The Watkins Copicat - A Brief Description

Article written by John Beer

During the late 1950’s Charlie Watkins had an idea to develop a self-contained audio tape echo unit. He enlisted the help of an engineer he employed at the time by the name of Bill Purkess. Together they developed a tape recorder device that would both record and play back at the same time. The idea was to record, then play back the recording a few millisecond’s later via a series of replay heads, thus producing varying lengths of delay effect. The name "COPICAT" is just pur-rrrr-fect for the machine. I think that was Charlie’s brainchild. Brilliant absolutely brilliant. Never could figure why it was spelt "Copicat" instead of "Copycat".   Could it have been a mistake? Any ideas?   Since writing the above Charlie told me that "Copycat" was the chosen name, but at that time there was a photocopier bearing the trade name of "Copycat"  after some thought from Charlie, the Y was replaced with an I  and the  "Copicat" was born.)

It was the development of this type of device that gave musicians of the era their very distinctive sound. In particular certain guitar players. The record buying public thought what fantastic players these guys were. But, if their echo unit broke down mid solo, they would not have looked so clever, I can tell you that from my own posing experiences!!! Any basic guitar player can play Apache, but without added echo it would sound pretty naff.

Musicians come and musicians go, they get all the glory for their records and sound, without a thought for the engineers and inventors who made it all possible. Believe it or not, incredibly, Charlie Watkins is still working hands on from his small workshop to this very day, producing Songbird amplifiers, and until recently, the new Copicat Gold along with his accordion related products. Charlie is also a keen and accomplished accordion player.  Still working well into his eighties!  Now if that's not a lifetime's commitment to the music industry then I just don't know what is.!  So your Majesty, I know you are doing a pretty good job as well, but  how about a Knighthood or at least an MBE for a living legend to honour his outstanding contribution to the British music industry.


C.W.  M.B.E.

~  update Sept 2011 ~

Charlie Watkins has recently received a well deserved lifetime achievement award from Audio Pro International for his services to the Music Industry.

Congratulations Charlie if any one deserves this, you certainly do.

To read Charlie's story in his own words, follow the link below, and then click the Watkins & Wem history link at the bottom of the page.

But don't forget to come back will you !

It is not my intention to go into the full history of echo chambers and the quest for echo for musical use. You can find all about that elsewhere on the Internet. I just want to try to explain in layman’s terms how a tape echo works. A little knowledge of electronics would be of benefit to you but is not essential.

The Watkins Copicat and other units like it use an endless loop of magnetic recording tape, which travels across a series of heads. When a signal from a microphone or an instrument is fed into the machine, it records the signal onto the tape loop as it passes over the record head. As the tape travels on, the recorded signal is then picked up by a series of playback heads. These play back the sound as the tape passes over each head in turn, creating the echoes in a way that only a tape echo can-can- can-can- can...........

The number of playback heads selected by the three pushbutton switches determine the number of repeats. The distance between each playback head determines the amount of delay between each echo

The playback heads of a Copicat are also connected to the Swell or Echo control that allows the user to set the volume of the echo, relative to the original signal. Another control, the Reverb, or on some units called the Sustain, allows the signal from the playback heads to be fed back into the system to be recorded again, creating a very distinctive feedback effect that adds more and more sound to the loop with each repeat. If fully activated, this control will produce a continuous spacey feedback noise that builds and builds. This type of effect was used as the intro and ending to the original 1962 recording of "Telstar" by the Tornadoes.

An echo effect that has few repeats and a very short delay between each repeat is often referred to as slapback echo. This unmistakable effect was the main ingredient of the 1950s rock 'n 'roll rockabilly sound.

The Tape And The Heads

The tape consists of a plastic backing coated with a thin layer of magnetic particles of iron oxide.

The record head consists of a coil of fine wire wound onto a small C-shaped magnet with its tiny gap in contact with the moving tape. The incoming signal having been amplified, produces a varying magnetic field in the gap of the magnet. As the tape moves across the recording head, the particles are magnetised in such a way that the tape carries a copy of the shape of the waveform being recorded. The play back heads are very similar to the record head, and in some instances they are identical. As the tape moves from the recording head to the playback head(s) the reverse happens. The magnetic copy of the waveform, implanted on the tape produces a varying magnetic field in the gap of the magnet within the playback head, inducing a minute electrical current. This varying electrical current is an electrical copy of the signal on the tape, and is known as the 'wet' signal. This is then amplified by the replay amplifier, added to the original 'dry' signal, and fed to the output .

The replay heads react to the signal on the tape, in much the same way as an electric guitar pickup reacts to a vibrating string.

Some of this signal is fed back into the record amplifier via the Sustain pot to be recorded again and again and again to produce the long repetitive echoes.

After the tape passes the last playback head, it travels around the loop and over the two rollers on the tension arm. On valve Copicats there is, on the outer edge of the tension arm between the rollers , a housing that contains a permanent magnet. As the tape passes the magnet the previously recorded signal is erased, ready for the process to start all over again.

