From goff747@aol.com Thu Dec 25 17:57:47 2003
Subject:Re: Speed Control Pedal


In a message dated 12/26/03 8:37:47 AM, rb.keele@verizon.net writes:

<< I viewed Dan S' response as a safety issue rather than a musical one. A
pedal unit designed for volume control most likely would have the low side of
the pot (ground) tied to the units case for shielding. Also, a pot designed to
handle line voltage usually has a plastic shaft and is mounted to insulate
the pot from any casing that could be touched.

The sewing machine pedals that you mentioned typically are wire wound
rheostats, stepped with a multi position switch and wire wound resistors, stepped
with multi position switches (or commutator) and coils or with a simple
autotransformer. They are designed for the high power of the motor. Also, when
controlling with a variable resistance it is necessary to approximately match the
resistor to the motor. Anyone that has ever done stage lighting with an old
rheostat (pre variac) board would probably remember that if you put a small bulb,
like a table lamp, on a dimmer rated for say 3000 watts, there would be
virtually no dimming.

Pedals that designed for volume control typically are designed with carbon
(or newer technologies) potentiometers. They typically can handle 1/10 watt to
1/4 watt. A motor directly controlled with one of these pots would turn to
useless carbon in less than a second.

I suspect that the control method that Sam was using was a control voltage to
a power controller in the box that was mounted in the Leslie. Jameco
Electronics http://www.jameco.com/ sells a Velleman DC Controlled Dimmer Kit
http://www.vellemanusa.com/ M/N K8003, for $22.95. The Jameco P/N is 128901CD. This
unit uses 0 to 10 volts DC which a volume pedal should be able to handle
safely and would probably be able to control the motor. (Assuming the motor
current is within the 3.5 amp limit for the controller.) From the toroid choke,
that I see in the picture, it appears that they payed attention to filtering the
switching buzz.

One final note. It is never a good recommendation to use audio connectors
(that are used for audio elsewhere in a setup) for line voltage. Phone
jacks/plugs are especially bad because they aren't really physically robust and break
easily. I have used XLR3 connectors for power when I'm not near other uses
for them. They are physically more robust (read physically safer) and since the
XLR series also comes in 4,5 and 6 pin models it is possible to use one of
them for power near an XLR3 audio system. That said, it is still best to use
connectors that are designed for line power when connecting line power. If an
accidental connection is made, the money saved by using existing audio cabling
and connectors could be eaten up with the cost of replacing one or more pieces
of equipment.

Ron >>

here's a genius when you need 'em.. the reostat/variac idea is what is
expected to be part of the equation. 1/4 phone plugs ( the kind you connect your
boards/guitars with to amps with) carrying 120 volts ac are accidents waiting to
happen specially since they short upon connect and disconnect. thanks for
helping to clear this up everyone and to you Ron for your research into this and
OHF Bob for bringing the mod up. this mod can be worthwhile even if only for
the treble rotor control, and if taken to a popular level could be part of of
a clone's design features if the manufacturers are listening. feathering
leslie control and its sound are techniques that are sought after by true leslie
artists and running a leslie is an art in itself although you'd have to pick and
choose the examples on records of this happening they are out there and many
never make it to a recording.