Leslie Model G37 Rotary Guitar Combo Amplifier

Once again I find myself haunted by a guitar sound in my head and go a-wanderin'. I know, I know: We’ve been down this particular road together at least twice before. Perhaps you can forgive me for launching upon another tone quest because in the end we can both benefit from my research and experience. I suppose the beginnings of this particular quest go back to the ‘60s when I heard George Harrison play his guitar through a Leslie on several Beatles songs as well as the song he wrote for Blind Faith, "Badge." Eric Clapton played through a Leslie on "Presence of the Lord" in Blind Faith, and Pete Ham of Badfinger used one on "No Matter What."

Then the first year I started to play guitar, my big brother Jim bought me a new album that really influenced me: The James Gang Rides Again. Through that album I discovered the subtle developments brought to the Leslie guitar sound by the team of Joe Walsh and his producer Bill Szymczyk. I loved the growly sounds Joe coaxed out of a Tele and a Leslie on "Cast Your Fate to the Wind" and that beautiful, emerging fade-in intro to "Tend My Garden." I had no idea how he got the sound but I knew I loved it. From there I followed Joe’s further work with the James Gang, then his solo career, and his career with the Eagles. Meanwhile, Steve Howe use the Leslie on YES’ Close to the Edge and Tales From Topographic Oceans albums and David Gilmour of Pink Floyd used it several times throughout Dark Side of the Moon and Wish You Were Here. And who can forget the Doobie Brothers’ song, “Another Park, Another Sunday” or Nazareth’s take on “Love Hurts?” By then I knew this was the product of the Leslie Rotating Speaker System. In the ‘70s I occasionally had the chance to fiddle around with my keyboard player buddies’ Leslies and tried to reproduce some of the sounds but I never seemed to have the money for a secondary amp of my own dedicate to this effect. In the ‘80s when I was starting a family, financial and space circumstances simply compounded that. Ironically by the time I had the money, I no longer wanted to lug around a monster Leslie 122 and to be honest, I didn’t really want to give up the space in my home studio for its huge footprint anyway.

But I still wanted that sound, so the first thing I did back in the 1990s was to track down a Hughes & Kettner Tube Rotosphere analog tube Leslie simulator. The Rotosphere has provided some pretty wonderful Leslie chorale (slow) sounds in my recording sessions and still does to this day. I've run it into a pair of amps and mic'd them and I've run a stereo modeler into it and have had excellent results. The pedal models extremely well the unlocking of the rotors' sync when either brake or speed controls are toggled. The pedal is still a respected standard amongst guitarists for its sound and does very well in the loop of an amp. You can read more about my early experiences with the Rotosphere and the physics of rotary speaker sounds, HERE. But even when running the ‘Sphere in stereo through a pair of amps it didn’t provide the same physical experience in person as playing through a real rotary speaker.

A few years later, a used Motion Sound AR-112 Sidewinder rotary guitar combo amp showed up at a nearby dealer with a couple of mechanical and electronic problems and a low price. I pointed out the problems to the staff and then let the amp sit in the store for a few months to soften up the price. Eventually, the store manager accepted my low-ball offer so I took the unit home and repaired it. Voile'! It worked just as well as new. The Sidewinder is a compact integrated amp with a guitar preamp, reverb, and a single rotor speaker system based on a 12" speaker. When I integrated the Sidewinder into my rig I found that a real rotary system provided a much more intense physical experience than the Rotosphere, actually spinning the sound around the room with its vertically-mounted rotor. However, having only a single driver and rotor, its spinning modulation lacked some of the depth and high-end complexity of a standard Leslie with two rotors. Nevertheless, it proved to be a fun addition to the rig. I made up for some of the missing complexity of the two-rotor system by running the Rotosphere in the loop of a second amp while the Sidewinder was spinning but the combined effect still wasn’t quite like a real two-rotor Leslie. It was, as they say, “Close, but no cigar.” So, as a real Leslie lover I always held out the hope that one day I would be able to get into a two-rotor system. You can read more about my experience with the Sidewinder over HERE.

The event that finally pushed me over the edge to the next move was seeing Joe Walsh perform at Charlottesville, VA, on his “2016 Toor.” For more background you can read more about that concert, HERE. Joe brought one of the new, smaller Leslie cabinets (a model 3300) and it sounded great. And I mean great. Coincidentally, I was seated directly in front of this amp and spent the whole time during the warm-up act watching its lower drum slowly spin at chorale speed. That concert and that amp convinced me that a Leslie didn't have to be huge and heavy to sound great. While trying to figure out what amp he was using by digging around the Internet, I discovered the Leslie G37, a unit they call The Guitar Leslie. Though it was introduced in 2009, there weren't a lot of related videos on YouTube. Neverthelless I soaked up all the videos and info I could find. No one offered this animal locally but a short while later the desire and the pennies came together when I was offered a fantastic general discount at an online retailer. Using that discount I made my order and the waiting began.

