The Musician's Room: The Hughes & Kettner Tube Rotosphere

Background . . . . Physical Info . . . . The Effect . . . . Notes & Hints . . . . Bottom Line

Leslie Model 122
Rotating Speaker System

The Leslie speaker system has contributed to some of the coolest guitar sounds ever: The Beatles, Joe Walsh, Eric Clapton, David Gilmour, Badfinger... all have featured the rich, swirling sound of Leslies on their guitars at one time or another. Since the Tube Rotosphere is designed to simulate the effect of a Leslie speaker system, it would probably be useful to examine a typical Leslie to learn the effect of the various parts upon the signal.

BACKGROUND AND THEORY: As you may know, Leslies are self-powered speaker systems, named after their creator, Don Leslie. The Leslie was designed to not only amplify, but also modify the sound of an organ. The original idea was to help create a "pipe organ" sound by multiplying the frequencies put out by the Hammond organ. The result was something else entirely! The unit's “works” live in a large, three-chambered box. The upper chamber contains a curved, rotating horn, which disperses the output of the tweeter. The middle chamber serves as a speaker baffle, containing the top-firing tweeter which plays into the bottom of the rotating horn and a downward-firing woofer which projects into the top of a “rotor” in the lower chamber. It also contains the motors that turn the horn and rotor and their controls. The bottom chamber contains the amplifier chassis and the bass rotor that disperses the output of the woofer.

Leslie 147 with back removed.
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In their original function as an amplification system for the Hammond line of organs, they were controlled from the console of the organ, with two speeds available from their AC induction motors and a momentary brake available as a function of DC current applied across the motor. Proprietary connectors were used to carry the signal and control voltages between organ and speaker system. Later, as owners desired to play other types of instruments through the Leslies, small combination preamp/control units were developed to allow any instrument with a ¼” jack to be connected.

While the rotors are in motion, the sound is amplitude modulated by the change in direction and proximity of the speaker. It is pitch modulated as the velocity of the speaker’s movement is either added to, or subtracted from the speed of sound propagation (the Doppler effect). The sound is timbre modulated as a function of the motion of the speaker and its frequency-response lobes. And finally, the sound is physically projected around the room by the motion of the speaker. Adding a further complexity to the whole modulation mix, the horn and rotor rotate at different speeds and change speed at different rates because of their different masses and separate motors. For interesting effects, keyboard players have learned to manipulate the “transitional” period between the slower and faster speeds, using the breaker and speed switches to cause the differential ballistics of the two rotors to complicate the modulation. The forty-watt amplifier is really under-powered for rock combo use so the 12AX7 preamp tubes and 6550 output tubes are often driven into gentle distortion, which can add a wonderful, musical growl and compression to the sound as well.

Through the years, there have been many, probably half-hearted, attempts to reproduce the sound of the Leslie electronically. While most of what the Leslie does can be analyzed as empirical data and reproduced via modern electronics, the problem has been that the various effects involved in the Leslie, as well as their interactions, have demanded a fairly large amount of processing. Until recent times, the electronics to mimic them all have been considered prohibitively expensive, given the market for this effect. A typical effect, then, only simulated some of effects and interactions, and the results have been universally disappointing. As of the 1990s, Hughes and Kettner have stepped up to the plate to produce what many consider to be the first really convincing Leslie rotating speaker simulator.

Meet The Hughes and Kettner Tube Rotosphere

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PHYSICAL: The H&K Tube Rotosphere is packaged as a rather large, well-built pedal. The front panel is heavy-gauge steel, the front “tread”, back jack panel, and bottom plate are cast aluminum, and the sides are heavy molded plastic. The bottom is covered with a near full-length rubber pad. Looking in through the window in the top of the pedal one can see a single, H&K-branded 12AX7 tube mounted in a ceramic base and held tightly by a retainer clip. When the unit is in operation, the tube glows gently and reassuringly though the window. Two densely populated circuit boards are also visible through the window, one mounted horizontally and one vertically, with eleven ICs and sixteen transistors visible. The German reputation for efficiency and precision is evident in these cleanly executed boards. The unit is powered by a proprietary, high-current wall-wart power supply and thus can not be powered by typical pedal board supplies.

