Fender Twin Reverb

This was one of the new (reissue?) amps. Pcb’s not the traditional fibre tagstrips. The reverb didn’t work and the amp distorted on the clean channel. The reverb is a fairly common fault, so I thought it might be worth giving a few in’s and out’s on them.

The spring line reverb in just about any combo is usually an Accutronics which was used in the old Hammond organs, and many amps originating from the ’60′s on.

The first thing to check is that it’s plugged in the right way round. Alright it’s brain death, but there is a tiny transducer at either end of the spring, the one that drives the spring has an impedance of a fraction of the one that puts out the reverb. It depends whether it’s driven by a valve (in which case it’s probably less than 10 ohms) or a transistor/chip (maybe fivetimes that). The other end might measure aound 200 ohms. If they’re plugged in the wrong way round, it won’t work.

If there are a couple of phono plug connections into the amp, a meter should read about what  we’ve just said. If they don’t (usually they’ll read either something or nothing at all) then it’s either a faulty lead (repairable) a faulty connection within the reverb tray (often repairable) or in the tranducers (throw it away). MAJ electronics on the web do them, and also a load of early H//H parts and other useful stuff. This Fender had a delicate break in the enamelled wire from the transducer. Tough on the eyes, but worked out ok.

The ‘distortion on clean channel’ fault was the power drop switch on the back. It’s a dual circuit (DPST) switch and I’ve seen them break down before. One side of the switch switches the HT (high voltage dc rail) and the other side switches the bias voltage to the output valves, to accommodate the dropped HT voltage. 500+ volts is a lot to break,  and it also has surge on it from the reservoir/smoothing caps. If it blows open on both circuits it stays at low power (so distorts early) and if it tracks across both sides it stays in high power mode. If one side blows and the other doesn’t, it fries your output valves, and if you put another set in without sorting the switch out, it’ll do it again.

One further point; make sure it’s a high voltage/high current surge switch 600 volts @ 2-3 amps should be fine. One further-further point. DON’T GO NEAR THE INSIDE OF AN AMP WITHOUT SWITCHINGOFF AND UNPLUGGING!!! Also discharge the caps or leave it for five minutes.

Echolette NG51 Tape Loop Echo

Well, here we are at last the Echolette NG51. Just looking at one of these things makes me drool. Sad, really.

In the last post we went to a lot of trouble to figure out how a ’standard’ tape loop echo does its stuff. Well, although this is broadly similar, there are a lot of very nice differences. The unit was of German manufacture, beautifully engineered, and, as we’ll see in a bit, a piece of art inside. And was, actually, pretty much entirely hand built. To build this today would cost you as much as a reasonable car.

Starting from the left, there are four small pots in a group. The lower pair are the input level controls for the two jacks below them. I modified this a little (this belongs to me, so I’m not knocking anybody else’s gear around). I’m normally dead against ‘mods’ of all kinds on vintage gear. The people who built these things in the first place were experts, with the odd genius in there somewhere. I think it takes a mountainous ego to ’improve’ them, especially as our current thinking is totally alien to the thinking of the ’50′s and ’60′s. On this machine, the gain was just too much for the output of current guitar pickups, so I altered the input resistor arrangement slightly. That was all, so nothing extensive.

The two small pots above the level controls are tone filters and a little confusing. There’s a bass clef sign to the left of centre on the pots, and a treble control to the right. But…..they mean that the bass is CUT to the left, and treble CUT to the right. So they have the opposite effect to the way we might expect. The three large controls along from that are:-

                             Repeat volume (adjusts the echo level); Sustain (controls the number of repeats); Tone (controls the tone of the echo signal.)

Next to that there is a jack input for line level (when used in an FX loop or auxiliary output on a mixer) and then a five pin output socket (DIN socket). There are two different output levels available from this socket, ground is the centre pin, the left pin is a line level out put and the right pin around 100 millivolts so the same sort of value as a mic, if you’re using it as an in-line effect into an amp, for instance. The three pin DIN socket above that is the footswitch.

Apart from that we’ve got a bank of four buttons. Power on, and two motor speeds. One other control is not obvious. The centre (sustain) control has a pull-switch arrangement which switches in an extra head giving a very fast echo.

   At the back there’s the mains supply input. This is original but a lot have been swapped for IEC (kettle) sockets. The voltage selector has so many voltage tappings you could probably use it on Mars.

