Randall MTS Module "The Cleaner"

Module was originally a Blackface which I have extensively modified to give me what I want/need out of a clean module.  With the gain turned right down and the level turned up its super clean and incredibly touch sensitive.  An amazing sounding combination with 6550 output tubes and GT75 speakers.  Crank the gain past about 5 and things start to get a little dirty, but the touch sensitivity and dynamics are still there in spades.  A flick of the grime switch will change the led from green to red and things start to get interesting.  Still clean and chimey played softly, but dig in with the pick and there's "grime" to be had.

• grime switch provides a slight gain boost along with some subtle, musical distortion - designed to give the clean channel a similar boost to putting a tube screamer in front of the amp and some subtle clipping.  A bit of grime, but even with the gain at 10, not super distorted and compressed;
• fat pull pot on the mid pot changes the slope resistor and tone cap to values that fatten up my humbucker pickups when split and run in single coil mode whilst still retaining that characteristic single coil spank;
• scrub pull pot on the gain tightens things up for solos and cutting through the mix.  Keeps the bottom end tight.

Design philosophies

• this is the touch sensitive, chimey but rounded and powerful clean sound I wanted.  Its not a "replica" of another amp's clean channel;
• the led should serve as a visual indicator in low light conditions when on stage;
• module has to sound good with all controls at 5/10.  This way, there's truckloads of adjustment available.  Sure, turning the knobs will reward you with a palette of tones and variety, but the base starting point needs to be good right off the bat, otherwise you're likely to run out of adjustment/flexibility in certain situations;
• there should be minimal clicking or popping etc. when flicking the various switches/options (although this is some times unavoidable);
• changing between the options should not result in a huge difference in output volume;
• layout/additional circuitry needs to be neat and tidy, without creating noise, reliability or maintenance issues.  Tube replacement needs to be easy, so nothing cluttering the space needed in an around the tubes.

Implementation

Grime switch

Changes Rc for V1A and V2B from 1k8Ω to 2k7Ω and 820Ω respectively as well as adding in a 22uF for V2B Cc.

Fat push-pull

Slope resistor changes from 100k to 33kΩ and treble cap goes from 270pF to 470pf. Mid pot is a B25k.

Scrub push-pull

Coupling cap goes from 0.022uF orange drop to 0.0044uF Cornel-Dublier polyester caps.  Gain pot is an A500K.

Misc stuff

A smattering of 1/2W carbon comps where they matter most (plate resistors).

Here's an in-progress gut-shot while I will still fine-tuning the final component values:

Tubes

V1 - JJ ECC803S - this is a long plate, fairly high gain 12AX7.  Sounds good in position V1 in The Cleaner.  Suposedly, the long plate designs are more susceptible to microphonics (esp. in combo amps).  I haven't experienced this but its something to listen out for.  Other good choices are the Mullard re-issue C4004 or the JJ 5751 for slightly lower gain and greater clean headroom.  I'm looking forward to building a second identical module for some proper A/B tube rolling testing.

V2 - JJ ECC803S sounds good here too, but not sure on longevity because of the special requirements of V2 in the randall modules.  The Shuguang 12AX7s are super robust and I've currently got one of these in V2.

Another is born - The Cleaner #2

I've made a copy of The Cleaner for the awesome Mattfig at Motor City Mods.  Just putting it through its final paces now (its the one on the right without the faceplate)...

Apart from the circuitry changes, I've added some information to the module that should be helpful for anyone using it.  Firstly, there's some suggested settings based on my personal favourites along with a set of blank settings so you can keep a record of your own (there's also a larger version of this on a card so you can review/record when the module is in the amp):

There's also a sticker on the tube shield which gives an idea of the tube combinations I've tested with the module and recommend.  The ones listed for V1 are in descending order of gain (I should really say that on the sticker!):

Doing the second one gave me a pretty good idea of the amount of work involved.  4 out of the 5 pots are replaced and 20 other components are changed out with another half dozen or so extra components being added.  Many hours of painstaking work is required to build one of these.

The Cleaner #3

The third (and probably final) Cleaner is taking shape.  Please excuse the nakedness.

Sound samples

I've put together a quick selection of sound samples to give a little idea of what it sounds like.  Amp is RM80 with 6550's and GT75's. Recorded with SM57 mic through a Scarlett 18i8 and into Reaper DAW. No effects or EQ applied. Saved as mp3 128kbit.