to try a variety of 12AX7's in the V1 position of this module.  So far, my clear favourite has been some old Mesa Boogie tubes that I pulled from my V-Twin rackmount pre-amp.  These tubes were anything less than inspiring in the V-Twin, but they give a really nice 'tight' sound in the Ultra Module V1 position.

Ultra module mods

C3 cap mod

Replaced the standard 0.0022uF C3 cap with a 0.001uF cap which tightens the bottom end and improves the pick attack for high gain use.  Also added a DPDT On-Off-On switch with additional 0.0015uF and 0.0022uF caps to give 0.0025uF and 0.0032uF options.  The 0.0032uF option fattens the bass up greatly and works well for lower gain usage. 

Tone mod - slope resistor and mid-shift cap

Matam on the mtsforum posted up some really helpful DIY mods for "Brit" based modules (like the Ultra) - mtsforum.grailtone.com/viewtopic.php?t=15466

This mod adds a switch enabling two combinations of slope resistor/mid-shift cap in the tone stack to be selected. Matam's mod switches between a combo of 56K 250pF and 33K 500pF.  On my version of the the Ultra , these components are R25 and C14.  Their stock values are 33k Ohm and 180pF.

I've added a DPDT On-Off-On switch which has enabled me to wire up the following tone stack options:

  • original Ultra 33k Ohm / 180pF;
  • 56k Ohm / 180pF; and
  • 56k Ohm / 510pF.

Gain lowering experiment

Adding a 100k Ohm resistor to R7 in an attempt to get a lower gain option on the Ultra did not really work.  I might try some lower value resistors at some future date.

1086 hybrid

I really dig the tightness of the 1086 module. For this reason, I've looked at several ways to implement some of the best characteristics of the 1086 on the Ultra module.  Here are some critical component differences:

R162.7k Ohm
4.7k Ohm
R23No part
47k Ohm
R24820 Ohm
1k Ohm
No part
c7No part

To hear the difference these component swaps will make, I want to put them on a toggle so I can switch between them.  Here's what I did:

  • used a DPDT ON-ON switch;
  • removed R16 and replaced with 4.7k Ohm part;
  • attached flying leads to either end of R16 so switch can add a 6.8k Ohm resistor in parallel with the new 4.7k Ohm R16 giving a total resistance of 2.77k Ohm when the switch is in "stock" mode;
  • removed C6 from the board (1uF 50v electrolytic);
  • soldered the (-) ends of a 1uF 50V electrolytic to one side of the DPDT switch and a 0.47uF 50V electrolytic cap to the other side of the DPDT;
  • added wires from C6 and C7 (+) to the appropriate (+) terminals of the caps on the DPDT;
  • added a ground wire from the GND end of R16;
  • replace R24 with 1k Ohm resistor;
  • attached wire from non ground end of R24 to switch via a 4.7k Ohm resistor.  Adding this in parallel with R24 drops it back to around 820 Ohm total resistance;

The effect of this mod is that with the switch in one position we have the stock Ultra setup of about 2.7k Ohm in R16, 820 Ohm in R24, a 1uF cap in C6 and no cap in C7.  With the switch in the other direction, we have a 4.7k Ohm resistor in R16, 1k Ohm in R24, no cap in C6 and a 0.47uF cap in C7.

Changing R24 from 820 Ohm to 1k Ohm takes a lot of gain and dynamics out.  Ended up reversing this mod.

Also attempted adding a 3.3k Ohm resistor in parallel when in 1086 mode.  This drops the resistance of R24 to around 770 Ohm.

Component types

C5aux treble cap (added in parallel to C14 when SW2 is installed and in left position).  Cap and switch Not present on Ultra.
C6cathode bypass cap
C7cathode bypass cap
C13cathode bypass cap
C14treble cap
R18link - in place when SW2 is not used
R25tone stack slope resistor

Related articles:

Randall MTS Recto Module

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Randall MTS Treadplate module

This is basically a later version of the Recto module with a few component changes.  Here's a summary of value differences.

Randall MTS Top Boost module

This module is Randall's take on a boosted Vox AC30TB.

Randall MTS Brahma module

Randall described the module as follows:

Randall MTS Modern module

Apparently the Modern is higher gain version of the Top Boost module.

Randall RM20B

Tube config