The 3.5V 150mA unobtainable bulbs

[Boater Sam]

Tried for ages to buy some of these bulbs for the Bush DACs and VHF80s without success.
200mA, 300mA no problem.
Have a VHF80 on the bench now, no bulbs, been robbed I assume.

Any ideas how to use LEDs as replacements?

I was thinking 2 LEDs back to back reversed in 2 old bulb shells with a shunt resistor but its tricky, the 2 original bulbs are wired in series across a 680 ohm resistor and a Brimistor in parallel in the heater chain.

Without any bulbs in there is 40V across the resistor. The valves are then under-run. Using 200mA bulbs, the valve filament voltages are correct.

Trying to get my head around the 100mA current for the valve heater chain, the volts drop across LEDs at 20mA, the required shunt value and of course not having any bulbs, I can't tell the current shunted through the resistor/Brimitor. And that will change as the Brimistor warms up!

Even with the correct bulbs they must be dim, 150mA bulbs on a shunt off 100mA chain seems wrong.

[Cobaltblue]

Hi Sam

Don't forget on a DAC90A the HT current goes through the dial lamps as well the shunt ensures they are slightly under run.

They however as dim as a TocH lamp

Cheers

Mike T

[Boater Sam]

Yes, I knew that, makes the calculations even more difficult.

I have seen TocH lamps brighter than DAC bulbs! Yes, I am that old, thank you.

[JacKam_]

Hi,

I know where to find 200mA bulbs , please open this link :

https://allegro.pl/listing?string=za...a-uni-1-5-0511

[ThePillenwerfer]

I didn't bother with any maths for LEDs for a DAC90a; I just did it and it they work.

I bent the LEDs to point towards the back of the light-box do give better diffusion. I later found they are even better if you file the tops flat.

[Boater Sam]

I have loads of 200mA thanks for the reply.

I thought of using 2 leds rather than 1 and a diode, they both light then. But will I get enough current to run the heaters properly? I think not.

I will have to play around with it and shunts across the diodes whilst monitoring the chain current

[ms660]

Looking at the Trader sheet it quotes bulbs as two 6.3 volt 0.1 amp for the Bush VHF80.

Lawrence.

[ThePillenwerfer]

A blown dial lamp can pass no current but the radio will still work so I can't see the fact that a LED passes less than a working filament lamp making any odds. It's easy enough to try and it's hardly a disaster is you do blow some dirt-cheap LEDs up. If it seems to work you can then take measurements to ensure that all is as it should be.

[Boater Sam]

That makes more sense Lawrence, I use the manufacturers sheet and it doesn't
give any details on the bulbs. Can't win, best choice, Trader or Bush?
But 100mA bulbs seem to be equally nonexistent.

The 6.3 volts makes sense too, as I said, the shunt pair have 40 volts across them with no bulbs in.
I was thinking Bush may have used up all the DAC90A bulbs up on the later AC/DC sets! They certainly disappeared somewhere!

[Radio Wrangler]

There is a dropper in the set and a 100mA heater string.

If you can get 100mA bulbs of almost any voltage, you can use a pair in series and adjust the dropper resistance to compensate. A few LEDs in parallel with current sharing resistors with a second LED in opposite polarity parallel with each one can also be planned to share 100mA. Add shunt one resistor in shunt across the leds (along with their sharing resistors) to rob some of the 100mA and dim thi lights as necessary.

David

[ms660]

Originally Posted by Boater Sam

That makes more sense Lawrence, I use the manufacturers sheet and it doesn't
give any details on the bulbs. Can't win, best choice, Trader or Bush?
But 100mA bulbs seem to be equally nonexistent.

The bulbs are listed in the manufactures manual also.....Parts List> Miscellaneous> 2nd item down.

6.3volt (or thereabouts) 100mA still available so far as I know.

Lawrence.

[stuarth]

LEDs are essentially constant voltage devices, at about 3V for white LEDs. If you just connect a shunt resistor across the LED, it will be very difficult to accurately set the LED current, and small changes in mains voltage and hence voltage across the resistor will dramatically change LED current.

It would be better to choose a shunt resistor to drop, say 6 to 10V at the operating current, and then connect an LED in series with a resistor across the shunt. Varying the LED series resistor will vary the brightness, you don't need to alter the shunt. A 62ohm shunt, and a few hundred ohm series resistor for the LEDs (using the circuit in post 5) should get you started. You could even use amber LEDs to replicate an under run filament bulb, but make it bright enough to see the scale.

Stuart

[Leon Crampin]

6V 100mA MES lamps were used in later bicycle "dynamo" lighting sets for the rear light. They were an update on the original 6V 60mA lamps used in the earliest lighting sets and gave (very marginally) more light.

I managed to find some of these fairly recently in a family run bike shop - on a card and branded "Ever Ready". They worked perfectly in the later Bush AC/DC sets.

Sourcing 3.5V 150mA lamps remains a bit of a problem if you want to retain incandescent lighting. One possibility is to find the older 6V 60mA lamps and fit two in parallel to replace two of the 3.5V lamps in series. I have not tried this, but it could be a reasonably non-invasive fix for the DAC90A scale lighting.

Leon.

[paulsherwin]

I have replaced lamps with 1W unlensed warm white LEDs, though not in a DAC90A. You need to allow about 20mA for decent brightness and they dissipate a lot less than 1W. The main problem is the AC flicker - you really need to rectify and smooth the supply somehow. Maybe you could wire them into the HT line somewhere?

I think mine cost 99p for 10 from the usual Chinese eBay sellers.

[Boater Sam]

I'll do a bit of experimenting, but I have to consider the Brimistor in the heater chain.
Perhaps the best solution all round is to get the heater chain correct and feed the illumination, whether LED or incandescent, from a small mains transformer or a capacitor dropper, or even the cathode of the output valve.
Sam.

[paulsherwin]

The advantage of using 1W LEDs is that they will be running a long way below their maximum ratings, leaving a lot of headroom for temporary overcurrent conditions. I think it's certainly worth ordering a bag of 10 and having a play - they'll come in useful for something if not this.

[electrogram]

I've used 6v 100ma bulbs with, after a bit of experimentation, a 180 ohm shunt. Better illumination than original arrangement but noticeable flicker in time with audio if you turn it up loud.

[stuarth]

I agree with Paul, the LEDs in normal operation will be operating well inside their limits (unless you like really bright dial lights!), so you don't need to worry about what happens as the Brimistor and the valves warm up.

I quite like the idea of putting the LED in the output valve cathode circuit. It's a bigger change to the set, but you'd get an indication when "that" capacitor started to fail😉

Stuart

Or my "solution" https://www.vintage-radio.net/forum/...d.php?t=129025

[Alf]

These bulbs seem to be available at RS