Put an LED in my B100R
I recently picked up an Ampeg B100R, and like many of them the power light was burned out. That seems to be a common problem. I thought about ordering a replacement, but instead decided to switch from the 'grain of wheat' bulb to an LED. I did this mostly because it seems the lights just don't last and an LED should.
WARNING: I did this and didn't kill myself, but if you even think too hard about it doing this mod it will lead to hair loss, chronic bad breath, your car will start to burn a quart of oil every 67 miles, and your dog will become incontinent. You've been warned!
Quick check with the voltmeter shows 37 volts across the bulb (maybe why they don't last?), and specs for the replacement bulb say .02 A. I picked up a blue LED from Radio Shack (5mm, 5 V/.03 A). Some quick math shows I need to add about 1600 Ohms to drop that 37 volts down to something safe for the LED (assuming I did my math correctly), so I picked up a few resistors as well. When measuring the voltage I also checked and noted the polarity (topmost connection on the circuit board is positive).
A bit of a tug on the wires pulled the old bulb assembly out of the tube it's mounted in. There's a small plastic bit that holds the bulb and the bit where the bulb leads and the jumper leads join together. The jumper leads are joined to the circuit board by spade connectors, so I used some needle-nosed pliers to carefully pull those off.
The leads from the original bulb are fastened to the jumpers by a crimp connection. I didn't have a good way to replicate this, so I snipped off the old bulb leaving as much of the bulb lead in place as possible and soldered the LED leads to them. Then I cut one of the jumpers and added two resistors in series (a 1k Ohm and 680 Ohm to get the total resistance needed), using some heat shrink tubing to cover it.
Carefully reinsert the bulb assembly back into the tube and reattach the jumpers to the circuit board, paying attention to the polarity again (actually the connectors look the same, so if you did get it backwards I think you could just swap them).
Ta-da! A nice blue light that should last the life of the amp.