All,
I have a Leslie Combo Preamp pedal that I use to connect musical
instruments to a Leslie 147 Speaker.

The pedal has a Pilot Light in it that indicates if the Leslie 147
Speaker is rotating at "Fast" speed.

A foot switch on the pedal closed and opened the circuit that
controlled speed. When the circuit was open, the Leslie spun "Fast"
and when the circuit was closed, the Leslie spun "Slow".

There are two wires running to the foot switch. Right before that
switch, the Pilot Light is located, connected to both wires that are
running to the foot switch.

When the footswitch is closed, the current flows through the footswitch
and not the pilot light, and the pilot light is not lit.

When the foot switch is open, the current flows to the pilot light and
causes it to emit light.

Here's the strange thing that I need help with. I noticed that in Fast
mode (with the pilot light lit from the open footswitch), an annoying
noise would come out of the Leslie Speaker. I, just by chance,
discovered that the Pilot Light was the source of this noise.

Some sources on-line talk about needing to ground pilot lights in
electronics, or it can cause a 120Hz buzz.

Has anyone every heard of this before? Could someone explain to me
what a Pilot Light is? Is it like a lightbulb, in that it acts as a
resistor in a circuit?

Thanks,
Joe

In article .com,
(Joe Waddell) wrote:

Has anyone every heard of this before? Could someone explain to me
what a Pilot Light is?

It's just a light used as an indicator; nothing more than that. If it is a
filament lamp (like a torch bulb) then I cannot see how it would cause the
interference you mention, but if it is a Light Emitting Diode (LED) then
it could be the source and I suspect it should have some sort of filtering
capacitor and resistor across it.

Joe Waddell wrote:
All,
I have a Leslie Combo Preamp pedal that I use to connect musical
instruments to a Leslie 147 Speaker.

The pedal has a Pilot Light in it that indicates if the Leslie 147
Speaker is rotating at "Fast" speed.

A foot switch on the pedal closed and opened the circuit that
controlled speed. When the circuit was open, the Leslie spun "Fast"
and when the circuit was closed, the Leslie spun "Slow".

There are two wires running to the foot switch. Right before that
switch, the Pilot Light is located, connected to both wires that are
running to the foot switch.

When the footswitch is closed, the current flows through the footswitch
and not the pilot light, and the pilot light is not lit.

When the foot switch is open, the current flows to the pilot light and
causes it to emit light.

Here's the strange thing that I need help with. I noticed that in Fast
mode (with the pilot light lit from the open footswitch), an annoying
noise would come out of the Leslie Speaker. I, just by chance,
discovered that the Pilot Light was the source of this noise.

Some sources on-line talk about needing to ground pilot lights in
electronics, or it can cause a 120Hz buzz.

Has anyone every heard of this before? Could someone explain to me
what a Pilot Light is? Is it like a lightbulb, in that it acts as a
resistor in a circuit?

Thanks,
Joe



A pilot light is nothing but a lightbulb, my guess is that the load of
the lamp is exposing a problem elsewhere in the circuit, probably a bad
connection or a dried out capacitor.

James Sweet wrote:
Joe Waddell wrote:

All,
I have a Leslie Combo Preamp pedal that I use to connect musical
instruments to a Leslie 147 Speaker.

The pedal has a Pilot Light in it that indicates if the Leslie 147
Speaker is rotating at "Fast" speed.

A foot switch on the pedal closed and opened the circuit that
controlled speed. When the circuit was open, the Leslie spun "Fast"
and when the circuit was closed, the Leslie spun "Slow".

There are two wires running to the foot switch. Right before that
switch, the Pilot Light is located, connected to both wires that are
running to the foot switch.

When the footswitch is closed, the current flows through the footswitch
and not the pilot light, and the pilot light is not lit.

When the foot switch is open, the current flows to the pilot light and
causes it to emit light.

Here's the strange thing that I need help with. I noticed that in Fast
mode (with the pilot light lit from the open footswitch), an annoying
noise would come out of the Leslie Speaker. I, just by chance,
discovered that the Pilot Light was the source of this noise.

Some sources on-line talk about needing to ground pilot lights in
electronics, or it can cause a 120Hz buzz.

Has anyone every heard of this before? Could someone explain to me
what a Pilot Light is? Is it like a lightbulb, in that it acts as a
resistor in a circuit?

Thanks,
Joe



A pilot light is nothing but a lightbulb, my guess is that the load of
the lamp is exposing a problem elsewhere in the circuit, probably a bad
connection or a dried out capacitor.

....or else the lightbulb has nothing at all to do with the noise you
experience.

How did the OP determine it to be so? I see nothing in the post to pin
the pilot light as the culprit, except that he says it's so.

The description of the lamp circuit is fairly complete, but how does
this relate to the audio signal path? What is this noise, how does it
sound? Is it a 120 Hz buzz or something else? You (OP) describe this
as a preamp pedal. Does it also contain active electronics to boost the
signal?

jak

...or else the lightbulb has nothing at all to do with the noise you
experience.

How did the OP determine it to be so? I see nothing in the post to pin
the pilot light as the culprit, except that he says it's so.

The description of the lamp circuit is fairly complete, but how does
this relate to the audio signal path? What is this noise, how does it
sound? Is it a 120 Hz buzz or something else? You (OP) describe this
as a preamp pedal. Does it also contain active electronics to boost the
signal?

jak


Well I had simply assumed that he had tried disconnecting the pilot lamp
and the problem went away,if not then that's certainly the obvious next
step.

Joe Waddell wrote:
Here's the strange thing that I need help with. I noticed that in Fast
mode (with the pilot light lit from the open footswitch), an annoying
noise would come out of the Leslie Speaker.

Ah, youngsters and their lack of familiarity with old stuff! (I say,
while stroking my hypothetical long white beard)...

If this is old technology, and if the foot switch is switching line
voltage, the pilot light is probably a small NE-2 type neon lamp and
series ballasting resistor. It does not conduct continuously but just
on peaks of the AC voltage when it is high enough for the lamp to
ionize. The voltage and current through it makes a nasty
spiky-square-ish waveform of about 200 volts p-p which can in fact
easily cause a buzz in nearby sensitive low level circuits. At the end
of each half-cycle it could also be oscillating in its negative
resistance region.

I would look for a shielding or grounding problem. Maybe replace the
neon light with a pair of back-to-back low current rated LEDs and a
series resistor to limit the current to a few milliamps or so.