Repairing vintage torch bulbs

Minimoog

Enlightened
Joined
Jan 13, 2005
Messages
771
Sometimes, like last weekend, I get a vintage torch through with an original but non-working bulb. For lights from the 1940's on it is easy to replace with a new one, but older torches sometimes have bespoke bulbs like the 'marble' 1.8 Volt bulb in the Rulag or 'S' and 'U' filament bulbs fitted to pre 1925 lights. Of course you can fit a new one but I have found that you can often rejuvenate the original bulb if the filament is open circuit and it is not sooted up. This will mean that the light stays original and you get to see the vintage beam as intended. Often the bulbs blow when people power them up on alkaline cells and the bulb just fuses with a clean break.

Here's what I do.

1. Fit the bulb securely (tight) into a bulb holder on flying leads at least 2 feet long. Ideally you want one from an old instrument panel with flying leads already fitted.
2. Connect to a regulated PSU with at three Amps output.
3. Set the voltage to what fresh batteries would be so for a 3.5 Volt bulb set to 4.5 volts.
4. Turn up the current to three amps and turn on power to the bulb.
5. Hold the wires adjacent to the bulb holder and keep the holder from flopping.
5. Using a hard plastic handled screwdriver, tap the holder (not the bulb) with the screwdriver handle. Choose carefully - you want a rounded one so if it does tap the bulb it wont break it.
6. Tap firmly in a few different directions - you will likely see a small blue spark which means you are doing it right - but needs to be a bit harder!
7. If you get it right the bulb will come on and stay on - and keep on working!

Main points are:

Make sure you have enough current to weld the filament back together. If the current dips even momentarily then the weld will not hold and will not make a lasting repair. Cold current is often very high and this is what welds the filament back together. Don't use batteries directly, they are just not controllable.
Use a small shrouded holder, not a batten type - you want to send as much shock into the bulb as possible without hitting it directly.
You can check your repair by looking at the filament at glowing point with a loupe. A good repair should be a slightly cooler spot on the filament. Sometimes you get an overlap, which is fine, but will have reduced the voltage rating of the bulb a touch.

Lastly, remember that the voltage rating of old bulbs is often falsely high for added brightness on old battery technology. I am almost 100% sure that older bulbs had tungsten wire that was current, not voltage rated - so 0.2, 0.25, 0.3 Amps etc. and so the bulb designer would ask for certain lengths to be fitted to give different voltages. So you can soon see what the actual voltage of the bulb is by looking at current drawn as when a bulb is lit the current remains fairly steady even with rising voltage, moving around the rating of the wire. Some bulbs will draw EXACTLY 0.3 Amps but some will be less. I have a 3.5 Volt 0.3 A bulb that draws 0.25 Amps at 2.9 Volts and is also bright enough by then so that is the true rating. At 3.5 Volts it is 0,265 Amps and too bright.

Hope this was of some use to you!
 

Minimoog

Enlightened
Joined
Jan 13, 2005
Messages
771
Just happy to share what I have picked up in years of bulb and torch collecting. A good power supply is invaluable for checking and trying out old bulbs so never scrimp when looking at buying one.
 
Top