The 31H Leslie

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Here's an article about our ProKeys 31H Leslie.  As you can see, this Leslie has been rebuilt, refinished, and has a unique steel caster plate which allows us to tilt it back and roll it around, something like a huge, top heavy hand truck.  This could be considered a road Leslie, as it goes out on plenty of gigs.  Because of its height, it barely fits inside the trailer opening though, and it's pretty bulky to move around, but it certainly sounds wonderful.  It uses all original audio components, including a Jensen F15LL Concert Series Field Coil 15", reconed to exact specs in 1997, an all original Jensen Hypex V21 driver, complete with its original sealed back shield, and a stock crossover network.  The Type II amplifier has had all the electrolytics replaced, and is due for some component upgrades and replacements soon.

When we first got this Leslie, it was really trashed.  Lacquer was peeling off in sheets, it had white paint on one side, and rust almost everywhere there was metal.  The motors wouldn't even turn, they were rusted solid.  Nothing seemed too badly broken, but it was in really miserable condition.  At first, we thought of just painting it flat black, but then I got curious what sandblasting might do to it.  After two minutes of blasting, I was glad I didn't paint it black!  The blaster took the lacquer finish right off, and "averaged out" a lot of the scrapes and nicks in the veneer.  We discovered that sandblasting isn't nearly as destructive as we first thought.  I had visions of huge holes blasted right through the sides - it just isn't so.  The blaster gun can be held as far away as needed, and you can easily control how much effect the sand has.

After blasting was complete, we started planning the addition of two speed motors.  Building the two speed motor mounts was an interesting project, as we discarded ideas one after another.  As you know, the 31 was made with fast-only motors, and the original metal mounts wouldn't work at all with the larger and heavier two speed units.  I decided to fabricate an upper shelf.  It would still be almost identical to the original, but very slightly larger.  I wanted to mount the upper motors right under the shelf, rather than trying to suspend them on the inside wall with any kind of bracket assembly.  And I did not want to reduce the large upper compartment to any degree.  The first decision was height - we needed a shelf slightly higher than the stock shelf was, to allow clearance for the taller motor stack with the slow motor attached.  Our final design uses three pieces of 3/4" plywood - a shelf and two supports - almost identical to the original.  We also added an additional support brace along the left inside wall of the box.  Very similar to the original shelf, the new one is simply a top plate and two end support plates.  Openings in the shelf were needed for the driver and the motor.  I used a very small Dremel router to work on the motor mount area.  It was necessary to remove plywood on the bottom of the shelf, above the motor mount area, to allow the pulley to clear the top of the shelf.  In this 31, the motor is mounted solidly to the shelf with no rubber shock mounts.  Some motor vibration noises are picked up with this kind of mount, but they're not excessive.  Some day, I'll put some time into redoing this mounting system, and use rubber to isolate the motors.  Once the top shelf started to take shape, the rest of the project became easier.  We used a standard 122 type upper belt tensioner in the 31, which solved that problem immediately.  Spacing didn't allow the tension spring to be mounted in the same way as a 122, so we mounted it on the opposite side.  By this I mean the belt isn't being tightened towards the back of the Leslie, it's tensioned towards the front of the cabinet.  The pictures will clarify it better.  Looks a little unusual, but it works exactly the same way.

The lower motor mount caused a few more problems.  We solved them by cutting a rectangle of plywood, and opening it up for the pulley and motor mounting bolts.  It mounts solidly in the right rear corner, directly to the existing cabinet corner bracing.  This location does NOT match the length of the lower belt, so we had to design some kind of a tensioning device for the lower belt.  Because the motor is mounted solidly, it can't be swiveled to make the belt looser or tighter as required.  We worked on this problem for quite awhile, and came up with a strange solution.  With the Leslie laying on its face, we made some measurements in the area that a tension pulley was required.  Unlike other Leslies, the 31 has a wide open bottom with a steel support bracket across its width.  This bracket is the mount for the lower bearing for the drum rotor.  The drive pulley for the lower drum is mounted below that bracket.  I machined a narrow, approximately 1/4", three or four inch slot into this bracket in the correct location.  Then we rigged an old lower pulley onto an axle, using a bolt, spacer washers, nuts, tubular metal inserts, and some ProKeys mojo.  We bolted this assembly in the slot in the steel rotor support bracket.  It's made in such a way that we can adjust the height of the pulley to match the belt, and the assembly can slide laterally to make the belt as tight as needed.  The tensioning pulley rotates freely on the axle.  Yes, it really is difficult to adjust the tension of the lower belt.  Fortunately, it's held the adjustment for several years now, and only needs an occasional drop of oil in the tensioning assembly if we hear a squeak.

