Adding 12vdc LED to tube amp

[POWDOG]

Hi all. Newbie to this forum and to LED lights. I want to add a 12vdc LED module to my tube amp to backlight a Plexi panel. The module contains 3 LEDs, needs 12vdc and draws 45mA. Is there a way I can wire this into my 6.3 volt heater winding? I think the circuit requires a diode and resistor, but like I said I have no experience with LEDs. If anyone knows how to accomplish this and could draw out a schematic I would be very grateful. Thanks for your time.

[eleventeen]

"3 LEDs" should only require about 1.2 volts [each] at maybe 15 mils [each] which is 1/30th what you are requesting, so I am going to assume that you have 3 STRINGS of LEDs, since "3 LEDs" would look like 3 pilot lights and in my imagination would not be enough light to light up a panel...though they would do better lighting up the edge of some plexiglass.


Yeah, you should be able to use a simple circuit known as a "voltage doubler" (easily findable on GOOG images) to power up your LEDs. Your AC source is the two wires of the filament winding. You need 2 diodes, two caps. Doublers are drawn in about 6 different ways, but they are the same circuit, 2 diodes, 2 caps. It will draw (using your numbers) 2 * 45 mils = 90 mils, very little add'l load on your tranny. 


LEDs are easy to use but they absolutely require a current limiting resistor in series with them. You must investigate whether whatever you have has such a C-L resistor internally, or what. Do NOT just throw 12 volts onto whatever you have without C-L, the thing will blow instantly. Hopefully, your doubler will produce something like 15 volts, you place a 66 ohm resistor in series with the supply (could be a 50 ohm, 75 ohm, etc) and drop 3 volts. The 15 volts will not blow it up, but a lack of C-L most assuredly WILL blow it up. LEDs are of course polarized, they have a + and a - and have to be wired properly to work. Wiring them backwards will not hurt them, they just won't work.

If you have a certain product you're installing, folks here can help you with this question of C-L. Do not ignore it. Post a brochure or link to mfr's literature if you have further Q's.

[TIMBO]

eleventeen has nailed it, If your string of LEDS are already set up to be connected to a 12v supply a simple PSU is easily made or most electronics store would have a simple off the shelf modules.

[eleventeen]

[POWDOG]

This is the LED module I am planning to use. I'm starting to think it might just be easier to use a 6.3 filament incandescent bulb.

[eleventeen]

I dunno about "easier". Probably not, all told.


First, unless it says different on the instructions on the back of that item, I would assume fairly confidently that your current-limiting resistor is in there. Otherwise people would be blowing these things up, and they don't. So that issue I consider settled.



Next, you have to tell me/us how many 6.3 volt lamps you think you will need. Is this a little 4 or 5 inch square of plexi like on an old B-15, or is it a full panel across the entire front of a Marshall-like head? The issue is, how many lamps do you suspect you would need? Those lamps draw 150 mils each if they are #47's. They draw 250 mils in #44. One, two, OK. If you need 6 of them, that could stress the PT. It would not surprise to find that #44's can melt plexi if they are close.


Frankly, the biggest issue I would have with 6.3 volt lamps is that they are kind of a pain to mount.

[PRR]

Gosh, I didn't know Philmore was still in business.

Connect those "12V DC" wires right to your 6.3V AC heater supply. Use clip leads. Clip one side, then just tap the other side to complete the circuit for a moment. I do not think it will be harmed. I am 50:50 about whether it will light. If it lights, it may be much dimmer than they intended. The internal resistors are scaled for 12V, and on AC it only lights half the time, even less than half. And an LED on an AC supply will flicker, worse than an old-fashioned fluorescent lamp. Not obvious just standing there, but if you move your head fast the streak in your eye is dotty not solid.

> might just be easier to use a 6.3 filament incandescent bulb.

That's so 19th-century. You have to keep a box of bulbs around. LEDs are SO much better in several ways.

It might be easy to get a 10-bag of white LEDs and some 47 and 100 Ohm resistors. Put LED and resistor in series. Put that across your 6.3V. A white LED drops about 4 Volts. Being simplistic, that leaves 2V across the 100 Ohms. 2V/100r is 0.020A or 20mA. That's a good bright light. Actually the 6.3V AC peaks to 9V, giving an instant of 50mA. However half of the AC wave goes the other way and the LED will not light and won't take much current. The average power will be well within the ratings. Technically we violate the Reverse Voltage rating (5V or 7V) but the resistor prevents blow-up. The 100r resistor should be ample light for most stage-work. If you need to light a large area, or play noon concerts outside, more LEDs is a good approach. If you need the MOST light per LED, you can try 47 Ohms, but LED life may be short.