 

There are quite a few parallels between what we see above and the idealised diagram of the last blog. The differences are these:-

The tape travel is in the opposite direction, so all the heads are in reverse order so far as their position within the unit is concerned. There are three record heads, and this makes it possible to vary the output of each repeat. The two playback heads can be switched from single to double giving a double echo as required. The roller furthest left is a single one (double in the iealised version) because the tape tension is much more accurately adjusted by the tension roller and spring on the left side of the unit in this case. Also the black rubber roller pinches the tape (pinchwheel) onto the capstan, so that’s a more effective design also. The head with ‘X’ marked is an addition. Its a permanent magnet and just cleans up the tape erase to the Nth degree. Another thought about the tension roller is that its face MUST be perpendicular to the chassis. The arm to which  it is connected I’ve often seen bent. Or should we say, not quite straight (back to the second hand car trade trade). If this is not really well aligned, the tape will move around and maybe even drift off the heads altogether at the extreme. 

Everything should be scrupulously free of grease (even grease from your fingers, so careful when you handle the tape).This shows the various presets, and if they’re not set at least somewhere near it might not work at all. The hum balance is probably the easiest to set approximately. Except its dead easy to sustain third degree burns from the valves as you’re doing it. Turn all the echo controls off, and turn the amp up till you can hear hum. By adjusting the hum balance preset (it moves the centre tap of the heaters effectively) you’ll get to a low hum spot. A pair of long nose pliers (genlty used) can save your fingers. The output signal can be set approximately by plugging your guitar (etc.) into the amp, noticing the sort of level you’re getting, then going for something like that while adjusting the pot. Handy if you’ve got three arms for this sort of thing. The controls would all be off except for the input pots which would be 12 o’clock.

The delay level is a bit more of a guess. With tone full on, echo volume at about twelve o’clock, sustain off and any one of the top pots (they’re the echo volumes) fully on and the other two off, set the delay level so that the echo signal is about the same as the dry signal. Easiest on the middle head (so middle repeat vol, on others off ) but a matter of personal choice really. 

Here’s a last parting shot before I remove to the water hole. (Tea hole). On this last pic you’ll see a heavy twisted black and white cable. There’s a good reason for this, but you can improve matter just by tightening down bolts. The transformer is bolted to the main chassis and the main earth (ground) is connected to that. As the heaters have a centre tap to ground (via hum balance ) any problems with the chassis connection between the transformer (that’s the big brown thing on the left of the pic, with wires all over it) is big bad news for hum and background noise. The heavy black/white cable actually straps the main and the sub chasis’s together, so the electrical connection of the frames isn’t really important. If that’s not done, slacken the screws off to the transformer chasis, and then tighten them down again. It will shift any claggy accumulations and refresh the ground connection.

I feel a much deserved tea coming on, or posslby somethng stronger when the sun is over the yard arm.

Tape loop delay basics

First an apology, in the last blog (Delay line stuff) I said that I would be doing a thing on the Echolette NG 51, but sadly, I was lying through my teeth. I realised that the NG 51 is actually different in many ways to just about every other tape echo, so I’ll get to that one next. That’s if I don’t lie about that as well, of course.

 

If you’ve ever seen a WEM Copycat (after you’ve taken the lid off it) it looks a lot like the above image, with a couple of differences. By working at it in this way, I hope we can get the idea of the principles of the tape echo.

The drive is very much as this, for all the tape units I’ve ever seen. The Wem Copycat doesn’t have a pinch wheel, and it doesn’t have an erase head as such, just the permanent magnet erase.

So how does it work? The capstan drive is usually just the shaft of the motor drive which in this case is just a direct mains a.c. motor. That drive would not normally give the tape a sufficiently stable motion, but Mr. Watkins (the inventor of the Copycat) attached a heavy brass flywheel to the drive shaft, and that sorted it out simply and effectively. So the tape loop is driven by the capstan (motor shaft) in the direction of the arrows. So if we took a point on the tape starting from the capstan drive, it would be taken around the two pulleys, which takes the tape past the erase head which is mounted on the tape tension arm (this has spring arrangement under the top plate which sets the tension.) By passing over the erase head, any residual signals (sounds) are erased before they get to the next bit which is the record head. This is a fairly standard schematic representation with the arrow showing that the signal goes onto the tape (record); whereas the next head has the arrow showing the signal being taken off the tape (play).

