DIY Leslie speaker built from scratch and my Hammond organ story

I have this “thing” about old Hammond tonewheel organs and Leslie speakers. It started many years ago when I was a child. On a summer vacation our family stayed at hotel that had an Hammond organ in the lounge. A man was playing it an I thought the music and the sound coming from this instrument was really amazing. Before breakfast the next morning I managed to have a closer look at the organ and the Leslie speaker. Behind the louvers in the Leslie cabinet I could see something moving… what was it? I did`n know how these thing worked then, but i do know now! Later I went to school with a guy that had a very rare Hammond model in his house. It was a “Concert model E” (1937-1942) with a tonecabinet (see picture below). This organ had “typewriter like” buttons for presets never seen on other models. My schoolmate later became a professional church organist.

My first Hammond organ was a chopped L100 wreck that had been the residence of a mice family. The organ didn`t work and was very cheap. I thought I was able to fix it, so I bought it. The mice had their nest on top of the tonegenerator and several frequencies was missing. The main amplifier had a broken cathode resistor for the output tubes. I managed to restore the missing frequencies by fixing some of the L/C filters on top of the tonegenerator. Later on I bought a 760 Leslie and hooked it up to the organ. This set was so beaten up and ugly looking that my wife would`t have it in our livingroom. I used this set for gigging for some time, and then I sold it whith a good profit.

My second Hammond was a 1973 B3 with a 147 Leslie, both in fair condition. I had this set from 1994 to 2003. It occupied 3 to 4 square meters of our living room but it was a fine piece of furniture as well as a great instrument! See the picture below:

Soon after I sold this organ I began developing the “Hammond abstinence”  I was looking around for a smaller model, preferably a M100 model that had the guts of a B3 but in a smaller cabinet that would fit the staircase going down to my basement. I came across an advertisement for a M102 in Oslo and went there to have look. What did I see? A absolutely mint condition M102 with a stuck tone generator. See the pictures below:

The organ hadn`t been used for a good ten years and it was a 110 Volt 60Hz model (you can see the transformer on the floor to the right in the picture). The seller shipped the organ from the USA to Norway when the family moved . The electricity in Norway is 230 Volt 50 Hz. No one had noticed that that the organ was 4 to 5 semitones out of tune and the scanner vibrato was very slow! When the seller became aware of this, he lowered the price quite substantially (to 1500 NOK) so I bought it.

The organ was placed in my basement, the generator was oiled and after one week it was spinning very smooth. I also made a crystal controlled 60 Hz motor control for it (but that`s another story) Now it`s in tune with the rest of the world!

There isn’t much action in an Hammond organ with only built-in speakers, so I decided to build a Leslie speaker of my own design, a two-speed Leslie without an amplifier.The organ has a good tube amplifier, two 6V6 in push pull configuration, giving 22 Watts into a 8 Ohm load. I`m not planning to gig with this set so 22 Watts should be sufficient for home use. I did`n want to use the fragile and brittle Jensen V21 driver because I don`t think it`s efficient enough and it`s overpriced. I got the rotating treble horn and driver from ebay (seller: micaelcasino). As you probably understand by now, I`m not a purist, so my horn are without sound deflectors, and diffusors, and the driver is much bigger and more efficient than the original Jensen type. I like that!

Time for some initial testing!

To achieve the right “chorale” effect the horn should rotate at 45 to 48 RPM and for the “tremolo” effect about 440 RPM which will give you a perfect vibrato at 7,3 Hz. I had some permanent magnet DC motors laying around, rated 24 Volt 2 Amp and some adjustable power supplies rated 12 to 30 Volt 3 Amp that could become useful for this project. I mounted the horn assembly, motor and power supply on a MDF board and used a industrial type 3 mm Nitrile O ring as a drive belt. The pulley are turned from a 30 mm PVC rod. To get the horn to rotate at 45 RPM (chorale) I had to apply 2,3 Volt to the motor and for the 440 RPM it required 20 Volt. These permanent magnet DC motors have a strong torque even at low voltages, so with a stable voltage the rotational speed is also stable. To get 2,3 Volt I had to make an additional stabilized power supply and feed it with 20 Volt coming from the “main” supply. I used a LM 138 circuit (5 amp positive adjustable regulator) for this purpose together with a relay for switching between the two speeds. It accelerates to the top speed in about one second and retards in approximately. 1,5 second. I`m quite happy with this setup so I`m moving on to the bass rotor.

Time to test my woodworking skills

I made the bass rotor of 8 mm (5 layer) and 4 mm (3 layer) veneer.

It`s screwed and glued together with the centershaft in place, in order to get the thing properly aligned. To get this thing balanced I made a very light jig which I could rotate the drum in. I used thick metal washers as balancing weights and placed them where I suspected they should be needed just by looking at the drum. This tweaking was time consuming but I managed to balance it in a way so that there is no noticeable vibrations when holding the jig in my hands while spinning it fast. I don`t want my Leslie speaker to be moving around on the floor when it`s spinning at full speed! That`s why I put so much effort in balancing this thing. The rotor has ball bearings in both ends and the center shaft are 15 mm copper pipe. A similar motor/power supply and drive belt, as for the treble horn, have been used for the bass rotor. Spin up time is approximately 5 seconds and it uses about 7 seconds to slow down, which is OK.

The cabinet

The cabinet is made from 26 mm MDF board covered with birch veneer and the louvers are cut with a Plunge Router and are 10 mm wide. Cutting all these louvers took about 8 hours………..The box is about the same size as a Leslie 145.

Here it is (with almost complete) but after some testing I found that a single bass speaker (pulled from a “beyond repair” M 100) could not match up with the efficiency of the treble horn. Hmmm…..The solution was to use two bass speakers until a efficient 15 inch bass speaker (like the JBL E140) would come my way. So now I use this radical twin bass speaker configuration that sounds really great, see below:

I later incorporated a 2nd order dividing crossover network, consisting of a 7.8 uF cap in series with the treble driver, a 3.2 mH coil in parallel with the treble driver, behind cap. Further a 5.2 mH coil in series with bass driver and at last a 12.5 uF cap in parallel with bass driver, behind the coil. This is the original Leslie setup that gives you 800 Hz dividing frequency with 16 Ohm speaker units used. Not shown on this photo.

I find it very convenient to be able to switch between slow and fast from the swell pedal so I ended up with this solution, see below:

Here is my current Hammond / leslie setup. Looks and sounds good I think!

DIY Leslie Speaker

Oslo 27.04.2008