Help needed please with 16mm sound film projector.

[raditechman]

When 35mm nitrate film was shown in cinemas, the projectors had a "fire trap" between the top spool of film and the bottom spool of film. This should confine any fire just to the amount of film in the projector head,(gate and sound head), all of which were enclosed in metal. The projector spools themselves were enclosed in metal. There was a fire shutter between the arc lamp and the projector film gate, and metal fire safety shutters on the projection box wall which could drop down between the portholes and the auditorium. Films reels spools not being shown were kept in a metal case away from the projection box.
16mm was always safety film.

[emeritus]

My 1940's edition of the "Ilford Manual of Photography" says that Nitrate was preferred for release prints because it lasted significantly longer than safety film. 1940's safety film became unprojectable after about 120 showings due to perforation tearing, whereas Nitrate became unwatchable after about 180 showings due to excessive scratching. Archival storage properties were not a problem with release prints as they would normally be scrapped before deterioration of the nitrate base became an issue.

[matthewhouse]

I'm back on this again, there is a problem with the amplifier that I am stuck on. The problem is poor output and quite a loud buzz (not a hum) on audio. I'm very new to push pull valve amplifiers, but I have been doing some reading to try to understand how they work.

From research I have worked out the 6SJ7 is a pre-pre-amp for the photocell and microphone input. The 6N7 is a pre amplifier from the 6SJ7 and gram input, this valve has two triodes with a common cathode. In this amp the two triodes are wired in parallel. This then feeds the grid of the 6C5, another triode, used as a concertina phase splitter, the anode feeding one 6L6 grid, the cathode feeding the other 6L6 grid.

Injecting a sine wave into the gram input, I get a nice pre amplified sine wave at the grid of the first 6L6. But at the other 6L6 grid I get a nasty jagged, feeble and distorted sine wave. I've not looked much further due to lack of time, but shorting I think it was the 6N7 grid to ground silenced the loud buzz, and removed the distorted waveform leaving just a small ripple of mains hum which could be heard quietly in the 'speaker.

Am I correct in thinking the buzz is being introduced at or before the 6SJ7, and that there is also another fault at the phase splitter, as the two outputs are not balanced. I believe this is achieved with careful selection of anode and cathode resistors. The voltage at the phase splitter anode is about 230VDC WRT chassis the cathode is only about 4VDC WRT chassis. From what I am reading it should be much higher. There is a carbon resistor in this area that used to overheat, it doesn't now which could be an indication of something.

Just looking this up and typing this out has made it a bit clearer on how this works, but does anyone have any pointers on the best way to locate the problem(s)?

Matty

[PJL]

I would ignore the buzz and focus on getting the DC conditions right. Is there a circuit anywhere? My Ampro uses a driver transformer so completely different.

The fault sounds like it is in the phase splitter bias. However, I would focus on the 6L6 first and check the output valves are biased OK and running at the same anode currents. A 6L6 can easily melt the copper out of your output tx.

[matthewhouse]

Hi, I've not seen a circuit, one would certainly help. I'll take a look at the mentioned points tomorrow.

[Electrone]

Although my query is not directly related to Matthewhouse's problem I am putting this query in this thread as it concerns exciter lamp supplieswhich is mentioned here
I am helping a friend to get a 16mm sound projector working. In this, the illumination for the sound PEC is taken from the projector lamp through a lens system and a prism. There is no 50hz hum on the sound and there does not appear to be a 50 hz filter in the amplifier. (If there was it would have a detrimental effect on the bass reproduction) I have a theory that the projection lamp (and indeed any tungsten lamp) will not give a 50 Hz hum because, as the lamp prduces light by the heating of the filament, when the AC supply passes through each zero point, the filament hardly has time to cool down, thereby keeping the emitted light constant. Yet I know most projectors use high frequency oscillators to supply the exciter lamp which seems to put a hole in this argument, so why does the machine above not produce hum? Can anyone see any flaws in my theory?

[Nicklyons2]

I think much of what you say is true regarding the thermal inertia of the filament lamp; I would be surprised if there was no hum at all though. Given the era the equipment was made in a little hum would have been considered acceptable ( as with UL84 record players). Also, unlike in a 'proper' cinema, 16mm projectors were often used in Church halls etc i.e. not (acoustically) separated from the audience and hence what residual hum there would be would be lost beneath all the mechanical racket. The expense of a sophisticated exciter supply most probably could not be justified in terms of cost, complexity, and sheer additional weight affecting the portability.

[kalee20]

Originally Posted by Electrone

I am helping a friend to get a 16mm sound projector working. In this, the illumination for the sound PEC is taken from the projector lamp through a lens system and a prism. There is no 50hz hum on the sound and there does not appear to be a 50 hz filter in the amplifier.

Is this a GB L516 projector?

This had such a system. On AC mains there is a certain hum - at 100Hz - though the lamp filametn's thermal inertia does keep it reasonablly low. In addition, this projector DOES have a 100Hz filter, only it's in the loudspeaker box not on the amplifier circuitry. It is merely a series LC network across the output transformer (also in the box), which shunts anything at its resonant frequency. Tuning at manufacture was by connecting extra capacitors in parallel, selected on test. Crude, but good enough!

[Lucien Nunes]

I agree that it's a case of what was good enough for the particular purpose. The L516 does not have the best frequency response known to man, later projectors with valve amplifiers were better and more reliant on having a hum-free illumination of the soundtrack.

Lucien