Headlamp Bulbs for Vintage Bikes

By LoneStar

 

Many riders of vintage bikes find the original lighting equipment a bit disappointing, compared with modern lighting that can scorch paint at 300 yards. I'm one of these, so have been looking into options for a brighter headlight on my Series C Black Shadow. This discussion is based on my own search, but I hope it also will be of use for those in similar situations.

By way of background, these bikes were supplied with Miller model 74CV headlamps, nominally 7" but with an actual reflector diameter of c. 6.5". The size means the reflector can't be replaced with the more common 7" units, which are too large.

The original Miller reflector used a bulb holder (part # 106B) accepting a large bulb with a BA15d base (Lucas 168); it was inserted from the front of the reflector. Modern replacement reflectors, though, tend to be designed for British Pre-Focus (BPF) bulbs, as used on most British headlamps from the mid-1950s, including the Series D Vincents. My bike has one of these units, also an original Miller 6V 50 watt dynamo - so I looked for a 6V bulb with a BPF design These were the alternatives I compared:

  • Tungsten original-style 30/24W BPF bulb made in UK by Alite - Lucas 312 equivalent
  • Halogen 35/35W BPF bulb, made in India - not branded
  • Halogen 35/35W H4-style bulb with BPF base, made in Taiwan by Emgo
  • LED bulb with BPF base, from Ebay seller in Spain - not branded

 

I ran four tests on these:
  • Aimed the beam at a sheet of white paper c. 6 feet away, and photographed the beam pattern on high and low.
    This was to establish the shape of the beam. It needs to be focused forward in a tight pattern, and the low beam should be cut off to avoid blinding oncoming traffic. Note the photos aren't an indication of relative brightness, since the camera adjusts automatically.

  • Measured the distance between the filaments and the bulb base flange
    The light pattern depends on coordinated design of the bulb and the reflector. The reflector is a parabolic shape, which reflects a point source of light (like a filament) directly forward in a (nearly) parallel beam. This requires that the filament be in a specific position relative to the reflector, which is achieved by the British Pre-Focus specification as applied to bulb, socket and reflector. The wrong filament location will result in an unfocused light output.

  • Measured the current draw at 6.5 volts, to determine actual watts used
    This is important for anyone running an original dynamo, where output is limited especially at low rpm.

  • Measured the relative light output with a light meter (cell phone app)
    The cell phone app registers light in lux. The sensor was positioned just above the headlamp and aimed at the white paper. The figures don't correspond to universal measurements like lumens, and are probably subject to variation based on factors like how tightly the beam was focused on the paper. Still, they should be a useful indicator of relative output.

 

Tungsten original-style 30/24W BPF bulb made in UK by Alite - Lucas 312 equivalent photo photo
photo

Low

photo

High

Filament Height, inches - Low Filament Height, inches - High Watts - Low Watts - High Output, lux - Low Output, lux - High
.844 .844 24.2 32.5 19 22
This was a 30/24W bulb, as specified on the Series D Vincents. It served as a benchmark, with the hope that something better could be found. Nothing wrong with it, just not very bright.

Halogen 35/35W BPF bulb, made in India - not branded photo photo
photo

Low

photo

High

Filament Height, inches - Low Filament Height, inches - High Watts - Low Watts - High Output, lux - Low Output, lux - High
1.00 1.00 23.7 23.7 16 16
This bulb, despite being made by skilled bulb craftsmen in India, had two problems: it was actually only 24W, and the filaments were in the wrong location. This resulted in a weak and unfocused beam - note lower output than the tungsten bulb.

Halogen 35/35W H4-style bulb with BPF base, made in Taiwan by Emgo photo photo
photo

Low

photo

High

Filament Height, inches - Low Filament Height, inches - High Watts - Low Watts - High Output, lux - Low Output, lux - High
1.045 - 1.155 .728 - .875 30.0 32.2 18 31
An H4-style bulb, with lengthwise filaments and a shielded low beam. One result of the design is that the low filament is wrongly-located for BPF; another is that the low beam shield cuts down light output significantly. Still, the high beam is pretty effective. I've had quality issues with these, though - the solder joint at the contacts is poor and tends to fail.

LED bulb with BPF base, from Ebay seller amazonacycles - not branded photo photo
photo

Low

photo

High

Filament Height, inches - Low Filament Height, inches - High Watts - Low Watts - High Output, lux - Low Output, lux - High
.75 - 1.10 .75 - 1.10 5.7 9.9 27 47
This is the most popular style of LED BPF headlamp bulb available. I bought it from Ebay seller amazonacycles in Spain. Similar 6V BPF units are sold (late 2021) by Paul Goff in the UK, and Clasic British Spares and Lowbrow Customs in the US. Most are claimed to work in either 6V or 12V systems. While they all look similar, there's no reason to think they perform identically. Relevant issues with LED bulbs:
  • Color temperature - This determines the color of the light; the higher the number, the more blue the beam appears. Standard tungsten or halogen bulbs are c. 3200K.
  • Watts drawn - Normally this is much lower than a filament bulb, so not an issue for dynamo capacity. It may be useful as a proxy for light output, though, compared with other LED units.
  • Light output - This is specified in lumens; generally the more the better. For dual-voltage units, is output the same at 6V and 12V?
  • Low beam behavior - How is the low beam produced? These have two banks of LEDs, so one might expect only one to be active on low beam - not always the case.
I found none of the suppliers had full specifications on their LED bulbs listed on their sites, and most had little detail to provide in reply to a query - disappointing. I chose the Ebay bulb because it has a color temperature of 3000K, and I wanted to avoid the blue tint found on most others - it stands out on an old bike. The bulb I bought has excellent output and low current draw, but it falls short in a couple of areas.

First, the high and low contacts are reversed relative to a standard BPF bulb. This isn't a big problem, since you can either reverse the wires to the bulb socket or just live with the switch being in the wrong position. More significant is that there is no dip in the low beam. High and low both employ both LED banks, so the beam direction is identical; the low beam is just dimmer. This could be a problem for night riding, but my main concern is a bright daytime running light - so I'm reasonably satisfied with the unit.

Rex's Speed Shop in the UK offers a different design of BPF bulb. It aims light forward through its own lens, rather than backwards to the reflector as is usual. You could paint the reflector black without affecting output. I haven't tested this, but would be curious about the low beam dip.

 

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Copyright 2021 by Dave Hartner