In order to get the best out of your Copicat, it is very important to keep the tape path clean and free of any brown oxide deposits from the tape.


Service info

Change the tape frequently! When I used to gig with a Copicat I would fit a new tape at the start of the gig, and replace it during the interval. If you leave it on for months the echoes become muddy and distorted, even to the point where the machine fails to work at all.

I have had Copicats in for repair with nothing wrong other than the tape ripped to shreds and the heads and deck in a filthy state.  Keep the tape path clean at all times with isopropyl alcohol cleaning fluid.

Just remember that you have a 2 ft (60cms) loop of ordinary recording tape of the type used on tape recorders of the era. That 2 ft loop is whizzing around the machine at a rate on knots it was never designed to do. It passes the heads hundreds of times in one hour let alone months of use. Now imagine that 2 ft section of tape being part of a 1800 ft reel, as intended, recorded on or played back on a reel to reel machine. It would pass through the process once in a blue moon, and show little signs of wear.

Suspect Heads

The following test procedure applies only to REPLAY heads not to erase or record heads.  Valve machines have no erase head. The RECORD head is the first on the left, closest to the tension arm, the remaining three heads are the REPLAY heads. They respond to the selector buttons in the same order.  The MK1V  SOLID STATE version's replay heads are the three on the right.  The same applies to the Super IC  IC300 & IC400 unless it has an odd looking brown coloured head on the extreme right near the tape guide. Many do. It is often thought to have its cover missing.  This is an erase head, the three, sometimes four to the left of it are REPLAY heads.

To prove if a head is working, remove the tape, push all the buttons down (plugged in to an amp and switched on), touch the face of each head in turn with a screwdriver or knife. As you do so, you will hear a pop from the amplifier. If there is no pop, the head you are touching is most likely faulty, or has a break somewhere in its wiring.

If you can hear a pop, and if the head is in reasonable shape, try a good clean with isopropyl alcohol fluid and polish the surface with a fine rubbing paper from a car accessory shop. It is black and known as "Wet & Dry paper" ask for 400 or 600 grit. The higher the number the finer the paper. Go for 600 or finer if you can get it.

I have replacement heads for most models.

How It All Works

You may prefer to print this article and read it at your leisure

The Schematic Circuit Diagram


Don’t worry this is not Grannie's knitting pattern, it just looks like it!  Some of you will fully understand it, to others it will mean nothing.  I am going to break it down in to individual sections in the hope that you will have some understanding of it, assuming you have the patience and sufficient interest to stay with me.

I promise I will not get too involved with the heavy side of electronics, I would probably only dig a hole for myself if I did. You will however need to know what the basic symbols on the diagrams are. This I will try to explain simply in this section. Where I can, I will compare electrical current to water because it flows in much the same way, but you can't see the stuff.

The first thing we will look at are the Copicat Valves.

2 of these - DOUBLE TRIODE


  1 of these, a TRIODE PENTODE


The pictures show the valves found in a Copicat. There are 2 double triodes  and 1 triode pentode, similar looking but very different. Straight away there is some confusion, because we, the British call these things VALVES, while the Americans call them TUBES. Who is right?   Well the answer is, both are right.

The Americans call it an ELECTRON TUBE because of it’s tubular shape and the fact that electrons flow between the components within it. Or a VACUUM TUBE because the tube is sealed and has all the air removed from it, a necessity for it to work. Hence the word TUBE.

We the British call it a VALVE because it controls the flow of electrons, in a similar way that a tap controls the flow of water. A tap is a water valve, our Copicat valves are ELECTRON VALVES.

Valves or Tubes, who cares? From this point on, and me being a Brit, I will refer to them as Valves.

Valves all have numbers to identify them.  Now for a little more confusion. American numbers are different to our British numbers. The first Valve in British terms, is an ECC83. The American number for it is 12AX7. New valves are often marked ECC83/12AX7. So 12AX7 or ECC83 they are both the same. As far as I am aware there is no American number for the 6BR8, but no doubt someone will put me right. Or maybe it is an American number, sounds like one.
Enough of that now and on with the job...

Circuit Symbols For A Triode / Double Triode Valve

A Triode is so called because it has 3 electrodes, the Anode is shown at the top. The Americans call this the Plate.

The Grid is in the middle, this is the control grid, it keeps things under control, this is the bit that turns the tap on and off i.e. the tap handle.

At the bottom is the Cathode.  All three are housed in a glass tube with the connecting pins at the bottom, the air is drawn out, and the tube sealed at the top where the little blip is.  In addition a small firework is put inside, it is ignited the first time the valve is powered up, and burns off any remaining Oxygen.  That is what causes the burn marks often seen on the inside of the glass.  It is not a fault in the valve.  The firework is called a getter.  You don't really need to know that, but now you do.

