Leslie Model 860 Rebuild

(11/23/2004)

Our latest project is a bit of a departure from the usual guitar and bass amp work we do at DST Engineering. One of my better customers, Jason Cohen, is an exceptional keyboard player with an up and coming Boston area band. His choice of amplifier for keyboards (old Fender/Rhodes pianos, etc.) is an old Fender Twin. Certainly not the typical rig you see these days for most keyboard players, but he really likes the old tube amp sound, and so too does his audience.

Jason recently came to me with a special request. He wanted to replace the solid state amplifier in his Leslie 860 with a tube amp. Jay was looking to capture the sound of his full size Leslie 147 but in the much smaller package of the 860. My first thought was, "why not just use a Leslie tube amp from an old 147 or even one of the replacements already on the market"?

After a bit of research it became apparent that, due to the size limitations of the 860 there were no vacuum tube based amps available to fit the smaller cabinet. After receiving the 860, I spend a couple of days staring at it and formulating a strategy. I decided to take the "old school" approach of separating the amplifier and power supply into two chassis. As luck would have it I was able to find some ready made chassis blanks that would fit the two available empty spaces in the Leslie cabinet. The power supply could be mounted on the bottom in the place originally taken by the solid state amp, the vacuum tube power amplifier would mount in the space directly above it and just to the left of the 12" bass speaker.

For this particular project large amounts of output power weren't required, and we decided that something in the 50 watt range would be plenty. This allowed me to base the amplifier on the existing DST U-series guitar amp design. The U-series amps have 2 output tubes and deliver in the 50-75 watt range. They can also work with any type of octal output tube. The customer's preference was to go with 50 watts from a pair of 6L6's as this is his preferred choice in the Fender guitar amps he uses.

Once the basic decisions had been made the rest of the job was just the logistics of making it all come together. As you'll see in the photographs everything was incorporated into the two chassis' including the motor speed control circuitry, which was placed in the power supply section. The only other addition was a new passive crossover network to separate the highs and lows at the output of the amplifier. This can be seen mounted just behind the tweeter horn.

There are 2 vent ports right above the tubes into the treble rotor area. Heat drafts up and the spinning rotor acts like a fan. Although the vents are large enough that even with the rotor off it stays pretty cool. There's also an intake port at the rear panel and on the deck near the horn driver mount.

The power amp is actually mounted on rubber grommet type isolators. The amp is the single board system I use in the U34 and was originally designed to float. Although we didn't float it in the U34 it does here. I was actually more concerned with travel than use. I'm amazed at how well damped the cabinet is. The rotors have these crazy torsion bars and floating mounts to minimize vibrations. I found this out the hard way when I turned the cabinet upside down. Important safety tip... Never turn a Leslie upside down. They don't like sideways much either.

To drive this setup I ended up modifying the Jason's Speak-Easy preamp. I added a midrange control, changed the tone stack a bit and separated the signal and control cable lines to increase noise isolation. I also upped the gain a bit to accommodate his Nord keyboard which has a very low output signal. This pre-amp setup allows the L860 to be controlled from the players seated location by foot.

The final step of the project was a complete vinyl recover job done by Jeff Swanson. The cream/oxblood combination was chosen by the customer as a nod to his love of classic Fender amps.

Even though this was a difficult project, it turned out very well. The customer's brother (who is also the band's drummer) pretty much summed it up: "Dude, it sounds just like someone put a 147 in the clothes dryer!"

Have a question or want more details? Contact Bob Dettorre, VP Engineering, DST Engineering
bob@dst-engineering.com