Large 12 volt bulb. - UK Vintage Radio Repair and Restoration Discussion Forum

Kentode · 26th May 2022, 1:31 pm

I couldn't resist buying this on the flea market this morning. It's 2 3/4" diameter and my first thought was automotive, but at approximately 29 amp's I'm unsure.

Anyone recognise it? Can I just strap it across a car battery? What might be the gas?

Radio Wrangler · 26th May 2022, 1:41 pm

High power and high intensity bulbs last longer done as low voltage high current filaments. Some gases act against evaporation of the tungsten. Done in small envelopes for automotive use they need high temperature envelopes like fused quartz, but a stationary one could have a bigger envelope and get away with glass.

Probably for a projector or spotlight.

David

turretslug · 26th May 2022, 5:11 pm

That filament has been deliberately made to be close to a point source, reinforcing the idea of being intended for us with lens and/or reflector assemblies. Also, it looks as though some care has been taken to isolate the filament from shock with those convoluted supports.

Brigham · 26th May 2022, 6:45 pm

That base is often seen on projector lamps.

Joe_Lorenz · 26th May 2022, 10:00 pm

I opt for the projector lamp first hand, too.
350 W in automotive use for lighting would be heavily overloading most ancient vehicle generator systems although that would be a bright lighting on any vintage car.

But maybe that bulb was intended for airstrip beacon lighting systems?

Regards, Joe

vidjoman · 26th May 2022, 10:12 pm

My thoughts were that it might be from a vehicle mounted spotlight used to illuminate aircraft for the gunners. I saw some of these vehicles shortly after the war and were an attraction in local parks etc. to show the work of the civil defense units.

broadgage · 26th May 2022, 10:13 pm

I suspect that it is intended for use as an aircraft landing lamp.
Such lamps are only used briefly as an aircraft is about to land.
A very substantial wattage is needed to give adequate lighting.

Trigon. · 27th May 2022, 9:52 am

On:

https://www.electricstuff.co.uk/bulbs.html

It's described as:-

Quote:

Osram 12V 350W, military, probably for small searchlight or signal lamp. bulb 2¾" dia.

Cheers

emeritus · 27th May 2022, 10:53 am

From drawings of projector lamp bases in the Focal Encyclopaedia of Photography, the lamp base seems to be a "Medium prefocus", which the article says was the most common type of pre-focus base. I came across a mains bulb with this base in a pre-war film strip projector.

Kentode · 27th May 2022, 12:24 pm

Quote:

Originally Posted by emeritus

From drawings of projector lamp bases in the Focal Encyclopaedia of Photography, the lamp base seems to be a "Medium prefocus", which the article says was the most common type of pre-focus base. I came across a mains bulb with this base in a pre-war film strip projector.

Thanks. I think it's more like (F) Mogul prefocus. Does the encyclopedia give uses for it?

emeritus · 27th May 2022, 1:22 pm

Not really, other than saying it is used where a precise positioning of the filament is required. The entry simply discusses the various types of base used for projector bulbs. Evidently the different-sized wings ensure that the bulb will only fit in one orientation.

Note that the base consists of two telescopically-fitting members. I assume that this allows the smaller one to be attached to the glass bulb and then the vertical position of the filament relative to the base can be finely adjusted and the two parts then permanently soldered or welded together. The same type of construction is used in the SCC-base 240V 300W projector bulbs used in my 1960's slide projector. Interestingly, I discovered that the filament of an old 12V 36W SCC car headlamp bulb that does not have thus adjustment facility, that I tried in the projector socket when I was a schoolboy, was precisely positioned at the same point in space as the mains bulb filament. As the proper bulbs only lasted ~25 hours, I sometimes used it instead, powered from linked low voltage secondaries of a large transformer dad had salvaged from an early TV that could deliver the necessary 3A.

VT FUSE · 27th May 2022, 2:03 pm

As a 13 yr old lad an old boy gave me a smallish search light from a ww2 Air Sea Rescue launch that used such a lamp. Can not remember now if it was 12 or 28 Volt given that the RR Merlin power plant would use 28V, but most probably would have a generator for 12V requirements.
Mike

Kentode · 27th May 2022, 4:39 pm

I've just tried the bulb and it can be fitted in two positions 180 degrees apart, unlike type G, but as Emeritus states there's an inner metal cylinder with the locking tabs and a sprung base contact. However, the outer fitting is of a composite not unlike Bakelite so no soldering.

