Looking After  Circuit Diagrams




Before we start, note that safety must be paramount. Old amps use high voltages and can kill. Do not put yourself at risk.

I accept no responsibility for you damaging yourself.


I have only ever had a very limited knowledge of electronics; I can just about do electrics, ie wiring motors, switches and lamps, but replacing the odd valve or capacitor, or fitting a solar cell, is about as far as I have gotten and I am forced to rely on others. So I thought that a basic guide to what do with old projector amplifiers could help you to avoid unnecessary damage to old amps. By forcing me to codify what I already know, at least in part, and to learn more stuff as I get additional guidance from those who do understand this stuff, it should help me too. Please excuse me if I get too simple for you I want to be as basic and simple as possible for beginners and for me. I am seeking only to help people avoid damage and overcome a few of the more common problems. For me, anyway, getting beyond that requires the services of someone who actually knows what they are doing electronically. And if you do, you dont need this.

My thanks to Paul Schimmel who has been my technical consultant for this.




The very first point is that you should NEVER CONNECT OR SWITCH ON AN OLD AMP without first taking precautions, both for safety and because some electronic components deteriorate with time, and powering up the amp with damaged parts could do extensive damage, causing far more trouble than following a few simple procedures first. The longer the machine has been out of operation the greater the risk. Always visibly inspect the internal wiring - old rubber insulation can just perish away leading to a real risk of electrocution and/or fire. Whilst inside, also inspect the condition of the large capacitors, any with bulges or leaks will probably require replacement. Note that many machines do not have a separate amplifier switch if you plug it in, the amp is on. (The 9.5 Vox incorporated a power connection into the speaker plug to pre-empt this problem, a useful safety trick - Pathé obviously knew about 9.5ers).

This is not good, for the reason given above, but also because, with valve amps in particular, if you switch them on without a speaker plugged in, this, too, can cause serious damage. (It is possible, however, to replace the speaker with a dummy load of appropriate value; normally 16 ohms is a good starting point). Another important factor is that no amplifier likes being connected to a speaker of a lower impedance rating than it is designed for (higher seems to matter less). This means you should not try to use a 4 ohm speaker with an amp designed for 8 or 16 amps. It is also important to note that if you wire two 8 ohm speakers together in parallel, the impedance drops to 4 ohms. Wired in series the total impedance is 16 ohms. So many machines have been separated from their original speakers that this issue becomes crucial. A lot of speakers are marked but, if not, the ohm range on even a basic multimeter will help. The multimeter only tests using DC and the displayed results will read approximately of the loudspeakers impedance (which is normally measured using a 1kHz sine wave). Eg around 6 ohm on meter is probably an 8 ohm unit, around 12 is probably 16 etc. Some amps - the Heurtiers, for example - can be adjusted, in the same way as for mains input voltage, to feed speakers of different impedances.

Yet another issue with older amps is that the speakers are often mains energised. This means that, instead of the permanent magnet modern speakers use, the speaker coil is an electromagnet, powered by the mains or even by a supply of up to 400v from the projector. Unless you are a complete expert or a Museum, this is one area where changing the original is wholly justified you really do not want 240 -  400v in your speaker cables. (Nor, of course, do you want to try connecting a powered speaker cable to a magnet-driven speaker). This means considering not only a new speaker, but also internal changes to the amplifier to match the output to the new speaker. More on this later.


OK, so what are these preliminary precautions of which I speak? The first step has got to be finding out as much as you can. There are various resources you can call on Cinerdistan is just one; some others are in the links section in Miscellany. Elsewhere, eg Oldtimer Cameras, you may have to pay for projector instructions or manuals, but it could be well worth it. However, for many machines you may be unable to get all the info you need and you just have to check out the machine as best you can. What are the projector and lamp voltages? Many older machines need 110v, but this may not be so obvious if they have lost their original transformer or, perish the thought, resistance. (The GB L516, and presumably its siblings like the K516, used a resistance connected to the side to reduce 240v to 110v for the lamp, but used mains power otherwise). Is there a speaker? Is it marked? Does it have a permanent magnet?

I have 2 GB L516 machines that have been adapted. Either can be connected to a box (left) which contains the necessary circuitry and transformers for the lamp and the speaker. One lead plugs 

amplifiers into what was originally the resistance socket on the side of the machine, to connect the lamp and its transformer. To avoid trying to shoe-horn additional stuff into the main body of the machine, the original speaker lead is used as far as the box, where various gubbins sort things out, so that a normal speaker can just be plugged into the box. This limits the high voltages to a short stretch between projector and box. 

