LED pilot light wiring.

[MarkG]

I just finished my crack amp last week. It sounds way better that I could ever have guessed something I built would sound but I'm already getting the itch to tinker. Although I'm competent at following instructions, that's where my knowledge of circuits ends. I'm trying to figure out how to wire in a nice blue LED that I have for a pilot light. I thought I could just tap the 1.5VDC power from the 12au7 socket, and all would be Jim Dandy. Not so much. I guess my LED needs a little more juice, it does work fine powered off a 9v battery. Any suggestions on where the best place to tap power from would be without messing with the sonic qualities of the amp?

Thanks,

Mark.

[dstrimbu]

I thought I could just tap the 1.5VDC power from the 12au7 socket, and all would be Jim Dandy. Not so much.

Hey Mark,

You're probably at risk of hosing up the bias for your 12AU7 if you tap in to A3 or A8 of the the nine-pin socket.
Have you considered a 120v pilot lamp? 

-Don

[MarkG]

I thought I might be able to use one of the LEDs I already have but I'm starting to think I need to figure out where it goes and then get the parts I need rather than try and use something I already have.

Thanks, I'll look into it further.

[Maxwell_E]

a lot of LEDs are about 5V's, depends though. You might be able to run them off the heater tap with a resistor in series to bring it down from 6.3.

[MarkG]

Thanks, that's what I'll do. I'll let you know how it works out.

[Grainger49]

When running an LED on a DC source you want to know three things.  The first two are the diode characteristics, forward voltage and current.  The other one is the voltage you have to use it on.

Then subtract the LED forward voltage from your available voltage.  Calculate the resistor you need by dividing that remaining voltage with the current the LED draws.  You probably won't come up with a standard resistor value.  So choose one with the closest value.  If you need to choose from two values choose the lower one.

[MarkG]

Thanks for the info. I'll have to go and buy a couple of LED that I know the values for. These were in a ziplock in the bottom of desk drawer, unlabelled. Now that I know how to calculate the resistor I need, I should be good to go. I am still a little scared about drilling into the chassis plate but that's for another day.

Mark.

[Grainger49]

You can probably check the LEDs you have.  Put 3 AA cells in series, that is 4.5V.  Then attach the LED with a 1k ohm resistor in series with it.  It will only light one direction and not pass any current the other.  You can measure the voltage across the LED and across the 1k resistor.  The current is the voltage across the resistor divided by the value, 1000.  The voltage across the LED is what it will normally drop.

The 1000 ohm resistor might be too big.  Maybe a 560 ohm will do.  But you get the idea.

[Laudanum]

Could always use a lighted switch.  Might take a little searching to find one that drops right in but Im sure Mouser or Digikey would have some.  That would take care of the on/off verification without having to drill or punch another hole.  Of course, it isnt retro so if looking retro is part of the equation ... nevermind  :-P

[MarkG]

More retro-modern is the look I'm going for. Several of my other components have a nice blue led on them and I was trying to match it. I think I have a solution that will work.