BTH1     BTH2

A couple of shots of a BTH 452. There was a series of similar machines, going back at least as far as 401, and including ones with mag sound as well as optical. All were the same basic design, but with many detail differences. I had four, each with a different valve set. They have irritating mains connections, ranging from the B&H Jones plug type on the 452 to seven-pin speaker type on the earlier machines. The pin configuration on the Jones plug is not the same as B&H use. The seven pin type have me utterly confused, as each pin has its own wire, yet there can only be a max of 3 wires in from the mains (or is it 110v for some with extra wires in and out for a resistance?). Does anyone have the workshop manuals and/or circuit diagram? I really don't want to do more damage than time and neglect has already wrought.

I am currently in the throes of trying to sort them all out. The amps all have the main section on rubber mounts to avoid projector vibrations affecting the sound, and are actually easy to remove once you figure it out - only about three screws and the entire unit lifts out.

I suppose a major part of the appeal of these machines is the decidedly eccentric layout. That curving film guide over the claw mechanism is not something I've seen anywhere else. A single-sprocket sound machine is also relatively unusual, with the lamp at the back reflected thru, as for many silent machines.

Chris McCabe has provided some useful info on these projectors:-

"The 452 was a mains voltage machine; the 450 and 451 (mag/optical with the extra preamps in a wooden plinth) were 110v. The Belling Lee 7 pin plug assembly was needed because not only did the transformer case supply 110v for the motor and lamp, but also the entire exciter and amplifier voltages, including a U52 rectifier valve in the case. Therefore, the projector main unit is not self contained. Unfortunately these power supplies are usually missing. I have a nice 451 in this condition. If you come across such a power pack spare please let me know. I do have the technical data and service book for the 450 series around somewhere if you like I will try to find it if you want the pin connections. Obviously one can lash up the machine to run motor and lamp from 110v just to see the wheels go round and the other circuitry could be provided but frankly although the machines look nice and contain a lot of engineering (like the gear driven fan!!!!) the results can easily be bettered by a 601." 

Bill Kilgour sends some pix of a very rare beast, a 451 with magnetic sound and a mag sound unit bolted on underneath.

bth3     bth4     bth5     bth6     BTH7 

I don't think I've ever seen a BTH transformer before; as Bill points out, the amp is not just powered from the tranny, its on/off switch is there, too. Even rarer must be the Xenon version I found in a mag.


This is a superb pic of an earlier model, courtesy Ron Ashton. 

BTH 301

Bill Kilgour has provided a write-up and many pix of a BTH 301, which looks like he's restored it to a very high standard. All the stuff in quotes below is Bill speaking.

" The BTH 301 16mm optical sound projector arrived in the autumn of 1947 from a company perhaps more famed for its professional 35mm cinema machines. With impressive black wrinkle finish and stripes running over the mirror box and lens holder, it had something of a streamline art deco appearance. BTH used similar stripes on their 35mm SUPA cinema machines. (These were single unit projectors, not made up from separate parts such as picture head, sound unit and base, which might be from different makers.)

BTH9     BTH9a     BTH10     BTH11     BTH12     BTH13     BTH14 



 BTH15     BTH16     BTH17     BTH18

A brand new 301 would have set you back £220, a tidy sum in those days. Competitors included the Bell & Howell 601 at £237 and the Ampro Premier 20 at £213. The BTH was not the quietest 16mm machine on the market but, to be fair, it was not designed for the coffee table in the living room. It was more at home in the back of a village hall, operated by the mobile film units that toured rural areas in the forties and fifties. I spoke to the owner of a business that hired out these machines, who spoke very highly of them and told me they would just run and run. If one did seize up, it would go in to the workshop, get doused with oil and be back on the road again. I got the feeling that, unless a machine actually seized up, it never would get into the workshop.


BTH19     BTH20 

The 301 can be quickly set up ready for use. With its single sprocket, the machine is surprisingly easy to thread, especially if you have experience of lacing an Ampro Premier 20, and the 301s simple threading path path is kind to film.The gate tilts fully forward for easy threading and cleaning of the independently front sprung runners. A thumb screw locks the lens in place. (TQ3 take note).

BTH21  The British engineering is of a high standard, using helical gears for the main drive. The mechanism turns smoothly on 14 ball bearing races (16 if you       include the motor). Speed change from 16fps to 24fps is effected by a gear lever placed under the single sprocket. There is no reverse gear. The take-     up  drive is through two large discs with a driving roller in between, a design dropped in the 401 series, possibly because no one could fathom out how     it  worked or how to adjust it. It sports a powerful A.C. capacitor-start induction motor at a time when the competition was still struggling on with           centrifugal sparking contacts for speed control. The mechanism Is very easy to turn over with the inching knob and rolls to a stop after switching off. Nice that.

BTH22 The lamp is a pre-focus, 115volt, 750watt with the light beam turned 45 degrees via a mirror box onto the gate. I'm never happy with reflected light but it  seems to do the job and lamp changing is very easy (when cold). Cooling air is forced round the lamp via a cowling from a fan on the main shaft, another  high speed gear driven fan sucks air in at the base. Fan speed is the same for 16 or 24fps.