There is one other important component to all our valves.  So important, the valve will not work without it, and so important it is rarely mentioned or shown on schematic diagrams or circuit symbols as above. It is a HEATER FILAMENT.  Commonly referred to as the "heater" this is the nice warm glowing bit you can see when the valve is working.  It is not shown, because any engineer would know it has to be there, it is in most cases simply wired up directly to the power supply.  Its only purpose in life is to look pretty and to heat the "Cathode" .  Not showing the heater circuit, helps keep the schematic less complicated, and less cluttered. 

You can find out from other websites, highly detailed explanations of how triodes and other valves work if you are that way inclined.  I don't want to go down that road here, just want you to get the basics.  Which are as follows.....

All the components need to be contained in a vacuum, therefore in a sealed container.  Glass does this job very nicely.  The cathode needs to be heated before anything can happen, hence the elusive heater mentioned above.  The Cathode is held at a negative - potential, when heated, electrons will flow from it towards the Anode which is held at a positive + potential.  The negatively charged electrons are therefore attracted to the positively charged surface of the anode.

When this happens an electrical current will flow in the opposite direction, from Anode to Cathode.  If we can control the flow of electrons, we can in turn control the amount of current flow.  This can be done by placing the third component , the grid, (an array of wires like you would put on the garden to keep the birds off) between the Cathode and Anode.

We already know the electrons are attracted to the positive Anode, so by adjusting the potential of the grid (now in the middle of the electron flow) to that of  the cathode, the electron flow will stop.  By raising the grid potential to a little above that of the cathode, some of the electrons will flow through the grid wires to the Anode, allowing a small amount of current to flow in the opposite direction.

Raising the Grid potential a little more will allow more electrons through to the Anode, in turn more current will flow.  So you see the grid is like the handle on our tap, we now have a valve.  All that is left is to put it to use in our circuit.   Dig dig dig......  

Fill a flimsy plastic water tank via a pipe with a kink in it, adjust the kink to a point where the water has a slight flow from the overflow pipe.  The overflow will be steady if the surface of the water in the tank remains undisturbed.  What is keeping the surface water steady?  The flimsy plastic tank.  Imagine the tank is our Grid, right now give it a good kick.  We have just varied the potential on our grid and caused a shock wave in the water tank.  As the wave rises more water will flow from the overflow, as the wave recedes less water will flow.  The kinked pipe will keep it all topped up and all will return to normal when the water surface settles down again.  The harder we kick it the more we affect the output from the overflow.  Not rocket science or as mysterious as it seems is it.

That is my basic description of a Triode valve.  Now you know as much as me - clear as mud in' it.

I will just mention a few other components we ought to know a little about, then we will look at that knitting pattern again. 


These restrict the flow of electrical current through a conductor, (wire) a bit like a kink in a water pipe.  The measurement of resistance is referred to as the OHM.  Resistors come in a large variety of sizes according to how much resistance we need in a given part of a circuit.  1 meg = 1 million ohms  while    100 k  = 100,000 ohms 10k = 10,000 ohms 1.5k = 1,500 ohms.  On older schematics resistors are shown as a zigzag line with its resistance value beside it. e.g. 1.6k dead easy in' it   Yea well, here is the twist, a modern schematic, designed to avoid confusion //?{ { @@ } } ?\\  would show the resistance value as 1k6.  or 10 ohms as 10R  or 1.5 ohms as 1R5 . Now, I know I'm  moving up in the queue a bit, but believe me, the old ways are still the best.

The higher the resistance value, the bigger the kink in our pipe.  Resistors have coloured bands around them which tell the resistance value.   The top 3 bands are the value bands the 4th is a tolerance band stating how accurate the resistor should be.  i.e. + or - 10%  a 1.6k resistor would be Brown = 1   Blue = 6   Red = the 2 Zeros.  Silver would be + or - 10%  I don't want to dwell on this, you can find the resistor colour code, and how to apply it out there on the web.

How a resistor looks

How resistors are shown on schematics

The top one is what is shown on ours


Not easy to describe, as they have different uses in the circuit and there are several types.  In relation to water, they are somewhat like a bucket, they hold or store electricity.  If the bucket leaks, we are in trouble.  I'll not go in to detail at this stage cause it's boring, and I would like you to stay with me a while.

Now a word of caution, that big can of a capacitor near the transformer has a nasty habit of jumping out and biting you.  Even if the machine is switched off and unplugged from the mains it may well still bite, so do not touch it or any part of the circuit until you are sure it is discharged.   Short it out with a screwdriver, short each of its pins to the metal outer casing.  Do not touch the blade of the screwdriver while you are doing it.  If it has any charge there will be a loud crack and a spark as you short it out.  Once you have done this to all the pins, it will be safe to handle.  If you are not confident, then leave it to someone who is.  Be warned this can kill.  Not the object of this exercise, I don't want anybody getting hurt.


Lets take another look at the knitting pattern

It is the same knitting pattern, but I have coloured in areas of it. These are the areas we are going to look at individually.  Nothing electrical will work without a power supply.  The red bit over on the right is the power supply, so we will take a butcher's at that first.