Then obvious stuff like are all the valves there? Is there any visible damage to wiring, eg rotted insulation. I dont much like relying on old wire even if it looks good, you don't know what's going on inside. The next step is equipment. Obviously a basic multimeter, so you can test circuit continuity. Soldering iron and the usual tools. And dont forget to record what goes where when it comesto any dismantling I often draw a sketch of wiring/connections. Even what valve goes where is something I find challenging.



In avoiding damage to old valve amplifiers, a Variac is highly desirable. If you dont know, its a specialised form of transformer that can output anything from 0v to full mains. The big ones are able to take the 10 amps or so that many lamps use, but these are expensive, unless you get lucky with a second-hand one. However, amplifiers dont consume much and a 1 amp Variac should be ample as long as you dont turn the motor and lamp on. The principal value of a Variac is in feeding the amp with a voltage low enough not to cause damage if there are problems, but it also helps to gently re-form the electrolytic capacitors; these can often be revived by an initial low voltage, where an immediate application of full voltage would not do this, leaving them non-functional and leading to serious damage.

I was luamplifierscky enough to acquire a low-power Variac from a friend, tho it was naked. A few years ago, I attended some metal-working classes at a local college. They had big metal slicing and bending machines, so I took the opportunity to cut and bend two U-shaped sheets, so that when put together they would cover all six sides of a cubic box. This was just on spec with no particular use in mind, but they provided a housing into which I could just about squeeze my Variac. The other thing you need with a Variac is a standard 100w house lamp (I suppose youll have to make do with 60w nowadays as 100s are no longer available, or 2 60s in parallel). This is wired in series with the output from the Variac (before it reaches the projector) and serves two purposes. First, it limits the maximum possible flow of current (and therefore the damage that might be caused) and second, if it starts to glow more than just a bit, you know something is going wrong. The mains input is the wire coming in from the right, the controlled output goes thru the lamp, via a standard connector so I can just take the lamp out of the circuit when I want.

Using a safe level of feed from the Variac, the valves will receive enough power to operate the heaters, so you will be able to see the glow. If you cant, something is wrong. Paul Schimmel guided me through testing a Danson in this way. One valve persistently failed to heat up; in the end it turned out that only one of the three machines we had available on the bench had a functioning version of this valve and applying full power would undoubtedly have done far greater damage. S-l-o-w-l-y bring up the voltage to the amplifier, using the Variac, whilst listening to the speaker and looking for a glow from all the valves. Stop immediately if there are any signs of trouble such as obvious burning smells, smoke etc. Hopefully as the voltage gets above 80% youll be rewarded with a bit of hiss and hum from the louspeaker. Keep advancing the voltage, once at 100% leave it to settle and maybe try putting a piece of paper in between the exciter and PEC. Hopefully youll be rewarded by a plop noise from the loudspeaker. If all seems well disconnect Variac and try a film. If sound is excessively humming, the main electrolytics may require replacement; if sound is distorted the paper coupling capacitors may be leaky. See below for more tips. Do not run an obviously faulty amplifier for any length of time in the hope it will get better: you run the risk of damaging the valves, the transformers and yourself.

AC/DC Amplifiers


Although Britains mains supplies went mainly AC from the 30s on, for reasons of versatility and/or cheapness and/or lightness some machines have what is known as an AC/DC amplifier. In this type of design the mains transformer is dispensed with and the amplifier connects DIRECTLY to the mains. This can be highly dangerous and live amplifiers should only be worked on if powered up via an Isolation transformer. Early machines often have unpolarized mains connectors and if you are unlucky the amp chassis will be live! Note it is normal to mount the amplifier chassis to the main chassis via insulators and fit plastic knobs so that the ordinary user is not exposed to danger. Projectors having AC/DC amplifiers include the GB L516, Ampro Educational and (in a very sophisticated form), the B&H 631 series.

Remember that a Variac is not a transformer in the conventional sense and offers no isolation from the mains.




We need to consider the types of valve you may encounter and what they do. Valves basically do four things in a cine projector amp:-

Rectification of the incoming AC current to provide the DC the amp needs;

Pre-amplification of the various inputs to the amp (photocell, Microphone etc) followed by further amplification until the signal is large enough to drive, inevitably:-

Power amplification, the output stage of the amp. Here the signal is given its final boost up to sufficient watts to drive the loudspeaker via the output transformer. Simple amplifiers may have just one power valve, more advanced designs will have two power valves working together in what is known as push-pull. Generally speaking any amplifier delivering more than about 3 watts will be push-pull.

In addition to the above some amplifiers also use a power valve to supply the Exciter lamp for reduced hum on optical sound.

Here is a list of the sort of valves you are likely to find in each of these categories:-

Rectifiers: 5Y3, EZ40, EZ81 etc

Pre-amplifier valves: 6SN7, EF37, EF86, ECC83 etc

Power output valves: 6V6, EL40, EL84 etc

There are also dual types such as the ECL82 which combine a pre-amp and power amp in one glass envelope. 