Let’s say we put a short sound onto the record head. This sound then becomes (effectively) a position on the tape, and this is taken around the loop. As it passes a playback head, at some time after leaving the record head, the delayed signal is picked off the tape. This happens for as many times as there are heads which is mostly three. This recorded sound then continues around the loop until it is erased by the erase head wherever and of whatever type it is. If we go back to the last blog, we’ll see that there is a loop which takes the delayed signal (in the case of a tape echo from the playback heads) and returns it to the record head. The control that usually sets how much of the delayed signal is returned is often called either ‘sustain’ or ‘duration’ or some such. By returning this delayed signal it again goes around the loop and provides further echoes as it passes the play heads. So we can get many more repeats than just the three from the playback heads.

Here are a few thoughts on the practical matters of tape loop echoes. 

The tape tension is really important. All the rollers should be kept lubricated. You can get some great bird squarks and rodent noises out of it if they tend to tie up, but that’s not always useful(?). The stuff to use is graphite, and not the stuff that has grease in it, because if that gets anywhere near the tape path, you’ve got a big cleaning job coming up. If you get a soft pencil and file it down a bit with a nail board or something, you’ll get enough to do all the rollers.

If a unit has been used for some time, you can get a small piece of fine abrasive paper, and clean up the capstan drive with it. They become polished after  a lot of use and start to slip a bit. You need to put a small piece of paper under the roller so that any of the abrasive stuff doesn’t get to drop into the motor.

Make sure that the heads are clean. Acetone or lighter fuel on a cotton bud will do it. Don’t use contact cleaner anywhere near it, because it leaves an oil residue and that is very bad news.

Degauss the heads often. That means you’ll need a head demagnetiser which are not a lot of money, and not hard to find on the web. Don’t go into Curry’s or Comet or Computers ‘r’ Us or wherever and ask them, because they’ll look at you as if you’d just grown three heads. (Not record heads). Don’t degauss the thing with it switched on, and don’t do it with the tape on, because you’ll get a glorious buzz that will be hard to get rid of.

Don’t operate your tape echo anywhere near a power supply; that’s one of those black plastic boxes that supply effects pedals and the like. They are very dirty radio frequency noise producers and you playback heads will pick the stuff up. It’s also wise to keep any leads away from those things. I’ll probably go into setting heads and the like but that’s a broad subject so probably a blog on its own.

Tea most definitely calls.

Delay Line Basics

As I’m prospectively doing a post on Tape loop delay machines (Echolette NG51 specifically) I thought I might do this as a sort of road map; preferably before we get completely lost, rather than after.

The triangle signs are often used in semiconductor electronics for an amplifier. Strictly speaking it’s usually a non-inverting discrete chip, which could have a number of these devices built into it. That sounds a lot like bullshit and doesn’t say much. Less strictly speaking, and cutting out as much bullshit as possible, it’s just some sort of amplifier. Something goes in and something comes out; the ‘out’ something usually  being bigger than the ‘in’ something. This is called GAIN and if it doesn’t sound familiar, there are other places on this site that deal with it in more detail. 

So, physically, these amps could b transistors, a chip (integrated circuit) or a valve. The input preamp on the diagram does just that it takes the input signal, which is likely to be, say, a few hundred millivolts and it amplifies this signal by quite a lot. It wouldn’t be unusual to see maybe 20 volts coming out of it for a valve preamp. The rest of the circuit needs these kinds of signal voltages to work with.

The very next thing we see is a connecting line (or signal path as it would generally be called) and it splits. The ‘S’ marked under the signal path just says that this is the straight signal, just a bigger version of whatever it was that came into the input (from your guitar, mic or whatever). We now need to ignore the signal path that branches off, and just follow the horizontal path. If not we all go insane. Well I would anyway.

Along this top path we next get to the input mix buffer. A ‘Buffer’ is a preamp, the main function of which is to prevent one part of the circuit interacting with another. It can be something called a ‘Unity gain preamp’ and that just means it has a gain of one, or the same in as out if you like. Its main function is the ‘buffering’ action we just described. In this case it also acts as a mixer, which we’ll come back to in a bit.

The next component of the Delay can be many things. It’s called the ‘Delay line’ and is the bit that does the delay-ing. This can be a magnetic tape loop (as in Roland Space Echo, or Wem Copycat, or Echolette, or Echoplex) or it could be a disc (as in a  Binson Echorec, or Schaller) or it could be a spring line ( as in the Acutronics  that you find in a ton of combo guitar amps) or a studio plate reverb or it could be a digital delay line. The big function of this bit is that the incoming (straight) signal comes out of the other side later than it came in. Depending on which of those devices it is largely by how much it is delayed that gives these different gadgets their character. A spring line will never sound like a tape delay for instance.