The picture shows the metal inner (red arrows) and what may be an alignment line (blue arrow) on the outer fitting. The two are seized together and as l don't have a runway I won't try to free them !

When I've got time I'll fit crocodile clips and try it on an old car battery. Watch The Skies!

broadgage · 27th May 2022, 5:13 pm

If my supposition about it being used in an aircraft landing light is correct, then correct alignment, not just forward/backward alignment but also rotationaly correct. So as to give the best beam shape.

The small size of the lamp relative to the wattage does to me suggest aircraft landing.
I suspect that if this lamp was operated continually in hot weather, in an enclosed lantern, that the glass would soften and deform.
An aircraft landing light is intended for very short term operation, and exposed to air flow at 100 MPH or so. Modern aircraft use par 36 lamps of improbable wattage relative to the dimensions, they also are only rated for short term use.
Projector lamps have a very compact filament for best optical control, but in a relatively large bulb, to reduce blackening, and to avoid the glass softening.

American type locomotive headlight bulbs were often of surprising wattage at very low voltage, but were also in a larger bulb. Usually 6 volt.

A searchlight bulb also has a compact filament but in a larger bulb.

Daylight signaling lamps came in two varieties, those in which the lamp was lit continually and the light controlled by a mechanical shutter, and those without any shutter but in which the lamp was flashed on and off to send signals in Morse or other code. This lamp does not look suitable for the first sort as it looks unsuited for continuous burning.
Nor suited for the second sort as the very short and thick filament would take too long to heat up and cool down. I have seen a 12 volt signal lamp bulb, it used 9 filaments electrically in parallel arranged in a compact grid pattern. for quick response.

26th May 2022, 1:31 pm	#1
Kentode Octode Join Date: Mar 2015 Location: Yorkshire, England. Posts: 1,301	Large 12 volt bulb. I couldn't resist buying this on the flea market this morning. It's 2 3/4" diameter and my first thought was automotive, but at approximately 29 amp's I'm unsure. Anyone recognise it? Can I just strap it across a car battery? What might be the gas? Attached Thumbnails __________________ Regards, Ken. BVWS member

26th May 2022, 1:41 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,867	Re: Large 12 volt bulb. High power and high intensity bulbs last longer done as low voltage high current filaments. Some gases act against evaporation of the tungsten. Done in small envelopes for automotive use they need high temperature envelopes like fused quartz, but a stationary one could have a bigger envelope and get away with glass. Probably for a projector or spotlight. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th May 2022, 5:11 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Large 12 volt bulb. That filament has been deliberately made to be close to a point source, reinforcing the idea of being intended for us with lens and/or reflector assemblies. Also, it looks as though some care has been taken to isolate the filament from shock with those convoluted supports.

26th May 2022, 6:45 pm	#4
Brigham Octode Join Date: Mar 2006 Location: Co. Durham, UK. Posts: 1,116	Re: Large 12 volt bulb. That base is often seen on projector lamps.

26th May 2022, 10:00 pm	#5
Joe_Lorenz Hexode Join Date: Dec 2019 Location: Hohenroda, Eastern Hesse, Germany Posts: 460	Re: Large 12 volt bulb. I opt for the projector lamp first hand, too. 350 W in automotive use for lighting would be heavily overloading most ancient vehicle generator systems although that would be a bright lighting on any vintage car. But maybe that bulb was intended for airstrip beacon lighting systems? Regards, Joe

26th May 2022, 10:12 pm	#6
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Large 12 volt bulb. My thoughts were that it might be from a vehicle mounted spotlight used to illuminate aircraft for the gunners. I saw some of these vehicles shortly after the war and were an attraction in local parks etc. to show the work of the civil defense units.

26th May 2022, 10:13 pm	#7
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,129	Re: Large 12 volt bulb. I suspect that it is intended for use as an aircraft landing lamp. Such lamps are only used briefly as an aircraft is about to land. A very substantial wattage is needed to give adequate lighting.