 The large diameter sound drum is part gear driven via a felt clutch. A pinch roller above the drum gently nips the film to cancel flutter, rather than press on the drum. The roller swings forward for threading. Adjustable slit focus is incorporated to allow for emulsion on either side of the film (this was early days). Above the exciter lamp is a thumb screw for centering the 4volt 24watt exciter lamp on the sound slit for maximum output. A well-used  adjustment.


One feature included, and I wish other projectors had this, is a sideways tilt screw on the rear right-hand corner, which saves looking round for a beer         mat.I dont like tilted pictures on the screen any more than I like muck in the gate; perhaps Im too fussy.


The intermittent unit is held in place by two bolts with shims to allow adjustment of gear meshing.For sound projection, the dual bladed shutter is used in the           single position, and to counteract flicker at silent speed, the extra blade is opened out. This feature was not available on the first  models. Only two claws move the film through the gate, not the three we now accept as standard but, rememberthese machines would be running near mint prints in the fifties, (mint 16mm print, dream on) so two would be plenty. The spring loaded claws slide over the film surface until lined up  with the perforations, unlike some machines that will then attempt to punch a new set of holes. 

BTH26     BTH27     BTH28     BTH29     BTH30     BTH31     BTH32      


For servicing, the 10watt valve amplifier can be quickly detached by removing one large screw and disconnecting two 4 pin plugs; the photo cell is withdrawn from underneath the machine. In fact, I have never come across a projector so easy to dismantle. The fuses are not in the projector but on the front panel of the 115volt mains transformer, along with a voltmeter socket. When not in use, the transformer lives in the speaker case with the large 12in. speaker. Transformer and speaker together? Try and lift that lot off the floor.


301's turn up on eBay from time to time but never seem to have the all-important transformer, now considered as rare as hen's teeth. The diagram should help those who have the projector but no transformers. The side cover gets lost as well. For some reason, sales of the 301 were never as good as they should have been, perhaps due to the popularity of the American-designed Bell & Howell 601. Now, some sixty years later, a British Thomson-Houston 301 in its majestic wrinkle-black livery is a nice addition to a 16mm projector collection. Thanks to a host of spare parts from Sound Services Ltd (who had acquired the entire BTH stock of bits in the 1960s), the 301 pictured is still working and projects a nice steady picture.

Bill Kilgour"


As a sort of coda to Bill's stuff about the 301, here are pix of a 301 with a blimp. Until seeing this particular machine, I had assumed that the metal machine shown above was all there was. In fact, there was a three-part blimp, presumably to reduce sound levels. It would seem that these rarely survive, and I can see why - it's all a little on the fragile side to withstand the slings and arrows of outrageous fortune. And the projector is fine without it, really - I can't imagine it actually did much good, and even without it, the machine had a handle for carrying and spool arms that folded out of the way.

 BTH34    BTH37      BTH35     BTH36     BTH38


 In the pix above, I have made the machine do a strip-tease, purely for scientific reasons, of course. The front section of the blimp in Pic 1 is secured at the top by a single catch, to the right of the carrying handle. The bottom of this piece is held by two of those partial hinge things, where what would be the centre section of the hinge is incomplete, so that when folded down, as in Pic 2, it comes free. The rear blimp piece and the cover for the speaker use the same kind of fitting. Note in Pic 1 the cut-outs for the sound controls as well as as the rather oddly-placed lamp and motor switches. Pic 2 shows the large cut-out along the top of the blimp component for the spool arm and the threaded film. The back section of the blimp is much more robust, being secured by two substantial catches to the base section of the blimp. This is pretty strong but, particularly if the front section is not in place, can be a bit wobbly since it is not secured to the machine itself. The top of this piece is left therefore with a degree of movement from front to back, which feels a tad insecure. Note also that this part of the blimp has to be folded back at least partially to allow the spool arms to be unfolded. Finally we have the base, secured to the projector from below by two substantial bolts. A cut-out is left for the adjustable front foot, and for access to the photocell, which is removable from below.


I have now found a workshop manual for the BTH 450/451, so here it is. Amendment slips have been added to some of the pages and overlay the original text. I have therefore given "with" and "without" page versions.

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These pix are from photocopies, so not as good as I would like. Originals or good scans thereof welcomed.

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This one is much earlier.


John Fisher is also into BTH and has provided a circuit diagram for an early BTH amp. As a sign of my usual care and attention to you lot, I have taken a v. tatty copy (half is shown) and tidied it up for you. Also an ad and a couple of pix of the machine. It strikes me as very basic and unsophisticated in both design and execution, but I suspect this gave it strength and durability.

 BTH40     BTH41     BTH42     BTH43     BTH44


From early to late. By 1963 there was a much more modern-looking machine, the 700/701, tho' by then going under the AEI banner.

bthaei1     bthaei2     bthaei3     bthaei4     bthaei5     bthaei6