1/ The Power Supply

You can see the incoming mains supply is fed to the transformer via a double pole switch.  That means it switches both the live and neutral wires of the mains lead. The switch is on the back of, and is part of, the swell control.  There is no earth lead fitted to these old machines, they rely on earthling from the main amplifier via the screen of output lead.  When plugged into the amplifier the screen is connected to the amp chassis, which in turn should be connected to earth.  It is done this way to avoid hum via an earth loop.  Today's electrical regulations, quite rightly so, would never allow this for obvious safety reasons.

The mains supply is also fed to the tape drive motor, via the toggle switch mounted to the left of the swell control

The Transformer has an input coil called the primary winding, with tapings for different voltage mains supplies.  There are 2 secondary windings. One supplies the high voltage required by the main circuit.

The other supplies the 6.3 volt supply for the valve heaters and panel lamp  As said previously the heater wiring and connections are not shown on the schematic.

Both coils are wound on a non conductive former mounted in the centre of a soft iron core.  The primary winding is energised by the AC mains supply, creating a magnetic field around it.  The magnetic field induces an electrical current known as an E M F  (electro motive force) in the secondary windings, producing voltages at the secondary connections.  The voltages are dependant on the turns ratio of the primary and secondary windings.  A transformer allows us to chose the voltages we require for a given project, it also provides isolation from the mains supply.  i.e. There is no direct connection of mains power to the circuit beyond the transformer primary winding.  A safety feature.

The mains supply and the transformer output are both alternating current (A C).  Basically the supply is constantly changing from positive to negative.  It changes at the rate of 50 times per second. 50 cps or 50Hz. this is known as the mains frequency.  In the U.S. the mains supply is at a 60Hz frequency.  By connecting an ac supply to an oscilloscope, (looks like a hospital heart monitor) it is possible see what the wave form looks like.   The drawings below show what the sine wave looks like.

I don't think I need to dwell on the heater winding any more, just to say that the 6.3 volt AC output is connected to each valve heater in turn, just as it is, but with a 100 ohm resistor connected from each end of the winding to chassis ground, for the purpose of hum reduction.  You can see the heater wires on the previous page, the tag strip picture, left side, running from the transformer to the first valve base, along the outer edge of the chassis.  They are the orange twisted pair of wires.  The twisting is also done to reduce hum by phase cancellation.  You can look that up if you wish, but I wouldn't bother.

I think it would be advantageous to have a copy of that picture here so we can refer to it as and when needed.

If you take another look on the left, you can see behind the twisted wires, an aluminium and plastic device secured with 2 rivets, and with 2 yellow wires attached.  This is the 1/2 Wave Rectifier, next above it is that crocodile of a capacitor.  Beware of it.  Above that again is the mains switch and swell control.

The 1/2 Wave Rectifier is connected to one end of the high voltage secondary, it converts the AC in to DC - Direct Current.  It will only allow the positive peaks of the AC to pass through it. As fig 3 of the sine wave diagram shows.

The output we now have from the rectifier is D C. D C because though not smooth, it is not alternating from positive to negative. If we look at fig 3 on the sine wave chart, you can see the line starts at zero volts climes the peak becoming more positive until it reaches the top. In other words the positive voltage is gradually increasing until it reaches its peak, at which point the positive voltage begins to fall until it meets the zero volt line. The voltage remains at zero for the time it takes for the negative going part of the cycle to take its course, but is blocked by the rectifier. Once the negative cycle is complete, the positive one starts again, passing through the rectifier. So we now have a very bumpy road, which needs smoothing out. We do this with a series of Resistors and Capacitors.

Now here is the watery bit. Our ½ wave rectifier, so called because it can only rectify ½ of the sine wave, is a bit like a single action water pump. Pumping water in regular spurts, rather than a nice smooth flow. One way to smooth out the water flow would be to squirt it into a bucket with an outlet pipe near the bottom. With the pump squirting water into the bucket, when full water will flow out of the outlet pipe in a fairly smooth way, because the bucket is full of water, allowing a continuous flow during the dormant periods. It holds a charge of water.  I told you earlier, a capacitor was a bit like a bucket. WELL, (excuse the pun), a capacitor holds a positive charge of electricity, to top up the supply during the negative going period when the waveform is at zero, recharging as the positive cycle starts again. So you can see how this has ironed out most of the bumps.

We now have a much smoother supply of D C power with only a slight shimmer. This shimmer is called Ripple. Going back to the outlet pipe of our bucket, we can reduce the ripple still more by putting a kink in the pipe, restricting the flow a little and producing a damping effect. Remember kinks and their likeness to resistors. Good, so now we put a resistor on the wire feeding the capacitor, creating a damping effect to the power coming from it. Now if we then attach that supply wire to yet another capacitor and resistor, the power output from that will be as smooth as a baby’s bum. That conditioned supply is ready to supply power to the oscillator section of our Copicat.  We are not finished yet, adding another resistor and capacitor, will supply the power requirements for the preamp, record amp and playback amp circuits.