Obviously, the earliest valves you are likely to find in a projector will date from the early 30s, unless you find a very early sound-on-disc machine. The earlier valves tend to be large, glass-envelope jobbies, or smaller, metal-envelope ones, with a large central pin, with a locating stripe down the pin, and four to six moderate-size pins of about 3mm diameter or so. Later valves are smaller and have thin, almost wire-sized pins, with a gap in the circle of pins that deals with location. There are all sorts of minor variations; some have a top connection as well, some are fitted with metal shrouds. One type has pips moulded into the side of the glass envelope to assist correct location. Some pix and more info are at the end. 



These are the most frequent cause of problems with old amps valves fail far less often. The problem is that, as they age, capacitors become less and less able to deliver their intended performance. This can result in loss of volume in the output or more serious problems such as allowing the wrong voltage to pass to a valve and overload it. A vintage amplifier will normally contain large Electrolytic capacitors used in the power supply circuitry and smaller paper capacitors used for coupling signals between the various stages. Electrolytics can suffer from drying out and lack of use can cause their capacitance value to fall. Often they can be revived by re-forming. This involves gradually ramping up the voltage applied whilst limiting the current in the circuit, using the Variac in the way described above). A reformed electrolytic should draw less than 1mA, but it may take 24hours or so to reach this value. If the capacitor is visibly leaking or bubbling then it should obviously be replaced, but it is cheap enough and usually relatively simple just to replace them all as a matter of course. The main smoothing capacitor is the one located immediately after the rectifier and the most likely to overheat and fail if the amplifier is not raised gently from its slumbers in storage. 

Sometimes valve amplifiers make use of multi-section devices having 2 or 3 capacitors mounted in one can with colour coded tags to identify the different sections. The colours are normally described on the can itself. The outer capacitor is the section intended to be connected to the rectifier. Paper capacitors, on the other hand, are prone to absorbing moisture from the atmosphere and becoming leaky. In particular a leaky capacitor connected to the grid of an output valve can ruin a perfectly good valve and/or transformer. Suspect paper capacitors should be replaced with modern Polyester or Polypropylene types with voltage ratings equal to, or greater than the part being replaced. To check for leakage, measure the DC voltage on the grid *after* the capacitor. For a typical output valve, this voltage should be below 1v. Any paper capacitor made by Hunts should probably be changed on sight as these are notoriously leaky.


With age, resistors can drift high in value. This doesnt seem to be so much of an issue in cine projectors compared to domestic radios. Presumably something to do with the latter being in heavy daily use. Resistors should be visibly checked for signs of overheating, any suspect can be easily checked with a multimeter (with one leg removed from the circuit). More detailed diagnosis will require a circuit diagram. For the absolute purist, there are originality issues here, too. I have heard that old radio buffs will go so far as to hollow out old, duff caps and fit modern ones invisibly inside to maintain appearances. This is a bridge too far for me, tho I admit I would like to retain the old design of an amp and the valves, which are very often available, maybe even the photocell. Bear in mind these will have been specifically designed for a specific projector and can therefore be good. They can also be poor, of course, but there is no guarantee that a non-specific amp will do that much better.

amplifiersHere is an example of a projector amp, a Danson. Originally Italian made, some parts of it came to be replaced or added in this country. This is one of a batch of no less than four that I had on hand. There are various minor differences, but nothing very major. The oddest feature of the Danson is its switching. There is a switch on the base of the amp, just inside the door. On one machine this is a push-to-make switch that presumably acts as a cut-out when the amp door is opened. Others have a standard toggle switch; this may, of course, be a customer modification rather than an original feature. There are switches on the front panel for sound (this actually operates the exciter) and projector. But the projector switch does nothing until the on-off switch incorporated into the tone control knob is switched on. The valves light up whether this switch is on or not, tho this could be just the heaters.

amplifiersamplifiersA couple of shots of the amp out of its shell. Removal is mostly straightforward, four screws from the bottom of the machine to undo and two connectors, a multi-pin job next to the tranny and a co-axial type, which is the feed from the PEC. BUT BE WARNED! It is essential to extend the front feet of the projector (used for tilting) to their maximum extent before moving the amp. You can see from the absurd capacitor standing right up from the rest of the amp how hard they worked to shoehorn the amp into a very confined space; another problem is that the big tranny tends to foul the socket for the front feet and, if not unscrewed, the shaft of the foot would prevent removal of the amp altogether.

