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Hot Wiring Your Guitar

The FTPA41's big brother and its possibilities have to wait a little longer, as amidst the mail I found a couple of requests for information. Firstly, to Mr. Moore in Yorkshire, my apologies for the delay - life is currently hectic. I have tried a passive mid-range idea for bass or guitar, and so far it's useless. My brainy colleague, Dave Petersen, hopes to get to an active unit design that might suit as soon as he gets out from underneath a pile of work, and I think I can suggest some useful alternatives in a future article where I shall look specifically at bass wirings.

Meanwhile, a brave beginner in Tyne & Wear is about to buy his first soldering iron and get stuck into his guitar. Courage brother, if you can spell reasonably, and can thread a needle, then you have sufficient wit and dexterity to wire your guitar any way you like.

A couple of thoughts on the iron. When I first started wiring guitars and melting my first capacitors, I used a little 15 watt Antex. As it wasn't very hot, I damaged fewer components than my tumblings deserved. On the other hand, the iron wasn't hot enough to solder to pot casings. A 17 watt alternative to this is available and while a beginner will find this useful as technique develops, more heat will be needed for proper shielding; like soldering on to copper foil, pot casings, string earths and so on.

My regular iron in my home workshop is an industrial type solder station with a transformer and interchangeable elements. I use a 370°C for general work on guitars, a 430°C occasionally for heavier stuff-earthing and so on, and a 315°C for light work like trying out different capacitor values on taps or pots. It's a fairly expensive set-up, and would be hard to justify for occasional work or a one-off job.

Antex make a 25 watt iron which they say has a heat capacity equivalent to that of a 40 or 60 watt conventional iron. I would advise a beginner to use a well known make such as this rather than to take pot luck in a bubble-pack D.I.Y. store or spend the earth on a professional set-up which could collect more dust than wear. This 25 watt iron is also available as a kit with a stand and some general advice on soldering. I think a stand, and the accompanying bit of sponge for cleaning the tip, is an absolute essential.

Desolder braid will also prove useful to the beginner, as in the early days of soldering, much excess solder may be splashed about. Desolder braid can be used to clean up mistakes simply by placing the braid against the solder you wish to remove, and heating it with the iron so that the solder soaks into it. I have found clamping action surgical forceps extremely useful for work in and on guitars, both from the point of view of poking a bit of wire and iron into a confined space, and from the point of view of heatsinking components and thin pick-up conductors. A good medical supplies house will usually stock a cheap line that will be suitable. Alternatively, smooth grip pliers will do. Personally, I prefer the clamping forceps because you can let go and leave them attached to the work.

The specific query from our friend in Tyne & Wear concerns the possibilities offered by a pair of Lawrence L 500's and a couple of push-pull pots.

Figure 1. Lawrence L 500 wiring.

The L 500's first. I first came into contact with these pick-ups a while ago when I was asked to look at them for another, sadly defunct, publication, and I enjoyed them very much, but found them a little too limited tonally for my own purposes. They are covered, along with Schecter three conductors, in my forthcoming book "Customising Your Electric Guitar". The L 500 is a rich, bar type high power humbucker which, set up right, will deliver the sort of full sound favoured by heavy metal freaks all the time, and most of us (not ashamed to admit it) now and again. The unit is wired three conductor and shield for overall pick-up phase reverse and coil tap. The ones I saw were sealed in epoxy, so it was not possible to add another conductor, nor did I find it possible to check out if the coils were wound in opposite directions or simply linked out of phase. I shall assume, for the sake of the diagrams here, that the coils run in the same direction and link out of phase - the effect using these colours will be the same either way. In Figure 1, you can see that it would be logical to take red as hot, black as earth, and white as the tap wire. Lawrence recommend tapping to earth via a capacitor, and if memory serves me right, the value is .02uF. It is (or was!) claimed in the accompanying wiring information sheet that this retained the humbucking character of the pick-up when tapped. I have said before, and must repeat, that this is not wholly correct. The .02 uF earth tap shaves off the treble frequencies from one coil, and thus as far down as the capacitor threshold at this impedance, the pick-up is effectively single coil, and thus normal single coil noise will be present from the highs down to this level. Below that level, some hum cancelling will occur. Thus, if you were to use .05 uF as tap capacitor, the background noise would extend to a lower frequency than if you used, say, .01 uF. Obviously, there could be quite considerable tonal variation. The value of a partial earth type tap via a capacitor is undoubted in terms of avoiding volume drop on switching from series coils to tapped mode, and it is possible to achieve some nice tonal variation and still retain enough power to distort an amplifier fairly healthily.

It is well worthwhile experimenting with different value tap capacitors as tonal results can vary quite considerably. During the prototype work for the circuit in the Vox Custom 25, I found that I could achieve a very passable "out of phase Strat" tone on a rear humbucker by tapping the coil nearest the bridge via one value capacitor on a 24¾" scale, siting the pick-up very specifically, but needed a wildly different value to achieve the same result on the same pick-up in the same place on a 25½" scale. I also found that slight variations in capacitor value could have the effect of moving the peak point of this tone from one string to another, and eventually settled on a value that landed it smack on the third, which is where the classic back and centre mix on the Strat shows to its best advantage anyway. Whether or not you can achieve this with a Lawrence I cannot say, much depends on your guitar's physical characteristics, and I do not feel that the Lawrence is as versatile as the Dimarzio X2N's which I invariably favour. However, experiment will certainly lead you through some interesting tones. Putting a couple of different values at a time into the circuit via a temporarily mounted (break out the P.V.C. tape) SPDT will give you a helpful A-B test facility. If you really get interested, mount up all the values you want to try on a double tag board, get yourself a couple of wires with hooded crocodile clips at each end, and try different values at different points in the guitar circuit. It used to be said that guitarists couldn't hear the difference between different value tone control capacitors - don't believe it.

Figure 2. Push-pull switched pot connections.

Meanwhile, back at the specific query, Figure 2 shows the back and front view of an Allen-Bradley push-pull pot. The top section is the normal rotary variable resistor, values available that I know of are 250k ohms and 500k ohms. The pot has attracted criticism in that the rotary motion can feel too loose for some tastes, and that having a nylon track, it can develop a static crackle. I have used one on an experimental base for some time now and have had no problems, but I do not like them for guitar as I suspect that they are linear (I have no confirmation or otherwise of this) as I cannot achieve the sudden sharpening up with a 1nF bypass capacitor that I can with, say, the Dimarzio 1M ohm or 500k ohm pots. I could argue the value of audio pots in other situations, but I have had excellent results with a 250k ohms linear pot for tone.

Whatever, the bottom half of this useful little beastie houses two pairs of contacts. Pull the pot shaft out and the lower pairs make, and the top pairs break, push it in and the reverse happens. Which reminds me of the time Fender first brought out a pullout top boost on the Twin - for months people were wandering around hopefully pulling the knobs off amps! A similar thing happened to guitars briefly when the Schecter custom guitars made their debut.

Figure 4. Pull switch used as double coiltap (on tone control).
Figure 3. Pull switch used as phase reverse (on volume control).

Figure 3 shows how to wire it up with a Lawrence for pick-up phase reverse and volume in a one volume, one tone and three-way selector guitar, with the pot as volume.

Figure 4 shows how to wire another in the same guitar for. simultaneous complete earth type tap on both pickups and as tone control. Wire B to earth would be replaced by a capacitor for a simultaneous partial tap.

The Allen Bradley push-pull pots are available in Britain from Chandler guitars.

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Speech Synthesis - Jabberwocky!

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Working With Video

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Nov 1981

Feature by Adrian Legg

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> Speech Synthesis - Jabberwoc...

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