Finally, the other end of the transformer secondary winding is connected to chassis ground to complete the circuit and becoming the return or negative part of the power supply. The negative connection of the three capacitors also connects to chassis ground. The three capacitors are in the single can you see above the rectifier in the picture .

That just about concludes the power supply.  I am now going to work from the other end of the knitting pattern, starting at the blue bit. 

2/ The 1st stage - The Preamp

Ok then, here it is.  The blue bit. This is the dual input preamp section.  Where we can plug a guitar and a microphone, or 2 guitars or mics into the input sockets, control the level of them and feed an amplified signal to the record amplifier and a signal without any echo to the output jack plug.

Still a bit of an eye full, but over to the left are the 2 input jack sockets, then to the right of the 1 meg resistors are the 2 halves of the ECC83 double triode, and a little further right are the gain controls, etc.

To simplify things a little, the next drawing is half of the above.  As the circuit for the 2 inputs are  the same, we can split the drawing in two, getting some of the clutter out of the way.

If we look at the drawing above, the input jack socket is on the left.  It shows a switch contact that will short out the 1 meg resistor when no jack plug is plugged in. The switch contacts open as soon as a plug is inserted.  The first 3 resistors, 1 meg, 1.5k and 100k, are chosen at those values, to set up the valve in its operating condition.  The switch contacts of the jack socket will ground out the grid of the valve, allowing no electron flow within it.  When an instrument is plugged in, the switch contacts open allowing the 1 meg resistor to set the valve at a midway electron flow condition, and ready to accept a signal from the instrument.  The wire at the top is the positive feed from the power supply to the valve anode, via a 100k resistor.  This is the kink in the pipe that feeds our tank with water at the correct rate of flow.  You will see that between the resistor and the anode, there is a connection leading to a capacitor.  This connection represents the overflow pipe from our water tank.  The cathode resistor 1.5k is there to set up the valve as just explained, while the capacitor beside it is a bypass capacitor for negative feedback.  We will not dwell on that now because it is complicated and not relevant.  The circuit would work without it.

If we now plug a guitar into the input jack, the action would first lift the grounding to the grid. Now turn up the volume knob on the guitar and strike a string.  The signal from the guitar is a small AC voltage that will travel along the wire to the valve grid,  having an effect on it like kicking our water tank.  The grid will change from a steady stable condition, to a wildly varying one both above and below our previously stable centre line.  As the grid goes more positive than it was, it will allow electrons to flow between the cathode and anode, also allowing current to flow in the opposite direction from anode to cathode.  This action will reduce the output from the overflow pipe - the tee connection between the 100k resistor and the anode.  As the voltage on the grid goes below the centre line the electron and current flow will reduce.  In turn creating a larger output from the overflow tee junction.  Sometimes difficult to understand, but, think water flowing through the tee, from top to bottom, not much will come out of the side junction.  Right now stick your thumb over the bottom restricting the flow, and it will piss out of the side one!  Now it makes sense don't it.  The output from the valve is a copy of what goes in, but is about 60 times larger,  it is also upside down, or 180 degrees out of phase with the original signal.  But don't worry it'll all come out in the wash!  More about that later.

A quick word about GAIN.  The valve has its gain set at about 60, by the anode and cathode resistors.  I am saying 60 as a round figure.  It means the input signal is amplified 60 times larger by the valve.  The anode resistor is 100k, the cathode resistor 1.5k.  Divide 100 by 1.5 = 66.6  There are other factors to take into consideration when calculating the gain of a circuit.  Nothing I am telling you is 100% accurate  but that should give you some  idea, like I said if you really want to do your head in, it's all out there on the net somewhere.

Up till now we know the signal path is from the guitar to the grid, where it stirs things up between the anode and cathode, and heads out through the tee junction to the next capacitor.

Below is the same circuit showing the signal path in red, with a sine wave superimposed over the red line.  The signal shown is a continuous note, not one as played from the guitar.

You can see the signal path as a red line from the jack socket, to the grid of the valve, then from the anode resistor, through the capacitor to the rest of the circuitry.  The signal superimposed over the red line, as you can see is AC alternating from positive to negative as it travels along its way.  You can also see the signal from jack to grid is small compared to that as from the anode resistor to the rest of the circuit.  Can you also see the larger sine wave is inverted in comparison to the smaller one? i.e. 180 degrees out of phase with the input signal. 

The capacitor used here is of a different type to the ones we dealt with in the power supply, much smaller by comparison. Usually one of two types were used in Copicats, Hunts, these were used a lot in Copicats. The outer wax coating tends to crack and the capacitors become unreliable. If you have any of these in your Copicat, and your machine is giving trouble, I would suggest you replace them all with modern polypropylene types, regardless of how they look. The other less common types are the mustard colour ones. These are of the highest quality, and rarely do they give any trouble. Both types are shown below.

Hunts capacitors as you can see.
Poor condition, change these regardless

Mustard capacitors usually trouble free
Normally no problem, good quality

My advice, if it says HUNTS replace it.