Joe's backline with the Leslie 3300 on the left

And while we wait for delivery, let's review the raw facts:

Leslie G37 Guitar Combo Amp
Single input channel offering two jacks with high and low sensitivies
Tube Preamp utilizing a 12AX7 tube for warmth
100 watt sold-state power amp
12" "Vintage V30 speaker" (Celestion?) and high-frequency horn driver
Two switchable tone channels, clean and overdrive
Available remote channel switching with the addition of an optional footswitch
Shared three-band EQ
Volume control for clean channel; volume, gain, and contour controls for overdrive channel
Independent bass (drum) and treble (horn) rotors, each featuring quiet, single-motor, solenoid-free DC servomotor-drive for high and low speeds
A pair of ganged trims allow you to fine-adjust high and low speeds.
Foot-switchable rotor high/low speed selection and stop/brake with included dual footswitch
Available continuously variable rotor speed control with optional foot treadle.
For best acoustic performance when the rotors are stopped, a photo-electric sensor system stops them facing forwards.
Medium-Density Fiberboard cabinet with black textured epoxy paint (Tuff-Coat?), corner protectors, and heavy-duty folding handles
Dimensions: 20.5” wide x 20.5” deep x 26.75” high
Weight: 100 pounds

Let's get one thing out of the way: in most rotary speaker systems, the speakers don't rotate. No, really. The problem of creating an inexpensive, reliable rotating electronic coupling that can carry the power of an amplifier to the speaker hasn't really been solved.* So what is inexpensive and reliable? How about placing a rotating baffle in front of the woofer and a rotating horn in front of the treble driver? Sold!

To semi-quote the pertinent bits of my article on the Tube Rotosphere (in case you didn't link to it), here is how Leslie Speaker Systems work and create their classic sound:

As you may know, Leslies are self-powered amp/speaker systems named after their creator, Don Leslie. The Leslie speaker was designed not only to amplify, but also to 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 (balanced by a plugged fake) which disperses the output of the tweeter. The middle chamber serves as a speaker baffle containing the upwards-firing tweeter driver 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. The middle chamber also contains the motors that turn the horn and rotor and their controls. In the original designs the bottom chamber contains the amplifier chassis and the bass rotor that disperses the output of the woofer.

Leslie 147 with back removed.
Click on the image to open a window with a larger view.

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 ganged AC induction motors and a momentary brake available as a function of DC current applied across the motors. Proprietary connectors were used to carry the signal and control voltages between the organ and speaker system. Later, as owners desired to play other types of instruments through the Leslies, small external 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 rotor. It is pitch modulated as the velocity of the rotor’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 rotor and its frequency-response lobes. And finally, the sound is physically projected around the room by the motion of the rotor. 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.

In 1965, CBS bought Electro Music, the parent company of Leslie Speakers. In 1980, the Hammond Organ Co. bought Electro Music and the Leslie name from CBS and moved production to their home in Illinois. In 1989, Suzuki Denshi bought Hammond/Leslie and made them a wholly-owned subsidiary. The company has kept many of the core Hammond personnel and maintained final assembly operations in Illinois, though some of the electronic subassemblies are built overseas. While many of the classic Leslie designs are once again being offered, they’ve also come up with some smaller-sized, lighter-weight, modernized versions of the Leslie speaker systems that make the classic sound far more portable. The G37 is one of these. Within the G37, the amplifier is relocated to the middle chamber with the speaker drivers and motors. The amp features a 12ax7 tube preamp, a 100 watt solid-state power amp, and comprehensive preamp controls to allow manipulation of your sound. A non-powered speaker-system-only version of this amp, the model G27, was offered for several years but has since been discontinued.

Leslie G37 Cutaway - click for larger version

The box arrived surgically between a pair of onrushing tropical storms that threatened to shut down the East Coast and at the very least, postpone delivery. Yikes!!! This thing is darned heavy. Being at the deep end of my fifth decade, I’m no longer a spring chicken and all amps seem to be getting heavier. When this monolith arrived I was also alone. I pushed the box into the center of the living room and took a walk around it, scratching my head and considering my options to get the amp out of the box.

