the red-blue pilot LED
In this project, I had the chance to test out an idea I've had for a while, which I plan to use in my own original guitar amp design and elsewhere -- now that I know it works. It's a more-informative pilot light for tube circuits. The idea is to use the red and blue LEDs from an RGB LED triplet (must be common-cathode), red indicating the heater voltage, blue indicating the plate voltage. At power-up, the light starts out red. As the tube rectifier begins to conduct, the blue fades in and the final colour is a nice shade of purple, for "ready to play". When the power is switched off, the red goes dark almost immediately, but the blue persists, fading out slowly: which gives a clear visual indication of when the high voltage capacitors are safely discharged. Of course, it could simply be done with two separate LEDs on the panel; using the RGB LED is just a cute gimmick. https://youtu.be/qPnR_FwKMtM I powered the red LED from the +11VDC heater li...