The capacitor we see at the anode resistor tee junction is known as a coupling capacitor.  Because it couples the stage we have just covered to the next part of the circuit.  Without going into great detail, it prevents DC from the power supply via the anode resistor from entering the next stage of the circuit.  It is a DC BLOCKING device.  If we check with a volt meter we will find 200 or so volts DC on the anode side of it.  The same test on the other side of it should read zero volts.  If it shows any DC volts on the other side, even just a few, the capacitor is faulty and should be replaced.  This is what is meant by a leaky capacitor.  You need to be careful deciding the capacitor is actually faulty, because the suspect DC could be coming from somewhere else.  The only way to be sure is to disconnect the end under test and check again.  Though the object here is to explain the workings of the unit, and not to fault find, I thought that was worth a mention at this point.    

Though the capacitor will block the DC element of the circuit, it has the effect of allowing the AC signal through to the next stage, the Gain Pot.  This is a rotary adjustable resistor, one of the knobs on the left of the control panel.  The variable resistor has a circular carbon track, of the resistance value marked upon it.  In this case it's 500k.  there is a movable contact called a wiper, attached to the spindle which gives you any resistance setting between zero ohms and 500,000 ohms 500k.  The full name of the device is a "POTENTIOMETER" abbreviated to Pot.  Not to be confused with the smokey stuff!!

Below is what a pot looks like.

The fixed resistance of 500k is connected from the output of the capacitor to ground, while the wiper is connected to the following stage.  With the pot set at minimum, the wiper will be at the ground end of the fixed resistance, where no signal will be sent out through the wiper.  Turning the wiper towards the other end of its travel, will allow the signal through, gradually increasing in strength until at maximum at the opposite end of its travel.   So it is a variable resistor, like a water hose you can squeeze between your fingers adjusting the flow of water from it.  The harder you squeeze the more resistance you add to the water flow, the less water comes out of the pipe.  Reduce the resistance, and more water will flow.   By varying the amount you squeeze, you are varying the resistance to the water flow, a variable resistance.

At the desired setting, the signal is fed to a series parallel network of resistors,  The signal, if any, from the second gain pot is also fed to a point in the same network of resistors.  This is the signal mixing stage.  Part of the signal goes through the 470k resistor to the control grid of the next valve.  Part of the signal is also fed through the 1 meg. resistor to the output jack , sending a signal with no echo effect to the amplifier of your choice.  That about concludes the first preamp stage of the circuit, except for the other 2 meg sustain pot at the lower right of the diagram.  This is I will talk about when we get to the final stages.

Just a word about that resistor network.  Due to the way they are wired, series and parallel, it is not possible to measure the individual resistors with an ohm meter when testing for faulty components.  You will get totally inaccurate readings due to a factor known as ohms law.  But due to another phenomenon, known as sods law, ohms law will not work out right either.!!

Ok, ready for some more knitting then?  Right here is the green bit.

3/ The 2nd Stage - The Recording Amplifier

This is the 2 halves of the 6BR8 the left section is the PENTODE part of the valve.  It is the heart of the "RECORDING AMPLIFIER".  The other part is a TRIODE just like 1/2 an ECC83. Enclosed in the purple line, it is the "BIAS OSCILLATOR".  We know roughly how the Triode part works, but not the Pentode.  Not too sure how deep my hole is getting, but below is the circuit symbol for the pentode section.

Phew  still long way to go yet!

As you can see, we still have an Anode, a Cathode , and a Control Grid.  Also there are two other components to the valve, a Suppressor Grid, and a Screen Grid.  So what's that all about?  To be honest the hole could get a little deeper here, so I have done my homework and looked it up myself !   

The Screen Grid is used as an electrostatic screen, between the control grid and the anode. This grid is held at a steady potential midway between that on the anode and the cathode or control grid, so tends to ‘shield’ the control grid from seeing the changes in potential of the anode. Hence it tends to reduce the effective capacitance between the control grid and the anode. The result is that less current is needed to drive the input signal variations of the control grid, making it easier for the valve to be used at higher frequencies than a triode.

The Suppresser Grid is employed to catch electrons that have been released from the anode. This grid uses just a few wires, placed near the anode, and usually connected to the cathode.  Most of the electrons coming from the cathode fly through the gaps in all the grids as they have been given a lot of kinetic energy by the cathode-anode potential difference. So most of them whiz past the wires in the grids. However most of the electrons knocked out of the anode will have a relatively small amount of kinetic energy, As a result, they tend to fall back to the anode, or drift into the nearest grid - which is now the suppresser grid. Having done this, they don't have any effect on the input current required to drive the control grid, and have little effect on the valve’s gain, etc.

So there it is, now you know as much as I do about the pentode!  Why do I do all this for free?  I'll never get rich, but what the hell If you're happy to read it, I am happy to pass on the little knowledge I have.

