TUTORIAL Surefire MN series bulb mod.

[Roland Gama]

In this picture tutorial I have replaced the bulb in the MN21 with a Philips 5761.
There are many others bulbs around which can be used.

Thanks for looking.

 

[Justin Case]

Very nice. Did you re-pot the 5761 in the MN tower? Have you seen FM's MN bi-pin adapter?

 

[Roland Gama]

Yes Justin, I have potted the 5761 with POP after shaving off a little bit of glass off the bulb here and there with a high speed dental turbine using diamond drill. Yup- dentist.
The main problem arose with the solder not holding to the bulb terminals.
Solved it by winding copper wire around it and then soldering it.

I have seen FM holders but the difference is this comes for free!!!
Besides it just takes a few minutes when you have the right tools around.

 

[wquiles]

Yes Justin, I have potted the 5761 with POP after shaving off a little bit of glass off the bulb here and there with a high speed dental turbine using diamond drill. Yup- dentist.
The main problem arose with the solder not holding to the bulb terminals.
Solved it by winding copper wire around it and then soldering it.

I have seen FM holders but the difference is this comes for free!!!
Besides it just takes a few minutes if you have the right tools around.

Nice work. It is great to see otherwise "scrap" turn into something useful

What is the "POP" potting compound?

 

[^Gurthang]

Roland,

Nicely done! A very well execuited mod. Any chance of a beamshot comparison?

 

[Justin Case]

The main problem arose with the solder not holding to the bulb terminals.
Solved it by winding copper wire around it and then soldering it.

Molybdenum is hard to solder because it forms a very adherent oxide layer.

What procedure did you use to get and secure the 5761's filament at the right focus height prior to potting?

 

[Roland Gama]

Feel elated getting a compliment from the maestro!

POP- plaster of paris( in my case 'dental grade' plaster)
Easy to work with, sets quickly, more resilient to heat than necessary for the job, tough enough to hold besides being supple enabling easy removal for the next bulb change.
Let it lose its moisture before using as it can fog the lens or let it burn a couple of minutes before placing it in the torch.

Beam shots done before by DM51 https://www.candlepowerforums.com/threads/204157

Roland.

 

[Justin Case]

What is the "POP" potting compound?

Probably Plaster of Paris, a common material that dentists use to make casts from molds.

 

[wquiles]

Thank you Roland and Justin - learned something new today

 

[Roland Gama]

Oops missed Justin's question on filament height.
I used the vernier calipers to measure the filament height from a standard MN21. It is 22mm from the base of the holder where it contacts the reflector to the centre of the filament coil. Also experimented moving the bulb in the reflector in different positions before coming to a definitive conclusion.

The philips5761 has a bigger spot compared to the osram64250 although the osram's filament is longer. I attribute this to the philip's broader coil.
Personally prefer the osram.

 

[Justin Case]

That is interesting. For various LED towers I have, it appears that ~23mm gives a better focus. If I back out the tower from the KT TH, I get a donut almost immediately, which suggests that 22mm isn't high enough.

For a WA01274 bulb in an FM MN adapter, the top face of the flange to filament center height also is ~23mm to get a good beam. There is some uncertainty in this measurement due to parallax and the glass bulb envelope's optical distortion. But I'd say to a high probability that the filament height is more than 22mm for optimum focus.

 

[Roland Gama]

Fully agree with you Justin. It's a bit finicky holding the vernier and the bulb to get a dead on centre measurement.
Neither did I delve too deeply into that dept.
Given the time I will machine some bulb holders having a threaded disc for filament height control.
The advanced engineering plastic used in the MN21 looks like semitron.
Have to figure out the glue used in it. Have any info on this?

Would be of interest to me to know which leds you have tried in the M6 reflector and how do they compare?

Thanks for the input

Roland.

 

[Justin Case]

I look forward to a definitive measurement for the filament height.

I've run a Seoul P4, Cree MC-E, and Cree XP-G in my various SF THs, including the KT4.

The XP-G gives the brightest hot spot, with the P4 second. The MC-E, when driven appropriately, delivers the most light. The MC-E produces a fairly large, bright hot spot, just not as intense as the others. All three produce nice beams in the KT4. The KT4 IMO has inferior thermal transfer performance vs a KT1, and this can impact using an MC-E tower driven at full power (i.e., 700mA drive per die).

The Seoul P4 is sort of the standard LED. Not the brightest, not the newest. But it works well, and installs easily. The XP-G may need some height adjustment, depending on what tower kit and what MCPCB you use. Also, if the XP-G is on a 10mm diam MCPCB, you have to trim it down, which is a hassle. The MC-E can be wired in 2S2P or 4P. That is extra work. You also have to grind down the corners of the MC-E's case to fit it through the KT4's opening, which is straightforward but somewhat tedious work (also has a risk of slipping and marring the dome).

 

[Roland Gama]

You are perfectly "spot on focus" Justin!!!

It's 23 mm. Did another 64250 and it's the tightest.


Thanks for all your help including led info.

Roland.