B.T.H Magneto-Generator Series 'A' Comet

[Michel]

Hello Simon,
Yes, I have the same BTH . On the other hand, the switch is wrong (3 positions only) .
I found a copy of PR18G switch without resistor . Does the resistor is essential? I guess we can add but where and what ?
Thank you.

 

[nkt267]

Now I know this is not what you have on your bike. But to give you an idea of similar systems here is what should be fitted to my 1932 Sunbeam. The dynamo is three brush with NO regulator but has a cut out relay to stop battery discharge.

this may give a better idea of how the swich works..John

 

[Villiers]

I know nothing about all this electrical stuff. My nom-de -plume of 'Villiers' is for good reason. But, (there is always a 'but') I do have Volume II of Modern Motorcycles by AW Judge. Published in the early to mid 30's it surveys all the proprietary lighting and ignition equipment of the day including all the BTH stuff mentioned above. Magneto's, dynamo's, headlights, headlight switches, markings on switches and what they mean and so on. There are also lots of pictures. (Villiers people need pictures.)
I can't effectively put it up here as the info is scattered across umpty-ump pages of three chapters, Chap 3, The Ignition System, Chap 4, The Electrical Circuit, and Chap 5, The Electric Lamps.
I would suggest a good local library search including the VMCC Library for a convenient copy to study at leisure would be worthwhile.
Alternatively I do live in the same city as Martyn of Oz Vincent Review and if he was interested we could possibly print these three chapters in that?
Cheers from happy two-stroke rider

 

[Simon Dinsdale]

Hello Simon,
Yes, I have the same BTH . On the other hand, the switch is wrong (3 positions only) .
I found a copy of PR18G switch without resistor . Does the resistor is essential? I guess we can add but where and what ?
Thank you.

Check the PR18G replica switch has severn connections on the underside. The series B & C Vincent's only required 4 connections and that is what I have currently seen for sale.
The original resistor was wire which was wound around a ceramic. The resistor was missing from mine, so I just used a modern high wattage resistor. I think the resistance was arround 3 ohms but would need to measure it to confirm. you could miss is resistor off, but when the switch is on CH it will charge the battery at a higher rate.
Simon

 

[Michel]

Simon, you mean SEVEN connections ?
No, only four connections... and I just buy it...

 

[Simon Dinsdale]

Simon, you mean SEVEN connections ?
No, only four connections... and I just buy it...

Michel
Here is a photo of the back of an original Series A Comet Miller headlight switch complete with the charging resistor.

Regards
Simon

 

[Michel]

For good, I'm loocking for one Miller headlight switch PR18G juste like the picture.
I'll pay good price (but not too much expensive, please). Someone could help me ?
Thank you Simon.

 

[Michel]

Michel
Here is a photo of the back of an original Series A Comet Miller headlight switch complete with the charging resistor.
View attachment 14512
Regards
Simon

I'm Lucky, I just found an original switch.
Does anyone know the resistance (Ohms) of the charging resistor?
Thank you.
Michel.

 

[Michel]

Michel
Just been taking some photos of my bike for you. Is this the BTH mag / dyno you have?
View attachment 14491

If so the original BTH (not Miller) cutout fitted on top of the toolbox. Here is a picture of mine. You can just see the circle in the top that shows the BTH symbol. The cutout can be replaced by a diode rated at 10A to 15A. The original BTH cutouts are extreamly difficult to find and I don't know of any replicas been made. The cutoput just stops the battery discharging back through the dynamo when the dynamo is not generating.
View attachment 14493
The orignal Miller headlight switch has 4 positions. Off, Ch, H, L. When used with the BTH dynamo, Off, disconnects the dynamo from the battery. Ch connects the dynamo to the battery through a resistor to limit the charge rate. H switches the headlight on and bypasses the charge resistor to allow maximum charge. L switches the side light on if fitted. The 3 position switch you have is a aftermarket copy which was never used on Vincent's and so will not work for this charging system as it is. There are replica 4 position switches available, I think the VOC spares sell them, but these only have four contact positions on the back which is what series B & C used. The switch you need has seven contacts on the back to allow for switching the charge resistor. You may have to find an original.
View attachment 14494

There is a possible alternative. Brian Werrett (in UK) has just completed restoring the Model PS for the club and was shown on the front of June's MPH. This bike also has a BTH with the cutout. Brian is thinking of getting an electronic regulator made by a company which will act as the cutout and also regulate the dynamo output so no charge resistor or switch in the Off or charge position will be required. This regulator will automatically work like the later bikes did, but it will only work on this type of BTH mag dynamo. If you send me your email address in a private message through the forum, I will forward it to Brian.
Regards
Simon
VOC Machine Registrar

Hi again,
On my switch there is no OFF. The positions are CH - D - H - L (left to right).
So I think that the load starts from the start of the engine (CH). Reason why there is the cut-out when the engine is stopped to prevent the battery from emptying in the generator.
I suppose that (H) and (L) are the lamps but what does (D) ?
Michel.

 

[Comet Rider]

Hello Michel
Usually it will be;
CH = Charge only
D = Side light
H= Main light High beam
L = Main light Low (Dip) beam

HTH
Neil