anyone know how to...

jtullfan

Member
2010-07-25 7:40 pm
Hey all-
So I have a tube guitar flea amp I built up a while back (little push pull 1w- based on 4 12ax7s (includes Ph. Inv.) and a 12bh7 cathode biased output tube. Had parts... gave it a go. Photos available to the intrigued. Anyway, I gave it a pilot light via LED, but want to figure out how (if even possible) to add another that functions as a level indicator (higher input level-> brigher led is the goal. Just one. I was hoping it was possible to do this IC free- a transistor at the most or is that not possible? I pulled the DC source for the led from the filament supply. Reason for LED: box was too small to accommodate a full size incandescent pilot lamp assembly.
 
You could probably do this with a single transistor, diode (to convert the AC waveform of music into pulsing DC) and a simple voltage divider on the input but have one of the resistors variable so you can set the maximum level. The LED will only stay at a specific brightness whilst a certain volume is played (basically, no persistence and could not be used as a peak level indicator).

To set the level for the LED, you would need to play at full volume and adjust the variable resistor so that the LED lights at its brightest, but is not pulling too much current (which would blow the LED). You would also need a resistor after the divider, before the base of the transistor to drop the voltage to within range.

The only thing about using an LED, is that for low volumes, it will not light at all because the forward voltage drop (usually around 2V) has not been overcome. In this case, it would probably be better to use a small, low wattage portable torch type bulb which would still glow at low volumes.
 

jtullfan

Member
2010-07-25 7:40 pm
Thanks everyone. Had no idea how this could be done. Say Jackand08, would you be willing to draw a schematic of how that would look? I think I got it but I'm not entirely sure. Good point about the Vf of the LED. I do have a 1.5V size Maglite bulb around somewhere. Suppose that would work?