Note that on this amp, the speaker socket is a jack that I fitted instead of the unusual 3-pin Danson type, which I don't have for this particular machine. I also had to replace the fuse carrier next to the tranny at the back; it was loose because the threaded plastic body was disintegrating. There was also a problem with this machine at the amp inputs behind those voltage change pins, with a loose connection stopping the machine working at all.

amplifiers amplifiersThese are before and after pictures showing the change of capacitors. It is easy to see that four tubular ones have been replaced by much smaller, square, red ones. Top left is a pair of caps; the replacement for one of them is, as you would expect, much smaller than the original. But the other is if anything larger, and I have no idea why. The three blue caps bundled together top right deal with smoothing for the early stages of the amplifier and are less critical to change. So if you are able to solder, the job is not too bad, tho later machines with printed circuit boards are a whole other kettle of fish.

When there were only 3 machines (the fourth came a bit later), we found only one functional EZ40 valve between them, tho the rest seem broadly OK. The valve layout is helpfully printed on a plaque. Looking at the LH pic, the front are 2 x EL41 and the aforementioned EZ40. An ECC40 and EF40 complete the line-up. The EZ40 is the rectifier providing DC power to the whole amplifier. The EF40 is a pre-amp pentode which amplifies the output from the PEC. The signal then goes to the volume control. The ECC40 is a double triode valve. The first section amplifies the signal from the volume control further and the second is the phase splitter used to divide the signal into two halves suitable for feeding the 2 EL41 power pentodes. These form a push-pull output stage giving approx 8 watts into the speaker.

Other thoughts

One way of dealing with all this is just to bypass it. I have a 9.5mm Super Vox that hasnt had an amp at all since I got it 40 years ago. Instead it has had a diode/solar cell, which I plug into an external amplifier. PA amps with microphone input are often sensitive enough to accept the output from such a cell without any amplification. I have, however, used a variety of pre-amps and a variety of amps. At one stage, I used a hi-fi amp, but they are really not suitable for this work in my view, nor are hi-fi speakers. I now use specific projector amps and speakers; at one time, I used the mic socket of a B&H 621, tho I think the amp got too hot without the motor fan in operation; currently I am using a B&H amp which is a 110v transistorised unit designed to fit in place of the original valve amp in the 620 etc series. I've built it a little box of its own and of course I always have 110v on tap to power it. You will find a great deal more about this topic, and possible suppliers, in the Lost Chord section under 9.5 sound. But remember, the First Commandment still applies;

Thou Shalt Not Bodge!

 If you cant do any good, at least do no harm .Try to ensure that if you do bypass it, the old amp is not damaged and could be restored by someone with more skill or more regard for originality. And if you remove parts, dont chuck them away. Often, old volume controls etc are very noisy and crackly in use; I am told that a spray with Contact Cleaner such as Servisol can help. It can probably help with other stuff, too, like valve bases, in & out sockets etc. Poor connections are one of the obvious problems of aged amps A standard irritation is the failure of projector manufacturers to use standard connections, whether for inputs or outputs. I keep several short leads on hand with a jack plug one end and a trailing speaker socket at the other. B&H are one of the worst offenders with their Jones plugs and off-standard speaker sockets and plugs. I keep meaning to get around to making up a short lead for them, too. Most recently, I managed to replace the Jones-type mains input receptacle on a 643 with a Euro type, just using a sheet of metal and the original holes to mount the new socket.


Valve Pictures


Valves are one of those infuriating and incomprehensible pieces of technology. There is a huge range of types and sizes the ones shown here are just a small selection of ones you might find in a projector amplifier. Its impossible to understand why they needed so many. Each manufacturer and/or country had their own numbering system, creating a need for vast lists of equivalents. There are thousands lying around, but the ones you have in large numbers are never the ones you need. Valves are always labelled with a special ink or whatever that is specially designed to fail rapidly in use, due to friction of handling and heat. So many of those valves you have are impossible to identify. Yet another area where the non-specialist has to plough thru masses of material to find a few grains of sense. One of the few good things about the Cold War was the backwards technology of the Eastern Bloc; for some years, valves were still available from the East when the West had stopped making them.


Note that some of these have a central locating pin, others use pin layout for location. !930s to 1950s.


The two on the left are metal; the other two coated glass. !930s 1940s.


The last of the Age of Valves. You may just see the pip on the glass base of the LH EZ40, reproduced in the metal base cover of the valve on the right.


Various covers are used to hold and stabilise valves, from the simple spring to a type which is very common but which I naturally couldnt find when I took this picture. It's a cross between the two covers shown here , a solid aluminium sleeve, with a spring at the top, and a base that fits over and twist-locks onto the valve base. NB springs and covers can be very tight I once broke the pip off the top of a valve with a spring clip like this.


While we were doing valves, I thought we might as well do Photo Electric Cells (PECs), which also come in a range of types and sizes; note top left one with the late small-pin valve base. Note that PECs should not be exposed to light for any length of time. If not mounted in a projector, they should be stored in a light- proof box.

Care & Repair