Questions about Pilot Lights (not gas pilot lights)

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

Reply to
Joe Waddell
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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.

Reply to
B. W. Salt.

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.

Reply to
James Sweet

...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

Reply to
jakdedert

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.

Reply to
James Sweet

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.

Reply to
Clive Tobin

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