Ampeg B15N Logo light

[Teleman55]

I was asked to change the lighting in this amp to give the plexiglass logo a blue hue.I tried a variety of bulbs which I tinted and blue light gels.Nothing worked very well.I tried a couple of Radio Shack high intensity LEDs rated for 5vts max.I tested them with a variable power supply at 3.6 volts and they seemed perfect.So I took a couple burned out bulbs,removed the glass and installed the LEDs,using heatshrink on the positive lead to avoid shorting.Put them in the amp and it was beautiful for about 2 minutes,then they burned out.Is there a simple way to do this?,I hate the thought of devaluing a perfectly working,original amp.Maybe the guy should just find some plexi in a suitable blue color and get it engraved with the logo.

[Enzo]

I am betting you did not include a current limiting resistor.

LEDs have a typical forward voltage drop - different colors are different voltages. If you apply exactly that voltage to them, you might get away with it, but any excess voltage just passes through it like a short circuit - current and all.

Most LEDs have a max through current rating something like 20ma. I suppose the new high intensity ones might well be higher, so check the spec on the part. In any case, when tunning off higher voltage, there needs to be a series resistor.

Get like a 9v battery, and LED like you intend to use, and a pot of maybe 1000 ohms. COnnect all in series with the pot set at 1000 ohms. If the LED lights, great, and if not, turn down the pot unti, it does. Monitor the voltage across the LED. The lower the resistance of the pot, the brighter the LED, up until it gets too bright and dies soon - as you discovered. But the voltage across the LED should remain more or less steady. That is the target volts.

Yes, you can run the LEDs in series and add their voltages.

Once you know the LEDs natural voltage drop, then what is the power supply driving it? I hope it is not AC, if so, put a diode in series with things so the LEDs only see DC.

Here is an example - change the number to fit your situation:

Red LED with 1.2v forward drop. 20ma max. We run it off a 5VDC supply. SInce the LED wants 1.2 volts, we need a resistor to drop the other 3.8v. That is Ohm's Law: R = V/I = 3.8v/ 0.020A = 190 ohms Standard values 220 and even 180 ohms would work fine.

Three LEDs in series? 3 x 1.2 = 3.6v total drop. Still 20ma since series circuits have constant current through them. That leaves 1.4v for the resistor to soak up. So R = 1.4v/0.020A = 70 ohms. I'd probably use 100 ohms and be done with it - LEDs don't HAVE to run at max.

Blue plexi is not a bad Idea. A local trophy shop could engrave the text.

[Alex R]

On the other hand... I had one of these in that lit up with a nice red glow. Someone had taken a felt pen and ran it along the bottom edge of the plexi where it sits above the two bulbs. Worked really strikingly well - didn't show up much in the transparent bits but lit the lettering up red. So try a blue felt pen along the bottom edge first. I guess all the light goes up through that edge, so it ought to work.

[Teleman55]

Alex,I tried a blue Sharpie but wasn't satisfied.Enzo Thanks for the information.I thought that both current and the fact that it is an AC circuit would be a problem.I figured that an LED being a diode might work so I experimented.You've laid out a nice lesson to put some information I have read before into practical use.Thanks