27th May 2022, 10:53 am	#9
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,337	Re: Large 12 volt bulb. From drawings of projector lamp bases in the Focal Encyclopaedia of Photography, the lamp base seems to be a "Medium prefocus", which the article says was the most common type of pre-focus base. I came across a mains bulb with this base in a pre-war film strip projector. Attached Thumbnails

27th May 2022, 1:22 pm	#11
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,337	Re: Large 12 volt bulb. Not really, other than saying it is used where a precise positioning of the filament is required. The entry simply discusses the various types of base used for projector bulbs. Evidently the different-sized wings ensure that the bulb will only fit in one orientation. Note that the base consists of two telescopically-fitting members. I assume that this allows the smaller one to be attached to the glass bulb and then the vertical position of the filament relative to the base can be finely adjusted and the two parts then permanently soldered or welded together. The same type of construction is used in the SCC-base 240V 300W projector bulbs used in my 1960's slide projector. Interestingly, I discovered that the filament of an old 12V 36W SCC car headlamp bulb that does not have thus adjustment facility, that I tried in the projector socket when I was a schoolboy, was precisely positioned at the same point in space as the mains bulb filament. As the proper bulbs only lasted ~25 hours, I sometimes used it instead, powered from linked low voltage secondaries of a large transformer dad had salvaged from an early TV that could deliver the necessary 3A. Last edited by emeritus; 27th May 2022 at 1:25 pm. Reason: typos

27th May 2022, 2:03 pm	#12
VT FUSE Hexode Join Date: Aug 2010 Location: Malvern, Worcestershire, UK. Posts: 344	Re: Large 12 volt bulb. As a 13 yr old lad an old boy gave me a smallish search light from a ww2 Air Sea Rescue launch that used such a lamp. Can not remember now if it was 12 or 28 Volt given that the RR Merlin power plant would use 28V, but most probably would have a generator for 12V requirements. Mike

27th May 2022, 4:39 pm	#13
Kentode Octode Join Date: Mar 2015 Location: Yorkshire, England. Posts: 1,301	Re: Large 12 volt bulb. I've just tried the bulb and it can be fitted in two positions 180 degrees apart, unlike type G, but as Emeritus states there's an inner metal cylinder with the locking tabs and a sprung base contact. However, the outer fitting is of a composite not unlike Bakelite so no soldering. The picture shows the metal inner (red arrows) and what may be an alignment line (blue arrow) on the outer fitting. The two are seized together and as l don't have a runway I won't try to free them ! When I've got time I'll fit crocodile clips and try it on an old car battery. Watch The Skies! Attached Thumbnails __________________ Regards, Ken. BVWS member

27th May 2022, 5:13 pm	#14
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,129	Re: Large 12 volt bulb. If my supposition about it being used in an aircraft landing light is correct, then correct alignment, not just forward/backward alignment but also rotationaly correct. So as to give the best beam shape. The small size of the lamp relative to the wattage does to me suggest aircraft landing. I suspect that if this lamp was operated continually in hot weather, in an enclosed lantern, that the glass would soften and deform. An aircraft landing light is intended for very short term operation, and exposed to air flow at 100 MPH or so. Modern aircraft use par 36 lamps of improbable wattage relative to the dimensions, they also are only rated for short term use. Projector lamps have a very compact filament for best optical control, but in a relatively large bulb, to reduce blackening, and to avoid the glass softening. American type locomotive headlight bulbs were often of surprising wattage at very low voltage, but were also in a larger bulb. Usually 6 volt. A searchlight bulb also has a compact filament but in a larger bulb. Daylight signaling lamps came in two varieties, those in which the lamp was lit continually and the light controlled by a mechanical shutter, and those without any shutter but in which the lamp was flashed on and off to send signals in Morse or other code. This lamp does not look suitable for the first sort as it looks unsuited for continuous burning. Nor suited for the second sort as the very short and thick filament would take too long to heat up and cool down. I have seen a 12 volt signal lamp bulb, it used 9 filaments electrically in parallel arranged in a compact grid pattern. for quick response.