Ah, the smell of new electronics in the morning! As I opened the top of the amplifier's container I noticed that the little white cardboard box inside the top that contained the footswitch was a bit, umm... mushed. "It shouldn't matter," sez I, naively, "Foot switches are designed to be trod upon!" Now, at 107 pounds net, this package was a monster and there were signs that the box had been on its head at least once. Sizing it up, I gently laid the box on its side and then wiggled out the amp from the box and stood it back up. I unpacked the power cord, grabbed a guitar, and plugged everything up. But you know, the footswitch's faceplate looked strangely, um... curved. Very stylish... but intentional? Don't think so.


I powered up the Leslie and waited for the preamp to warm up and, glory and trumpets, we had sound! But when I punched around on the rotary "ON/OFF" and "SLOW/FAST" switches we had no motion. After fooling around with it for a bit to no avail, I looked up Hammond USA customer service and gave them a call. Their helpful service manager, Ray, walked me through a set of steps to function-test the unit. The rotary controls switch via latching contact closures through a 1/4" stereo jack: tip and sleeve trigger fast speed, ring and sleeve trigger slow (chorale) speed, and open is off/brake. Armed with that info I tested the unit. Yep, the rotors did work but the footswitch was, indeed, as dead as the proverbial doornail. To make a long story short, Hammond graciously and cheerfully sent a return authorization and a shipping label for the crushed pedal and set the wheels in motion to replace it. Meanwhile I grabbed a stereo 1/4" plug, an on/off/on toggle switch, a piece of three-way cable, and a plastic box, and wired up a temporary switch rig to allow me to operate the thing until the replacement footswitch arrived. The switch is wired Chorale/Off/Fast, not the best arrangement, but it is actually quite elegant for a collection of pieces from the junk drawer and a few minutes with a soldering iron. The plastic box originally housed a lavalier mic. This makeshift switch held me the two weeks until the footswitch arrived. I'll be keeping it as a backup.

The "fingerswitch." A tip of the hat and a chocolate chip cookie to Joe in the shop

But wiring up the switch revealed that the unit had another problem: While the upper horns behaved normally, the lower rotor wouldn't stop when "OFF" was selected. I did some investigating by shining a flashlight through the louvers in back and found the sensor that was supposed to stop the rotor. It would try to stop but then the drum would launch into another searching revolution. However, if I shined the light directly on the sensor or the drum in front of it the drum would stop. Ah, the sensor was working but couldn't sense the rotor's motion. After a little coordination between Leslie's service manager, Ray, and the local Leslie technician, Ralph at KBD Systems, I was able to take the unit in to the local shop for repair. After checking all the voltages present at the various points in the sensing circuit, Ralph found everything to be electronically okay. Head scratcher! After further investigation he came to the conclusion that the sensor simply wasn't physically aimed properly at the drum. He loosened the screws and adjusted its point of aim until the unit performed normally. Voile'! Thanks to Ralph at KBD Systems and to Ray at Leslie, I'm back on the road. It took a while but the unit is fully back in spec.

The Leslie G37 is just a little larger than an upright dorm room refrigerator - you know, like the little Marshall Amp fridges? This thing is built like a tank though, with very solid walls and heavy corner protectors. The amp is painted with black, textured epoxy paint that is rough and tough enough to take off the skin from your knuckles if you idly brush them against it. Moving the unit is easy enough for two guys using the two heavy-duty swing-out handles. There are no rattles or clunks when moving the amp that might belie loose bits inside. And by the way, with no footswitch connected, the default motor logic is STOP. Makes you want to keep a backup like my little switch setup handy, no? The top rotor horn is molded from ABS plasic and the lower rotor is a one-piece styrofoam molding. In chorale (slow) mode the mechanics are dead quiet. At fast speed there is bit of wind noise from the spinning drum and horn, just like the larger units. In fact, if you place your hand by the rear upper vents you can feel a distinct breeze. However, in contrast to the older, larger units, there is no switching noise when either changing speeds or braking because the two rotors are powered by DC servo motors. A pair of photcells stop the drum and horn facing forward so that the unit can be used as a stationary amp when the rotors are stopped. Because of this the company suggests that you not mic it from the rear or sides but instead from the front. That is good advice, as long as you plan to stop the rotors while recording. If not, the classic Leslie mic positions will work fine.

The Front: it is a bit of a blocky little spud, isn't it?
But somehow cute.

