I have a burnt out dial bulb. The radio is from the 30's and the bulb is powered by 5VAC from the main transformer. It's a screw mount. Was wondering whether any screw mount of the same size would work from jaycar? The diameter is not necessarily the problem but it's the thread that I'm worried about. Has anyone done a replacement before? If you don't think Jaycar will work, can you nominate a supplier?

Possible part is

http://www.jaycar.com.au/productView.asp?ID=SL3211.

Most dial lamps have an MES base, which stands for Miniture Edison Screw and is a standard size.

It is odd that your lamp socket is fed from the 5 volt supply though. If that is the case make sure the radio is off and unplugged before changing the lamp or your fingers will feel a lot more than 5 volts. Most rectifier valves of the era contained directly-heated cathodes which collect the high tension DC to feed the radio circuit with.

Normally the dial lamps are fed from the 6.3 volt supply which feeds the other valves which have indirectly heated cathodes.

The lamp shown on the link you supplied is fine physically. I would recommend choosing a voltage which is just above the supply voltage.

I am also suspicious. A lot of those early ones ran off of a 2.5V rail and were "floating" above the chassis.

Can't recall one on the 5V rail? would be nasty with B+ floating on it.

Could it be a 6V bulb (likely on its rim) & have been moved due to lack of 2.5 V bulb?

Circuit will be the arbiter.

Marc

MES was certainly correct. I measured the voltage and it turned out to be 2.1V, so I got a 2.5V 300mA MES bulb from Jaycar. Works perfect! The supply for the bulb is AC fed directly from the transformer.

Come across these while searching for LED lights for my car. Might suit some situations eg a replacement transformer that is running a little hot for comfort. Warm white is offered with various colours which would suit those radios that have colours for the different SW bands.

http://www.ebay.com.au/sch/i.html?_nkw=mes+6v+led&_sacat=0&_sop=12&_dmd=1&_odkw=mes+6.3v+led&_osacat=0&_trksid=p
3286.c0.m270.l1313.

My interest would be in their voltage requirement. Can they actually handle AC?

Marc

I haven't actually tried them yet. However at least this one is rated for AC, typical 6V, Max 8V


http://www.ebay.com.au/itm/LED-BULB-MES-100-WHITE-6V-Light-6-Volt-100-degree-/220593961981?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item335c6cbbfd.

They are diodes after all so should just rectify the AC; but am not familiar with their technical aspects. Could just add a diode in series anyway.

I believe they have an inbuilt half wave rectifier diode.

If there rated for AC, then there must be the ability to handle the reverse current. Unless there is a full wave rectifier built in; Which would not be dificult with sub minature components etc.

These would be good if they can be applied to some of those radio's that should have round globes, but love burning holes in the cellulose of the dial when fitted with the long ones.

Marc

I think it's PI voltage that would be the issue.

I read somewhere they're half wave so that they can be used for AC/DC. I imagine that current is limited by a resistor as per usual with LEDs. Would be interesting to break one open.

I can't find a local reseller. Pity about the high UK postage. I have bought single $1 items from China with free postage.

PIV was not what worried me. If there is a low, or no cap and it's circuit is permanently loaded that is not an issue.

What is an issue is the reverse current when the AC swings the other direction. This is where a single diode would need to block rather than conduct.

There are regulators like LR8 that can operate from around 5V to 400V input, and they are not big.

Marc

Following up this, I had a power selector knob on a portable, which had been replaced in the past by a brass knob with a hollow spindle, possibly a gas cock or similar.
Instead of filling that and painting over, I decided to put a new LED in the hollow centre, as a power-on indicator.

The original lamp circuit was rated at 2.5 VAC 300mA, and typical bare LEDs are rated at 3 VDC. The only issue is current limitation to avoid burning out the LED. But when I tested with a pot in series, I found the LED added to the lamp circuit would not light up with any resistance in series, but lights up quite safely and sufficiently in simple parallel to the AC lamp circuit. Polarity doesn't matter. As a diode, it just acts as its own half-wave rectifier. The 50Hz cycle is not visible. The 50% duty cycle may actually help with heat dissipation. I haven't tested a 6.3 VAC lamp circuit but would expect a small resistance would be enough to protect a 3v LED. 12v LEDs, sold for auto applications and others, should not need any current limitation on a 300mA circuit.

Maven

It was supplied with 5 Volt Centre Tap, that's how the mistake was made.
Mine had two globes and they did them both in parallel on
the same side of the 5 Volt winding the big dummies.

I thought an LED would look odd in a valve radio,
as if it would stand out, and you'd look straight at the LED
when looking at the radio.

They have come a long way with colours though,
and there are now oranges and Sunset reds that might work.

Here's what the latest Jaycar neons look like:

Image Link

You could probably do a mock magic eye with the green one in a driven circuit.
There's a blue one as well, but I don't see how that's suitable.