"Dial" light and battery valve radios
 

Hi all,

I am having a bit of fun building a battery valve radio and 'need' a dial light, I am thinking of using the 'bias derived from a resistor in the HT- lead' volts to power a couple of white LEDs in series, with a resistor added to get the volts right if needed. This arangement will give a more constant voltage than a plain resistor can anyone see any problems doing this?

Re: "Dial" light and battery valve radios
 

That should work fine, and seems like a pretty neat idea to me. What's the HT current of your radio? Standard 5mm white LEDs will be happy with up to 20mA, but you'll probably find they're annoyingly bright at that current. I've got one on a piece of equipment I've built here used as a power-on indicator which is fed with 0.5mA and it's brighter than I really wanted!

You can always parallel up LEDs to share the current amongst them. Doing this is supposed to run the risk of thermal runaway (LEDs have a negative temperature coefficient of forward voltage) but it's routine practice in LCD backlights and appears not to cause any problems there.

The voltage across an LED is largely meaningless - they're really current-driven devices. Connecting them in series with the HT is a really good and efficient way to light them.

HTH
Chris

Re: "Dial" light and battery valve radios
 

Bear in mind that white LEDs generally don't look right in vintage equipment. They are actually blue LEDs coated with a phosphor, and give a very cold light which is very unlike traditional dial bulbs. Yellow or orange LEDs may be a better bet.

Re: "Dial" light and battery valve radios
 

Good point. 'Warm white' LEDs are also available which have a much softer colour. They're also not so bright, which might actually be an advantage.

Another thought occurs to me: is this the set with a push-pull output stage? If so, how will it be biased? If it's not class A, isn't there a risk that the LEDs will brighten on loud passages as the HT current increases? This could be considered a feature...

Re: "Dial" light and battery valve radios
 

I've got a RGB LED here. It's one watt per LED too. I've had it for a couple of years and keep saying that when I get around to it that I will make up a control unit for it so I can 'dial up' any colour I want.

Re: "Dial" light and battery valve radios
 

Who said it was vintage? They may well be yellow or amber in the end, LED white is a bit harsh.
A big capacitor will cure that! And if I use a pair of LEDs in series with a resistor across one the AGC would make one brighter/dimmer (perhaps)

Re: "Dial" light and battery valve radios
 

I would imagine your strategy would work if the L.E.D.s were in series with the I.F. valve anode load (I.F.T.). The current would ,I guess, be about right if you use low current L.E.D.s The output stage anode current would swamp any perceived variation due to A.G.C. action if they were in the H.T. line I suspect. Tony ;)

Re: "Dial" light and battery valve radios
 

Sounds like it should work.

Well worth looking for warm white LEDs too. So long as the device itself is hidden from view, it's unlikely that anyone would realise that it's actually an LED in there rather than one of those "grain of rice" incandescent lamps.

I'd suggest if it's in a position where failure of the LED s/c could cause problems that a part from the bigger manufactures (Cree, Nichia, Osram, Philips/LumiLEDs etc) is used, as I've found GaN based LED's of cheaper origins (i.e the dealers on eBay selling bags of thousands for not very much money) more prone to short circuit failures.

If it's easily replaced and such a failure won't cause problems this is of course less of an important consideration - so the price would be more relevant!