[MadMax]

FWIW, I've been using LEDs in all of my 'on' lights and wiring them to the 120v AC. I wish I knew how to post a circuit diagram here, but it essentially has a 1k resistor in series, then a paralleled LED and 1N4001 (opposite current flow directions), followed by a high voltage, non-polarized 0.47uF cap in series. If it flickers (and I'm sure it does) I haven't been able to detect it.

MadMax

[EL34]

They could be 3.6 volt LED's in series with a small current liming resistor
Most of the 12 volt LED tape lights I have are run that way

Probably would be easier if you just got my ready to go led here.
Nice chrome bezel
use a 180 ohm resistor in series with it to operating it on your 6.3v heaters

http://hoffmanamps.com/MyStore/catalog/Lamps.htm

[POWDOG]

Hey guys. I put together a voltage doubler and ready to power up the amp. Just have a few more safety questions. Do I need to power up slowly to form the electrolytics like I do the filter caps on a new build? Since I wired this LED circuit parallel to the regular 6.3 volt heater string, do I still ground the 6.3 center tap? I used a pair of diodes, not a bridge recto. Thanks again. Cheers!

[eleventeen]

" Do I need to power up slowly to form the electrolytics like I do the filter caps on a new build?"


Not if they're new and if the cap ratings are well over the expected volts. LEDs do not care about how well filtered the supply is. You're aiming for 12 volts. Hopefully you didn't buy 15 volt caps (which would be fine) but instead bot at least 35 volt caps and don't have to think about the issue.


Since I wired this LED circuit parallel to the regular 6.3 volt heater string, do I still ground the 6.3 center tap?"


Yes! The 6.3 CT is tube-hum related. 99% of amps need either the real wire CT or the synthetic one. 99% of amps require this. The LED thing has nothing to do with anything except lighting something up. There's no need to even connect your LED-supply ground to the rest-of-amp ground. All you care about is 12 volts supplied to those LEDs.

[POWDOG]

Muchisimas gracias. I don't know why it takes so long for things to click with me anymore, but I get it now. I will post pics of the results when I have them. I like the relative complexity of the circuitry necessary to make this work, but in hind site, it's SO much easier (and cheaper) to just use a 6.3 bulb. Hey man, I'm old! But the LED has a pretty big cool factor. Thanks guys.

[eleventeen]

I can appreciate the "authenticity factor" of a #47 bulb but they aren't 19 cents any more like they used to be, and of course they tend to burn out at irritating moments. And heat. You can hate solid state with a passion or just not care for it but LEDs, especially as silly cheap as they are, are very hard to dislike.

[jjasilli]

Thanks to all those who posted here.  LED's have always befuddled me.  Running them off AC, DC, series resistors.  It all seemed exotic and opaque.  I get it now!  (BTW this is also useful for model train lighting -- another hobby my grandkids got me into.)

[EL34]

It's way easier if you take the led's out of a series arrangement and use them in parallel.
Then you can use the 6.3 vac heater supply


Most likely you have 3 x 3.6 volt LEDs
3 x 3.6 = 10.80 volts with a small current limiting resistor

That's how all my 12 volt LED tape lights are run
The ones you see people using on cars are all set up the same way


Use my LED calculator here to figure out what you need for a current limit resistor
http://el34world.com/php/LedCurrent/LedCurrent.htm

[HotBluePlates]

I want to add a 12vdc LED module to my tube amp to backlight a Plexi panel.

I'm late to the party, and the photo doesn't accurately capture what it looks like, but take a gander at this. It's really more of "side-lighting" than backlighting.

Let us know if you finally got your amp panel lit up & how you did it. There are a few ways to skin that cat, each has its potential drawbacks.

On my amp, the control panel is clear and the preamp chassis is painted a solid color behind it. The panel is reverse-engraved from the side of the panel sitting against the chassis. The LEDs are 5 or 6 individual LEDs in parallel, hooked up to the 6.3vac heater wiring with a current-limit resistor in series with one lead to the parallel set of LEDs. You can see the LEDs in this post, which kinda looks shoddy, but is concealed from view between the chassis and cabinet. The whole panel takes on a faint glow (because the acrylic material is not perfectly uniform throughout), but only the lettering lights up brightly.

I copied what Standel did in their original amps in the 50's, though they used #47 bulbs at either end of the clear faceplate. Of course, Matchless did a whole other method of light-up lettering in their amps which is excessive work & bulbs & current & hummy wiring by comparison. Others made light-up faceplate by having a clear panel which had a colored layer of plastic (or paint) on the front surface, through which lettering was engraved to reveal the lighting.