Bell & Howell 626 16mm cine projector

[boater sam]

I looked everywhere, either too old or too new. Most odd, there seems to be nothing for 626  model at all.

[Kalee20]

I've seen one of these amplifiers - printed-circuit board but on a metal baseplate the same size as the previous 606 amplifier's chassis.

I'm rather interested if a circuit diagram comes to light, how the sound exciter lamp is powered. B & H were fond of using a 6V6 as an oscillator and driving the lamp from HF AC (the light source needs to be steady and ripple-free, so 50/60Hz AC is no good). Whether with the extra space, as a result of the miniature valves, allowed use of a metal rectifier and smoothing cap, to run the lamp from DC, I'm curious to see!

[boater sam]

I've seen one of these amplifiers - printed-circuit board but on a metal baseplate the same size as the previous 606 amplifier's chassis.

I'm rather interested if a circuit diagram comes to light, how the sound exciter lamp is powered. B & H were fond of using a 6V6 as an oscillator and driving the lamp from HF AC (the light source needs to be steady and ripple-free, so 50/60Hz AC is no good). Whether with the extra space, as a result of the miniature valves, allowed use of a metal rectifier and smoothing cap, to run the lamp from DC, I'm curious to see!

The whole thing is weird. They run on AC or DC but I have yet to find a rectifier! I suspect that one of what I thought were all UL84 is actually a PY82 or similar, the 4 valves are all the same size.

There is an EF86 next to the photocell, the exciter lamp was very close so I would guess that the EF86 is the "modern" equivalent of the 6V6. There are a couple of circuit diagrams with three 6V6 valves which may be almost the same as this one.

Watch this space, its too cold in the workshop today, if I could find it in the fog!

[Kalee20]

The three-6V6 amplifier uses two in push-pull, the third is just a power oscillator generating HF for the exciter lamp (3W, ISTR).

The EF86 in yours, Sam, will no way be the equivalent for the 6V6. It'll be the first stage of amplification following the photocell. (Probably a 6J7 or an EF37a in the original).

[boater sam]

Checked all the valves, definitely 4 UL84, no rectifier!  There must be a metal or semiconductor rectifier somewhere but this is a 1957 product.

There are also 3 transformers, in an amp that was designed to work on DC?  I can understand an output transformer and another for the exciter lamp generator, but what can the third one be?

[Mike Watterson]

Splitter to drive pair of UL84 in push-pull.

[Kalee20]

Or is one just a smoothing choke?

[Mike Watterson]

That was a thought I had too, but an AC/DC model more likely to use just R C PSU filtering? Chokes are expensive.

[Kalee20]

They are more expensive than a resistor, yes - but as the amplifier is part of an expensive projector its cost is a small fraction of the whole.

And voltage drop is lower, and smoothing is better.

[Ed_Dinning]

Is the unknown transformer for a mic input? These were sometimes used on a projector for PA purposes

Ed