On the front panel are the "bypass", "breaker", and "slow-fast" switches, the "drive", "output", and "rotor balance" pots, and the "tube sat" (saturation), "effect on", and "slow" LEDs, as well as a pair of LEDs which throb to show the speed of the high and low rotors. You and the band will be treated to a marvelous little light show of winking LEDs. The back panel features a pair each of ¼” input and output jacks for true stereo, a mode switch (guitar-keyboard) a ¼” stereo "remote" jack, and a jack for the wall-wart power supply. The manual explains utilization of the effect in stereo and mono modes well. The only drawbacks I’m aware of are the use of a dedicated wall-wart (normal nine-volters won’t work) and reports of footswitch failures. I bought my 'Sphere used. Guess what? Before the sale, the dealer replaced a failing footswitch. The unit does offer a remote footswitch jack that allows you to switch the speeds and bypass the unit from another footswitch if necessary. Other than the one switch, this particular pedal has held up extremely well. The previous owner fitted up a road case for the unit that may have contributed to its longevity. By the way, because the unit's manual was missing, I downloaded a copy from the Hughes and Kettner website. Thanks, guys!

THE EFFECT: H&K has created an extremely accurate reproduction of the Leslie’s sound using analog circuitry. They’ve given the low and high end their own simulated rotors with differing rates of speed change, just like the real deal. The controls replicate those of the real unit and are simple to manipulate. As most Leslie users can tell you, there is some technique involved in getting the most out of the Leslie speaker. For a little more complexity, you can create the transitional speed effect or just jog the rotors a little so they aren’t tracking together by momentarily punching the breaker switch or toggling the speed switch. On slow speed, while playing stereo through two amps, you can really hear the motion as the simulated bass and treble rotors turn independently. The bass rotor has a strong amplitude modulation effect on the signal, just like the real thing. Some keyboard players have complained that it is a little too strong, but I find it to be perfect for guitar work. Remember also that you can use the rotor balance control to help with that. The 12AX7 tube preamp does a fine job of simulating the growl of the amplifier, and that growl enhances the motion effect as it swirls around. The rear-mounted “guitar-keyboard” switch allows you to change the impedance and EQ of the unit to suit the two different instruments. The LED system allows you to know and set the status of the unit before kicking it in. For that matter, watching the undulating high frequency and low frequency rotor lamps is almost worth the cost of admission.

NOTES & HINTS: I’ve heard people complain that the unit is too noisy. Try to remember that even the very best Leslie is an analog, tube driven, electro-mechanical beast, which creates lots of noise and noises. In fact, some of the distinct character of the Leslie was derived from its flaws. The Rotosphere is a Leslie simulator, a one trick pony. It makes no claim to be anything else, including a chorus pedal. Keeping in mind the nature of the real Leslie can allow you to make the ‘Sphere sound better and more realistic.

The unit is best used in the series effects loop of your guitar amplifier, thus offering the unit enough gain to operate at its best and placing it at the end of the processing chain like the real Leslie was. Placing it in the loop also eliminates most, if not all of the effects of its soft bypass system. Use the “Tube Sat” LED and the “drive” control to set the volume of the unit. For a clean sound, you should be able to see some flickering of the LED. For a dirty sound, crank the drive until the LED is lighted more often. Set the output control to give you unity gain when the unit is switched in. Set the rotor balance to get a balanced sound from whatever amp you are using. Attempts to brighten the sound too much or to use the output control as a signal booster will yield hiss. When the controls of the unit are properly set to recreate the Leslie sound, this simulator is quieter than a hot-rodded and well-maintained Leslie could ever hope to be.

If you are attempting to get a realistic Leslie sound, you'll probably want to send a clean sound into the Rotosphere and use its drive circuit for any distortion you want. You won't want to overdrive your power amp too far either, because this will muddle the effect.

THE BOTTOM LINE: Okay, we know nothing beats the sheer physical impact of hearing a real Leslie in person. But after years of waiting, we finally have a compact, accurate, easily operable Leslie simulator in a robust package. When played through two amps, the motion simulation is very convincing. The sound is marvelous, rich, and full of motion: everything we’ve come to love in a Leslie. The tube preamp is the capstone to the whole unit that creates the gentle, growly distortion and compression that give the Leslie much of the milky, swirling effect. With a little practice, the speed and breaker controls can be manipulated just like the real ones. I have to admit that it has been *years* since an effect device impressed me this much. If you play this unit through two good amplifiers and close your eyes, the Rotosphere will definitely take you there.

Hughes and Kettner

Rotosphere MKII Manual Online