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FWIW,

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Some amplifiers do not have a coupling capacitor in the first stage. Since the first stage is usually a very high gain, the small amount of DC on that control grid will upset the bias and possibly allow the grid to become positive with respect to the cathode.

The end result is audio clipping.

{ Experience aligning a stereo solar cell using a Dolby test film and a dual-trace scope is probably essential first. With this experience, if you don't have a Dolby film and scope handy, it is possible to come doggone close with a 9KHz test film and multimeter ...}

Then, on the cheap a stereo setup for a drive-in booth and surrounding area if you have benches, etc. could be built up with opamp chips [2 on 1 chip about $1] for your preamp stage, feed outputs of these to the L/R inputs of a stereo amp. For the mono signal to FM transmitter use another opamp stage with 10K isolation resistors to each of your preamp outputs.

This is radically oversimplified; other parts will be needed with your opamp chips and if you don't have audio-circuit build-up design experience try to get local help from a techie to do it; still the cost of parts is almost zilch, it's practically like making something out of nothing. A Dolby CP-50 manual is full of apps using opamps if ya wanna see complete working circuits.

I've been out of this for so many years and don't even know if stereo FM xmtrs were ever used in drive-ins or not. Hope I'm not about to get shot for suggesting this...!

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If I design something, the goal is for it to run forever and never need service; because I want to go on to something else and not keep running back and fixing stuff. This was always, and still is, the design philosophy at McIntosh Labs.

But then, nobody's perfect. Last month I had to replace the stepping attenuator [volume control] on my C27 preamp. First repair needed since 1974. Did have to replace a couple of #1866 lamps along the way, tho.

The machine I am currently working on is a Brenkert BX-60 on top of an RCA 9030 soundhead. I've got Kelmar mono cells in the soundhead, and I do have a resistor bridging across the leads of the cell.
In the soundrack I've got a Numark DJ mixer with built-in phono preamps, a Numark twin tray CD player, and a Ramsey FM25 FM Stereo transmitter.

Once I got eveything wired up, film threaded, and the exciter lamps on, I flipped the switch and the sound from the projector is there... but very muffled and noisy - along with a nasty Hum. Grounding the sheilded cable to the mixer helps on the hum, but the sound is still muffled. Are both exiter lamps susposed to be on at the same time or just one? I don't see a way to light only one.

What am I doing wrong?

there's part of your problem. Does it have XLR inputs for microphones? If so, try connecting the solar cell to the balanced in using two conductor + shield cable and connect the ground(shield) to the mixer side ONLY. This should take care of most if not all of the hum.

Are you using AC or DC to power your exciters? Sometimes running AC will cause a nasty hum with exciters.

Switching can be done is a number of ways: you can leave both lamps lit and switch the outputs of the cells, or you can leave both cells hot and switch the power to the lamps, or you can switch both together.

>>the sound from the projector is there... but very muffled and noisy <<

Check three things:
1. the cleanliness of ALL the optics from the exciter to the cell.

2. Condition of the slit lens (is it full of oil or dirt or both?)

3. Focus of the optics in relation to the cell.

Correcting any or all of the above three should eliminate the muffled sound.

As for the system you are planning on using for the projector sound:

First thing I would do is can the DJ mixer and get some kind of a system designed for cinema use.

Aaron