So, how does the G37 sound? Let's just say that there is nothing on earth that sounds like a real Leslie with lower drum and upper horn rotors, and this is indeed a real Leslie. I was concerned that the smaller cab and woofer size might limit the bass response but I shouldn't have been. The unit has plenty of power and bottom end and easily fills a large room. When using the clean channel and the lower sensitivity jack, the 12ax7 tube preamp design removes any shrillness that might be expected from a solid state power amp and imparts something of a Fender-ish glassiness to the high end that makes the clean sound shimmer marvelously. Think "Another Park, Another Sunday.” The next step up in gain is switching to the high sensitivity jack which gives your guitar a little more "in your face" sound. The next step up from there is the drive channel which adds a very useful contour control that allows you to either boost or cut smoothly in the upper midrange to help the guitar cut through. When you switch to the overdrive channel, the second stage of the 12ax7 rounds out the high end in an interesting way adding a second voice to the amp. If you goose up the gain to produce overdrive at the level of, say, “Breathe” from Dark Side of the Moon (which I’m listening to on headphones at seven AM as I type this) or “Love Hurts” by Nazareth, or even "No Matter What" by Badfinger, it sounds quite nice. However, when pushed much harder than that it quickly becomes too bratty and splatty to be of much use to me. I use an external overdrive or a modeler to move into real growl and swish and it sounds great. Maybe when the unit exits warranty I can try an NOS 12ay7 tube to calm down the overdrive channel a wee bit. However, one thing becomes apparent as you play with different amounts of gain: as you increase the gain you actually hear a less pronounced high rotor sound.

Official shot of the Control Panel

On a completely subjective level: Ooooh! My! This thing is swirly. I’m a Leslie swirl fanatic and this is great Leslie swirl. The clean channel sheen and complexity are outstanding. Les Pauls and ES-335s, Strats and Teles all sound great through the unit. As one would expect, the single-coil pickups yield more high-end shimmer and the humbuckers yield more girth and roundness. With the EQ and Speed Adjust controls you can easily find a sweet spot to make things just right for either of these guitars. The treble horn rotor does indeed add a complexity to the sound that single-rotor systems simply can't equal. Something I had forgotten over the years is that the two rotors rotate in opposite directions, adding even further complexity and modulation than a single rotor system can produce. That makes the sound much more interesting when added to a darker, humbucker-equipped guitar.

The Back: I almost wish the control panel were on the front...

I played some YES ("The Remembering, High the Memory") with a 12 string guitar and the unit produced that signature high-speed warble Steve Howe used. I played "Another Park, Another Sunday." I hooked up my Tele to the Line6 HD500X modeler with a distorted Deluxe Reverb model and a pinch of reverb and delay to see how it sounded with the Leslie and to try Joe Walsh's version of "Cast Your Fate to the Wind" from The James Gang Rides Again and "Meadows" from The Smoker You Drink... I switched to a Les Paul and played Nazareth's "Love Hurts." I played Badfinger's "No Matter What." Now, my wife is a classically trained first soprano with a discerning ear whom I have gently led astray into rock n' roll. As I was working through the various songs she walked up, stopped to listen, and said, “You know those parts you are playing? They finally sound just like the records.” Well, there you go. You can't fool her.

Over the years I have written several songs featuring guitar parts with the Leslie sound in mind. Now I can finally put those sounds into the songs that have waited for so long.

* David Gilmour got his hands on the one rotary system that features a rotating speaker, the Maestro Rover, and became fascinated with its slightly different sound. The unit had problems with reliability, though. Through his chief technician, Phil Taylor, Gilmour commissioned three custom rotating speaker arrays dubbed "Doppolas" that featured an array of actual rotating speakers. You can hear and see them in operation on the Pulse concert videos from the 1994 Division Bell tour. They sound lovely. Unfortunately, even with a sizable investment in their development, the coupling that carried the signals from the stationary cabinet to the rotating speakers turned out to be quite unreliable. They haven't made an appearance since.

Q. Does the Leslie G37 sound like the larger 122s and 147s?
A. The G37 sounds just like the larger ones except that the onboard overdrive isn’t as smooth. On mild and medium settings the overdrive channel sounds great but through a solid state power amp the 12ax7 preamp gets pretty splatty, pretty quickly when you push up from overdrive to distortion. However, the G37 takes pedals well and sounds just like the old Leslies when you use an overdrive that sounds in the ballpark of the original Leslie tube amps. I’ve picked up the Analog Alien Joe Walsh Double Classic for this application and it does great. More about this pedal over HERE. Now admittedly, there are Leslie enthusiasts who compare the resonance characteristics of the various construction methods and woods just like guitarists compare tone woods and this does in fact feature a composite MDF cabinet. But it still sounds darned great.

