New Spin for a Leslie
modifying the input stage on a classic rotating speaker

Laughton Electronics Here is an old vacuum-tube device I modified. The goal was to provide a low-level unbalanced input and to find alternative means for dealing with power and motor-control connections, which were originally configured to connect to a Hammond organ.

The Leslie 145 is an amplifier-speaker combo with rotating baffles that give it the classic, tremolo electric organ sound. But the owner of this particular unit wanted the freedom to use it with electric guitar and other instruments (and of course organ as well).

There were three main aspects to deal with. The most obvious matter was to add a preamp stage that would allow the unit to accept a low-level unbalanced input signal via a standard 1/4 inch jack. This entailed adding an extra tube (a 12AX7 dual triode) to act as an input buffer and phase inverter.

The second issue was dealing with power switches and power connections. Something was needed to replace the original setup, in which a switched AC source simply arrived via a cable and octal plug from the organ. (The same plug carried the balanced audio lines and the motor speed select.) My solution was to add a female EIA power connector so that a standard, removable (and replaceable) power cable could be used; this also provided a safety ground connection which the unit otherwise lacked. I installed one toggle switch for Mains switching, and added another to provide a Standby function as well.

The final issue was some means to provide remote switching for the fast/slow control for the baffle motors. I decided to leave the options open, since the actual switch could take many forms: a footswitch, for instance, or a toggle located on the organ console. I wired up a low-voltage, solid state circuit that accepted its input through an XLR plug that mounts where the old octal socket used to be. The idea of using XLR was that an ordinary mike cable could be used as an extension for the footswitch circuit, although it's maybe a bit unlikely that that'll ever happen. Anyway, there it is.
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