

Some of us build tube amps, so here is a circuit fragment that may useful. It demonstrates how to power an LED panel indicator from the AC heater winding.



While it is possible to drive the LED from the high voltage dc that is powering the audio, it is better to place the indicator across the heater windings since more current is available there and a smaller current limiting resistor will be required.This simple circuit looks much like the standard version for DC except that a reverse biased diode is placed across the LED. This is to prevent the LED from being damaged by the alternating polarity which would have it reverse connected 60 times each second.

Select the value of R1 for the required brightness of the LED. As shown here, there is about 10ma supplied to the LED which should be plenty bright. You can change the resistor value to alter the brightness. The forward voltage drop of a green, red or amber LED is about 1.8v and calculation is:

Resistor = (HeaterV – LED drop) / current, or (6.3-1.8)/.01 = 630 ohms. The 680 ohm resistor shown on the drawing is close enough.

You would work the calculation the same for a 12.6v heater winding: (12.6-1.8)/.01 = 1080 ohms.

You can use my LED resistor calculator to experiment with different current draws.

We have selected a resistor based on the rms heater voltage, which is satisfactory for most uses. If you decide to draw the maximum current from the LED for which it is rated, you should check the peak voltage values to make sure the LED is not exceeding its maximum but for an average use with 10 or 20 ma. on the LED, you don’t have to worry about it.

Lifespan of an LED will be much longer than with an incandescent bulb and there are more choices of colors and mountings. You could even put a white LED inside a jeweled cover to make it look similar to the traditional style panel lamp.