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Leslie 2-speed conversion – MTCLogic


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MTCLogic1 MTCLogic1


3 versions:
MV-22: for 6H 22 type Leslie
MV-47: for 6W 47 type Leslie
MV-PP: Plug-n-Play version

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Product Description

So you have a single speed Leslie and would like to have 2-speeds. You have a choice of finding used 2-speed motors and fitting them in, or using an electronic 2-speed controller. 2-speed motors might be more authentic but they are not easy to retro-fit into cabinets that were not designed for them. Certain common modifications can alter the tone of the Leslie adversely. To prevent this requires cabinet skills beyond the average hobbyists.

Even if you could accomplish this feat, you still have to deal with the adjustments and parts that wear associated with 2-speed motor stacks.

Enter the MTCLogic 2-speed controller…

Typical speed controllers use a method of varying the duty cycle of the AC power to control speed. Current State-of-the-art commercial controllers use a technique of varying the frequency and voltage for speed control. Since we are forcing motors to run outside their normal range, it requires carefully varying the voltage as we change the frequency to keep the motors running smooth and from over heating. Our controller does exactly that. Using a variable Frequency and Voltage we are able to accurately control the motors while minimizing the harsh side-effects normally found with typical speed controllers.

Our controller not only uses this state-of-the-art speed control method, the heart of our controller is a computer! A CPU running a program designed to give optimal performance. All functions and controls are contained in this firmware.

  • Convert older single speed Leslie’s to 2-speed using original motors.
  • Replaces old mechanical relay.
  • Adds stop control with or without brake function.
  • Uses Industry Standard Motor Drive Technology.
  • New technology means quiet operation and motors that do not overheat.
  • Can even work on 2-speed models eliminating the slow motors and the tedious adjustments and wear.
  • Leslie’s that are single speed; 21H, 22, 22H, 22R, 31H, 44W, 45, 46, 47, 51.

This control board is mounted in the Leslie amp replacing the mechanical relay and giving 2-speed operation to single speed Leslie’s. Using Pulse Width Modulation (PWM) to change both frequency and voltage to add slow speeds to single motor Leslie’s with smoother operation and cooler running motors. This is the state-of-the-art method used in commercial motor speed control applications.

This is not the generic duty cycle method found on light dimmers and in-expensive motor controls. That method has problems such as low torque, motor heat and noise.

Our method changes the frequency and voltage to optimize the motor current at all speeds. Even though AC motors run best at their designed frequency, they can be made to operate outside that limit and with careful control of frequency and voltage, operate smoothly without excessive heat.

Best of all, our controller is software based! Any and all parameters can be tweaked making future updates possible as well as special customer specific features.
The original version of the RTCLogic controller was a great success and we installed over a dozen in our area some years ago until the product became unavailable. Those units are all still in operation and have never had one problem. I have used one myself for several years and it has been flawless.

Now, redesigned with brand new code, it not only has come back to life, but with improvements as well. So far we have even quieter operation at slow speeds and a better braking action.


When using electronic 2-speed controllers the motors and rotors should be in clean and lubed condition. (Shouldn’t they be anyway?) The belts should be in good condition and properly adjusted. It has also been discovered that by measuring the resistance of each motor, and placing the lower resistance motor on top gives better results in keeping the two rotors running near the same speed.

By the way, you can flip the field coil of the lower motor to cause it to turn in the opposite direction from the upper. This is in keeping with the operation of the 2-speed models which have the lower motor assembly located above the speaker shelf. Many say this sounds better.

So, add the MTCLogic board, flip the lower motor field coil, and you now have a 2-speed Leslie with stop and opposite turning rotors just like the later 2-speed versions. In my opinion, the older models sound better anyway. Having the older version but with 2-speeds is the best of both worlds!


MV22 – Replaces the relay under the chassis for a 22H, 22R.
MV47 – Replaces the relay under the chassis for a 45, 47.
MVPP – Plug-n-Play version requiring no installation.

Note 1: The PnP version does not replace the relay and does not have the stop function.

Note 2: Other single speed Leslie’s (31A, 31W, 31H, 21H, 50C, 51, 51C, 44, 46) are electronically compatible with our controller but have less room under the chassis for normal mounting. The controller can however be mounted under the chassis with some creativity which we can help you with. Otherwise, the PnP version will work on any model.

Special custom programming available on request.


A new software version for all three controllers is now available. It offers a “coast to slow” feature that is selectable by removing the on-board jumper. When selected, it allows the upper and lower rotor to “coast” for 2 seconds before the slow speed takes over. This along with the brake cycle being removed lengthens the slow down time and creates a nice effect.

The updated CPU is available to any MTCLogic owner.

Demo Video

Additional Information


MV22, MV47, MVPP

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