So we’ve now got a delayed signal and this has lost a lot of its initial amplitude (volume) which means it has become attentuated (less). I feel a desperate cup of tea coming on.

Because it has lost much of its original signal strength we have to put it into the next pre amp, and then out into another and then to outside world; and if it isn’t dizzy by the time it gets there, it will be after this next bit.

We’ve so far ignored the two delay loops, but we need to know what they’re all about. The first split off after the first preamp and went to the output mix preamp. This mixes the straight signal (also called the dry signal) in with the delayed (wet) signal, and there would almost always be some way of varying the wet/dry mix at this preamp. We’ve also got another signal path that takes the delayed signal and feeds it back to it’s own input. This is how reverb duration is done; the delay line (whatever it is) has its own delayed signal turned around and re-delayed, if you like. This is an example of a particular kind of feedback loop, and would be tightly controlled or the whole thing would become unstable and whine, wail, whistle; and maybe all together.

For a plate reverb or a spring line the output signals are very complex and any kind of feedback is usually avoided, because it’s no easy matter to keep it controlled with these devices.

That about covers it. I hope I can take an Echolette NG51 to bits soon and make all this stuff a bit more real. TEA TIME!!!!!

Valve operation part 8 (Low pass filter)

 

Following on from Part 7 which was devoted mostly to the  basic HIGH PASS filter network if we compare the diagrams above with those in the previous post, the only apparent difference seems to be in the positions of the resistor r1 and the capacitor c1. As before, the two ciruits, A and B look different, but are in fact functionally the same.

So what does a LOW PASS filter do, and how does this work? Well, a LOW PASS filter does what it says it does (which is pretty refreshing in an age that specialises in bullshit, don’t you think?) It passes low frequencies. This means that any high frequency signal applied to the input will be attenuated by some degree at the output. ATTENUATION means drop in amplitude; and AMPLITUDE  simply means how much of it there is, usually in terms of volts or millivolts.

Depending on the values of r1 and c1 (we’ve not touched on values much, yet) the high frequencies will see c1 (whose impedance is Xc) as a low value, and r1 will be constant as its value is the same for all frequencies. As the frequency goes up, the impedance (Xc) of the capacitor goes down (it’s an INVERSE RATIO) so at the extreme, Xc would be zero, so nothing would appear at the output, because its all going to ground through the shorted c1. It never quite works out like that, but it can get very close. At the opposite end of its operation, for very low frequencies Xc is close to an open circuit, and so it filters next to nothing.

These two filter networks have a particular SLOPE PER OCTAVE. On the previous LOW PASS  filter, as the frequency increases the output falls by 6db per octave; and for the above HIGH PASS filter the output falls by 6db per octave. The OCTAVE bit is easy to explain. A frequency an octave higher is twice the figure. So for a 100 Hz signal, an octave above that would be 200 Hz.

The db bit is more involved and is a logarithmic comparison figure (?). To keep it so that it at least means something until we can get to think about it later, 6db gain is twice as much; and 6db attenuation is half as much. So if our signal into the HIGH PASS filter is, 100 Hz; and we get say 1volt at the output, then an octave higher signal at the input (200 Hz) will produce an output of two volts.

Below is a graph of the way the High Pass filter operates. Approximately.

The vertical axis is in volts and is telling us how much is coming out of the network. The horizontal axis at the bottom tells us the frequency at any point and this is doubling as we travel right; 100, 200, 400 Hz etc. The horizontal line at the top of the graph represents a constant voltage signal input of 6 volts (but it could be anything). The actual numbers of anything in this graph don’t mean a lot. They would vary with the resistor and capacitor we happened to choose. Neither does the shape of the ascending line do what it should, quite. It should be a smoothly ascending curve, but as I can’t draw one, it isn’t. But it still gives the general picture. If the voltage out at 100 Hz, is (say) 0.5 volts which we can read off on the vertical axis, at the frequency an octave higher than that (200 Hz) we’re up to 1volt. So the output has doubled. An octave above that (400 Hz), and the output is up to 2 volts, so double again; and at 800 Hz at the next octave we’re up to four volts.

Before we get to the next octave which would be 2x 800 Hz (or 1600 Hz) and 8 volts output, we run out of volts. Because the signal going in, we said was 6 volts, that’s as much as we can get out, so the filter levels off and can have no more effect. Think of everything in reverse for the low pass filter arrangement, and that’s about it. Time for tea.