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 line (LED cathode at ground), through a 10k resistor.  I powered the blue LED from the same +HV line which goes to eyelet #16 on PC-5; resistors are two 220k/2W power resistors, in series.  (The power rating is overkill, dissipation should be less than 1/2W; and with a 1M resistor, dissipation would be under 1/4W so a regular resistor could be used, but I didn't have one on-hand.)


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