Looking at the schematic diagram, you can see we have an anode load resistor, and a cathode resistor with bypass capacitor.  Like the preamp valve we need these to set the system up.  The 330k resistor and 0.1uF capacitor are the set up components for the screen grid.  The suppressor grid is internally connected to the cathode. i.e. the connection is inside the valve.  As with the triode, the signal is fed to the control grid, producing the kick required to stir up the electron flow between the cathode and anode and the reciprocal current flow.  If you have been able to absorb much of this, you will remember I told you the output signal from the preamp triode was now inverted.  This inverted signal is fed to the grid of the record amplifier pentode.  During the 2nd process of amplification, via the pentode section of the 6BR8, the signal is again inverted, making it the correct way up again.  The signal is now in phase with the original input signal.  This amplified signal passes through the 0.1uF capacitor connected to the anode, through the 100k series resistor, where the signal is then mixed with another signal from the oscillator, which brings us to the next stage, the knitting enclosed within the purple line.

4/ The 3rd Stage The Oscillator.

The oscillator provides the recording bias needed to give a clear recording due to the non linear response of the tape.  This is somewhat complicated. I will skip the finer details for now and explain after we have covered the basics of the circuit.  This is the Triode section of the 6BR8 Here we have no cathode resistor or anode resistor.  The cathode is connected directly to ground, while the anode is connected to the power supply via the primary coil of the oscillator coil, the coil is in fact a small transformer with two coils.  The secondary coil, the top end is connected to the power supply at the same point as the primary, the other end to a 0.002uF capacitor in series with a 47k grid load resistor to ground.  The junction of the capacitor and resistor are connected to the grid of the triode.  The capacitor value 0.002uF  together with the 2200pF capacitor connected across the primary coil is chosen to help tune the oscillator to the correct frequency, it also prevents any DC from the power supply from appearing on the grid.  As the 0.002uF capacitor charges, it causes a change in the grid current, thus giving the old water tank a little kick, causing a change of current flow at the anode, which induces a change in the coils of the oscillator transformer / coil, the process is then repeated due to another change of charge state of the 0.002uF capacitor.  The result is a high frequency oscillation of about 44kHz.  This is way above the human hearing range, and is of sinusoidal form.  In other words it is a sine wave. If you could hear it, it would hurt your ears, much like a microphone does when it feeds back.  That is also oscillation.  The resultant signal is fed out through the anode-connected 200pF capacitor where it is then mixed with the signal we wish to record coming from the record amplifier.  The mixing of the two signals is known as MODULATION.  The modulated signal is then fed to the recording head, where as the tape passes over the head a copy of the modulated signal is implanted on the magnetic particles of the tape.  Below is an oscilloscope example of the three signal forms we have just covered.  These are actual traces I have taken from a working machine.  The signal from the record amplifier is sourced from a signal generator set at 1kHz.  Incidentally, 1kHz is the normal test signal used for audio equipment, because it is smack in the middle of the human hearing range. (Not a lot of people know that.  Duh)  here we go then the three traces.

That is pretty much the end of the road as far as recording is concerned, the next stage is to recover the recorded image and to play it back.  Before that I hear you saying why bother with the oscillator, why not just record the signal as it is?  In the last paragraph, I briefly mentioned linearity problems with the recording tape, and that I would try to explain more about this after we had covered the oscillator circuit.  Now is the time for more hole digging.

5/ Recording Bias

Magnetic recording tape has a non-linear response at audio frequencies, in other words without bias the recorded signal will be horribly distorted. The Bias is a high-frequency signal of about 44 kHz on a valve Copicat. We can avoid the effects of this non-linear response, by mixing the signal we wish to record with the high frequency bias signal. This is called a modulated signal. Biasing partially demagnetises the tape and the remaining net magnetic induction is the difference between the positive and negative half-cycles of the recorded signal. This differencing operation further cancels some of the non-linearity. (Dig dig dig.)  Ok so now I hear you asking, how do we remove the bias signal at playback?

At the second grid to ground, is an R.C. filter (Resistor Capacitor) comprising of a 47k resistor and 0.005uF capacitor.  It is called a low pass filter, allowing the lower frequency audio signal to reach the grid of the valve, whilst filtering off the much higher frequency bias signal.  In this configuration the filter is sometimes called a "bias trap",  as shown on the next schematic enclosed in the red circle.


6/ The Playback Amplifier

The first stage of play back, is to recover the signal recorded on to the tape by the record amplifier and head.  This I am sure you know is done by selecting one ore more of the three replay heads.  As the heads are the first link in the chain, I would just like to briefly touch on the subject once again.  the text and picture below, I found on the website  I have modified the text to suit the Copicat.

The basic idea involves an electromagnet that applies a magnetic flux to the oxide on the tape. The oxide permanently "remembers" the flux it sees. A Copicat's record head is a very small, circular electromagnet with a small gap in it.   Illustration below.

A  = the tape  

This electromagnet is tiny -- perhaps the size of a flattened pea. The electromagnet consists of an iron core wrapped with wire, as shown in the figure. During recording, the audio signal is sent through the coil of wire to create a magnetic field in the core. At the gap, magnetic flux forms a fringe pattern to bridge the gap (shown in red), and this flux is what magnetises the oxide on the tape. During playback, the motion of the tape pulls a varying magnetic field across the gap. This creates a varying magnetic field in the core of the replay head inducing a signal in the coil. This signal is sent via the selector switch to the grid of the playback amplifier valve.

