These were Randall's first attempt at a Mesa Boogie Rectifier module. There have been a number of variants since the original Recto module, each called something different:
- Treadplate; and
- Grail (George Lynch signature module version).
SacredGroove in this thread gives some specific information on the component differences:
- This is what I've noticed:
Recto--C3/.0047uf, C14/470pf, R1-3/470ohm
Rectified--C3/.0022, C14/470pf, R1-3/470ohm
Treadplate--C3/.0022, C14/300pf, R1-3/150ohm
A lot of people change C3 to .001 or .0015 for tighter bass attack.
Grail--C3/.0005uf, C14/470pf, R1-3/150ohm
OKStrat had this to say about the effect of these component mods:
- Changing R1-R3 to a lower value cuts gain a little bit. Dropping the value of C3 tightens up the low end but too much can affect your 'thump' also. It lowers the gain slightly on the low end, which helps keep the module from sounding too tubby/flubby.
"Original" Recto module
I've got what must be a fairly early example of the original Recto module. It has the chrome plain face plate and an "Eden Electronics, Proudly made in the USA" sticker on the bottom. The Key tone shaping component values are as follows:
C3/.001uf, C14/470pf, R1-3/150Ohm
The module does not look like it has had any of these values changed.
Component summary table
- V1 - R16 - 1.8MOhm
- V2 - R29 - 1.8MOhm
For info on cathode resistors see www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers.
Originally 0.0022uF, I've tried 0.0015uF and 0.001uF and prefer the 0.001uF the best.
I've also tried various values in the C14 position. 470pF sounds good, 300pF sounds better, 250pF results in a noticeable drop in gain which is not great.
The Recto module really likes the Chinese (Shuguang) 12AX7's. The gain and bite from these tubes is exactly what's needed to bring these modules alive.
More like the real thing
The defining characteristic of the Mesa Boogie Rectifier preamp is the V2B gain stage which is extremely cold biased by a 39kOhm cathode resistor (resulting in very asymmetrical clipping). Also, in later models a 220 to 800pF of high pass cap around the plate resistor (to bleed off some of the highs) and a 470pF cap to bypass high to mids around the interstage attenuator resistor were added.
If you take a look at the Randall Recto etc modules, its pretty clear that there are some components you can change to make the circuit closer to that of a real recto. See the pic below for the components I changed out on mine. I'm pretty happy with the results. Components with values in bold were changed to the values in bold. Cap CN1 was also added across R21.
Note: Mesa Boogie amps are renowned for having their tone circuits after the first gain stage. The Randall MTS modules do not copy this aspect of the Boogie amps. The Randall MTS tone stack is in the usual Marshall/Fender position at the end of gain stages. As a result, values that affect frequency (like C8, CN1 and C11) may have produce different end results, so often directly copying a portion of another amps preamp circuit will not necessarily produce the same sounding results.
Warpedmusician has come up with some interesting ideas on changing the values of the cathode resistor in the Rectifier's cold clipping stage - warpedmusician.wordpress.com/2016/01/17/dual-rectifier-cold-clipping-mods/#more-3197.