Q. Why didn’t you just buy a more modern Leslie simulator pedal? They are MUCH lighter than this monolith and New Soviet Men use them.
A. In my experience, even when they are run in stereo, pedals can't match the complex aural effects of a rotary cabinet when you are in the room with them. Yet. But don’t think I’m a Luddite: I work a whole lot with digital modelers and processing gear. But I want the real sound for my recordings.

Q. Weight?
A. Yeah. It is built like a tank. At 100 pounds it is still stinkin’ heavy and easily violates my standing forty-five pound amp limit by a factor of two (2.22). I won’t be casually humping this one to gigs. For that I'll hold on to the forty-pound Sidewinder amp. If I were going to gig this thing I'd definitely buy a road case with wheels. I will be using this amp for recording, though. While is it heavy, it is still far smaller and lighter than the original Leslie systems. Still, I'm glad my wife got me a hand truck a while back.

Q. So, is it worth the weight and cost?
A. I suppose it depends: Are you an industrial-grade Leslie swirly aficionado? Are Leslie simulator pedals letting you down? If so, I can tell you that in person, NOTHING sounds like a real, two-rotor rotary system. NOTHING. So, for me it was absolutely worth it. Your mileage may vary.

Q. Does it play well with acoustic and twelve string guitars?
A. Absolutely. An acoustic with a pickup sounds great plugged into this amp. Just take the time to dial it in with the EQ.

Q. Can you use a modeler as a front-end for the G37?
A. Absolutely. In fact this is becoming one of my favorite applications for the amp. You'll want for the modeler to be able to output its signal at guitar signal level because this is a guitar amp. You'll also want the modeler to be able to switch off the speaker emulations because, of coure, this is a real speaker system. Once you figure out how to set the EQ it sounds very, very nice. I was just fiddling around, using the Tele and my modeler set up as a Fender Deluxe as the front end grind through the clean channel and absolutely NAILED Joe Walsh's sound from the "Cast Your Fate to the Wind" portion of "The Bomber/Closet Queen." Growly and swirly and lovely. The Marshall 1987 sound is pretty amazing as well.

Q. Are the remotes proprietary?
A. Only one out of three is. The channel switching footswitch (FS1-TL) is a simple latching contact closure with a tip/sleeve 1/4" plug. I plugged in a Marshall latching footswitch and it worked fine. The rotor speed footswitch (FS-10TL) is a three-wire latching contact closure with a tip/ring/sleeve 1/4" plug. The wiring logic is: Tip shorted to sleeve yields fast speed, ring shorted to sleeve yields slow speed, open is off/brake. On the pedal, the right-hand switch selects tip or ring to sleeve for fast or slow and the left-hand switch simply interrupts the sleeve line for open and brake. By contrast, Hammond/Leslie tells us that their V-20RT continuously-variable motor speed control treadle is proprietary. As a reult, use of a volume control or expresion pedal will damage the Leslie unit.

Q. Does the unit have any quirks?
A. Yes. Besides the razzy overdrive, switching channels appears to be done by FET. The result is that there is a fraction of a second of silence as you switch from the clean channel to the overdrive channel. Another quirk is the speaker - it arrives badly needing to be broken-in, as witnessed by a sort of hash sound in the background and a slightly pinched response that goes away after some hours as the speaker is played in.

Q. Is there a protective cover available for the amp?
A. Hammond USA offers a heavy-duty padded cover for their Studio 12 Leslie Amp, the keyboard version of this amp that shares the same cabinet with the G37. The product number is HS13A-A18 and it is available where the Studio 12 is sold, such as Musicians Friend. Because the Studio 12 and G37 share the same cabinet, the cover fits the G37 perfectly.

Q. Does any artist use this amp?
A. As I closed out the research for this review this question occurred to me and I set out to find if anyone was using this amp. As mentioned above, there were originally two versions of this box, the G37 powererd amp that I've reviewed here and the now-discontinued G27 unpowered version that contained everything except the amp. Imagine my surprise when I discovered that David Gilmour used a G27 powered by an Alessandro Redbone Special amp during production of his latest album, Rattle That Lock, and for the final sessions for Pink Floyd's Final Album, The Endless River. That info comes courtesy of GILMOURISH.COM and the DAVID GILMOUR GEAR FORUM.

Picture courtesty of the David Gilmour Gear Forum. Leslie G27 with rotor back panels removed
and Alessandro amp are on the right. Also note the old tweed Gibson Ranger and Fender Twin.

UPDATE 05/24/2020 Hammond is now classifying the G37 as part of their new Compact line. It looks identical and the specs are the same.

at HammondOrganCo.com