Thanks to "How Stuff Works" for that superb explanation


Now the final stage of the knitting pattern, the purple bit.

There it is, to the left we have the 3 playback heads followed by one side of the 3 pushbutton switches,  feeding the signal recovered from the tape loop to the first half of the final ECC83 and its related circuitry. At the output of the second half of the ECC83 is a clump of resistors, selected by the other side of the push button switches and feeding the signal to the output and feedback circuit.  As before I will split the schematic in two and deal with each section in turn.  

At first glance, the three pushbutton switch seems a complicated affair, but it is actually quite simple.

The Switch

There are 3 actuators to which the buttons (not shown) are attached.  The actuators have a return spring and a latching mechanism to hold the button/s down when in the ON position.  A second press unlatches and returns the button to the OFF position.  Each actuator operates 2 sets of switch contacts, 1 on each side of the switch unit.  Below are some images of the switch contacts.

Centre top is the fixed contact board attaches to the body of the switch, shown with the solder tags at the top.  Below it is the moving contact, attaches to the actuator.  When the actuator is down, it bridges the centre fixed contact to one of the outer contacts, as shown in the right hand image.  The image to the left is the reverse side of the centre parts.  The 2 parts you can see make up 1 switch set.  Each actuator operates 2 switch sets, one each side of the unit.  Therefore there are a total of 6 switch sets to the switch unit.  As each side of the switch operates in different parts of the circuit, I will try to explain it in 2 parts.  The side that has resistors attached to it I'll deal with later.

The side with no resistors, is the side that selects the required combination of replay heads.  It is wired in such a way, that with all buttons down, all 3 replay heads are selected.  If we study the schematic, shown with all 3 heads selected, we can see that the 3 heads are all wired in series. And that setting any button to the off position, bridges the pair of switch contacts the head is connected to, creating a short circuit, thus rendering the affected head inoperative.

So you see the switches don't actually switch heads on an off as you would expect, it shorts out the ones you don't want to use.

Below is a schematic of the first stage of the replay amplifier, on the right of it, I have drawn a simplified schematic of the heads and switches.

The simplified switch drawing to the right shows heads 2 & 3 selected, switches open.  Head 1 shorted out, not selected, switch closed.

Having selected our required replay head or heads, the magnetic waveform deposited upon to the tape by the record head is passed over the replay head or heads, and induced into a minute electrical alternating current, which is an exact copy of the audio recording together with the bias signal, both of which are fed via a wire from the pushbutton switch to the grid of the first half of the final ECC83 valve, where it is amplified.  The amplified signal passes from the first anode to the second stage of amplification via the coupling / DC blocking capacitor . *

The grid of the first stage is kept at or close to ground potential by the selected or shorted heads.  The 3.3k cathode resistor, sets up the valve bias, (not to be confused with the recording bias) which adjusts the valve to it’s required operating area.  The 25uf parallel capacitor is a bypass cap which holds the DC part of the circuit as required for the valve bias, but allows any signal at the cathode to be shorted to ground/ bypassed., preventing negative feedback from affecting the grid.

 * The output from the anode and coupling cap, like the first preamp stage is now inverted, i.e. upside down compared to the input at the grid.  The positive going peaks of the wiggly waveform at the grid are now negative going at the output / anode.

 The second stage valve bias is set up in the same way as the first, but with a different value cathode resistor of 2.2k  instead of a 3.3k due to the fact that the grid is held down by a 1m resistor instead of the playback heads.  The signal from the previous stage 0.05uF coupling cap passes through a 470k resistor to attenuate / reduce the signal to a suitable level to avoid overdriving or distortion of the next stage.

 Also connected to the grid of the second stage is a 47k resistor in series with a .005uF capacitor, circled in red on the schematic just before the switch pictures. This is known as an R C filter (resistor capacitor) the values of which are chosen to filter off the high frequency recording bias, which has done it’s job and is no longer required.  The RC network is also known as a "LOW PASS FILTER" , it allows the lower audio frequencies to pass to the valve grid, and at the same time it grounds the higher frequency bias signal.   In tape recorder circuits, a high pass or R C filter is sometimes called a BIAS TRAP.

Ok, with the bias signal now out of the way, we are left with the inverted, recorded audio / echo signal that we do want, this is now fed to the grid of the second and final stage of the replay amplifier, which is the equalisation bit, and probably the most complicated part to try to explain.

The Equalising section.


Still with me?  You must be as mad as I am, but thanks.

Please feel free to email your comments. Is the article interesting to you, a complete waste of time, or just plain boring?

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I have now written most of the remaining text for this article, which still needs a little editing, and the pictures to go with it.  the paragraph above is part of it, I am hoping to complete this page during the next few weeks.


Last updated 17th September 2009



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Copyright © John Beer     Jan 2007 All rights reserved