After we were satisfied with the mechanical arrangements of the motors, we made a few other changes to the cabinet.  The 31 Leslies used wood screws to fasten the back panel onto the cabinet.  We replaced them with threaded Tnuts and stainless machine screws and washers.  Wood screws were also originally used to bolt down the 15" speaker.  Again, we replaced this system with Tnuts.  But we do this a little bit differently.  We mounted the Tnuts inside the bass compartment, and threaded stainless screws up through the shelf, from the bottom, using flats, locks, and thread sealer.  (see the next paragraph).  This method presents eight bolts sticking up which go through the mount holes in the speaker frame.  Once the speaker is in place, we use flats, locks, and nuts to really torque it down solidly.  We use this mounting method in all of our Leslies, and never have loose speakers.  I should include a warning here:  if you choose to use this mounting system in your own Leslies, be very careful when you reinstall the 15" speaker.  It's very easy to poke one of those screws right through the cone if you're not paying careful attention.

Here's a little trick that you may find pretty useful.  In a typical Leslie, there are several screws threaded into Tnuts, which serve as mounting posts. Examples are the upper motor mounts and the tensioning arm mount.  Very often, these screws work loose, and their washers cause a vibration noise inside the Leslie when certain notes are played.  I've searched around for weeks trying to track down some of these noises.  And they drive you crazy!  Try this: remove the screws, buy some flats and locks, and when you replace the screws, seal them into the threads with Hot Stuff or Krazy Glue.  They probably won't ever cause vibration noises again. 

Back to the 31H project - All inside surfaces of the upper and lower compartments were painted with a semi gloss black exterior paint.  The bottom drum rotor was also painted - it was originally a strange silver color.  The paint allows us to wipe up dust with a damp rag, much easier than raw wood.

A friend of ours welded the right angle caster bracket.  Basically, it's just a steel "lip" that's attached to the back of the Leslie with two casters mounted on flanges.  We used long and very heavy wood screws into the sides of the cabinet to attach this plate.  The bracket is made so the casters do not touch the floor until the cabinet is tilted back several degrees.  Because the top of the Leslie is open, (we don't use the upper back cover), we can grip the top and maneuver it much like a hand truck.  It's very top heavy, and can tilt back on you pretty easily.  Currently, the Leslie does not stand exactly level.  The caster plate raises the back of the Leslie 1/4", so next time we take the Leslie apart, I'll trim 1/4" off the rear "legs" then reinstall.

Finally, we did some sanding, and finished the cabinet in Tung Oil.   We didn't stain, seal, or fill the wood, we just wiped Tung Oil into the open grain.  The end result has plenty of visible open grain and is not glass smooth.  Some people don't like this appearance very much, but in our situation, I think it makes a lot of sense.  Even though we do our best to prevent nicks and scratches, they seem to happen occasionally anyway.  This kind of finish allows us to do a touch up that blends right in.  All that's necessary is to wipe or spray some more Tung Oil into the scratch and it seems to disappear instantly.  For Hammonds that travel a lot, I think it's far better than looking like a battle ax.

We recently refinished three of our other ProKeys Leslies using PPG Omni AU, an automotive two part clear coat.  Current plans are to spray this clear finish on the 31, right over the Tung Oil.  The Omni finish is very hard, resists nicks and scratches, and it's also very glossy.   Sometime soon, I'll take the 31 apart and shoot the clear on it.  Hopefully, there will be no conflict between the two coatings.

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