SHOEHORNING A STRAT

ADDING ONBOARD ACTIVE CIRCUITS TO A STRATOCASTER

 

 

This is a long rambling discussion about some issues involved in bastardizing...uh, customizing a Stratocaster with an internally installed stompbox effect, specifically the defiled Boss Super Overdrive SD-1, shown above. It's not a how-to article, and it comes after-the-fact, so it's not decorated with lovely guitar guts pictures. Sorry.

I enjoy tinkering with my guitars and electronics. I'm not an engineer and couldn't say why specific component values are used in a schematic, but I can study 'em and figure out what stuff generally does. At least enough to be able to make some guesses which have actually ended up fixing stuff. With this level of ignorance, circuit design is out of the question, but that's not an obstacle to stuffing battery-powered circuits that other people have designed into a guitar. If you like the concept of active circuitry and don't want to get your hands dirty, you could buy a Line 6 Variaxe off the shelf, or one of those cute Fernandes gadget guitars. There are also DIY products designed especially for this purpose -- you can easily add an off-the-shelf EMG active tone control circuit assembly, or with more work, install a Graphtech Hexpander/Preamp circuit for your guitar synthesizer. These aren't really electronics-centered projects and rely mainly on common sense and mechanical skills. There's some satisfaction gained from a "shoehorn" project: It's fun, challenging, and personalizes the guitar more deeply than decorating it with strips of electrical tape. Ideally, it also adds a useful feature to your guitar. The downside is that it usually devalues your guitar, and it's not something that you'd want to do to a vintage collectible or if you love your guitar for its potential resale value. Oh yeah, I guess it doesn't do anything for the value of the stompbox either... (LOL, I wonder how much the "Made in Japan" plate is worth to some sucker?)

 

THE PLANNING PHASE

SELECTING THE CIRCUIT The big question is: What to put onboard? While an onboard PodXT might be kewl, that's not really practical or in the spirit of things (it's kinda big & kinda AC powered). In the past, I've added a wacky parade of simple stompbox effect circuits to my Strats, including an MXR Distortion+, Maxon Phase Shifter, Envelope Filter, and DynaComp (not all at the same time!). These were all fun to have onboard, but the most useful one for me was the Distortion effect.

While Fuzz/Distortion has uses in certain types of music, Overdrive is a more natural extension of the electric guitar sound, with a wider variety of applications. Most guitarists who play popular music like to have overdrive somewhere in their signal chain or amp to give the guitar a fuller sound, sustain, or extra bite as needed. Not all amps have decent overdrive channels and you can't count on being able to overdrive an amp's output tubes at the volume level you're playing at. Fortunately, manufacturers have done a decent job of creating an overdrive sound in the convenience of stompbox silicon. Having this available on a guitar would mean that you could plug straight into any amp without any extra fuss and dial in the amount of grit you wanted, for that instant "at-home" feeling. There's also the intuitive factor: With this installed in your guitar, you could switch between the normal clean sound and the gritty sound as intuitively as selecting pickups, without looking. It would also give you fingertip control so that you could blend into overdrive as easily as managing the volume. And if you want to, you could always put other effects between the guitar and amp, including another overdrive stompbox, for even more distortion!

There are some practical considerations which guide selection of a specific circuit. First is size and the fit: Most of the early MXR stuff that I'd installed in the past were simple '70s circuits, assembled on small, sparsely-populated circuit boards that could be trimmed to fit within the Strat's controls cavity. Relocating a circuitboard's components, if necessary, generally wasn't a big deal. A modern, densely-populated VLSI circuitboard isn't going to be nearly as cooperative with your efforts to reconfigure it.

Secondly, there's the issue of how operationally complex the effect is. Many modern effects offer scads of features and control parameters. For onboard installation, there's an obvious problem of mounting all the controls, and then actually being able to use them while you play. An effect with a bunch of control parameters works best in a floor stompbox or rack, where you preprogram the settings in advance of performance. At performance, footswitches give you simplified selection control over groups of those preset parameters. Oh, and it's probably not a good idea to install a tube-driven effect either: Those usually require AC power.

There's also the issue of aesthetics. I prefer that my Strats look pretty much like Strats, with the original knobs and not a bunch of extra switches and knobs. One of the most difficult things in the planning stages is figuring out how you can best accommodate that by using specialized ganged and switched pots for double duty and eliminating controls that aren't absolutely necessary. These things narrow the field of prospective overdrive pedals considerably.

Semour Duncan pickup booster Initially, based on the controls count, I considered installing a DOD "Yngwie Malmsteen" circuit (OD & Volume pots) since they were fairly cheap. However, I ran across a Seymour Duncan Pickup Booster (with a single volume pot) first, so I installed that. It wasn't a difficult conversion, although the circuitboard did have to be stripped of all its soldered-on jacks and switches, and drastically trimmed to fit within the controls cavity. I installed a mini DPDT paddle switch in the guitar's scratchplate to enable effect bypassing and used a switched pot in the lower tone control space to control the boosted volume and switch the battery power on/off.

I liked it okay... The Seymour Duncan Pickup Booster circuit does exactly what it's advertised to do: It provides up to 25 dB of clean, uncolored gain. But it was kind of boring. It would overdrive some effects and amps, but it would make some (like a Twin Reverb) just much, much louder. What I really wanted was a circuit that would let the guitar feel at home whether driving an ultra-clean Twin Reverb or an ultra-distorted PodXT patch at a variety of volume levels. I happened to have an old Boss Super Overdrive (SD-1) circuit installed in an unused Frankenstein'd box.

BOSS SUPER OVERDRIVE SD-1 Word is that the SD-1 is similar to the more popular (and pricier) Ibanez Tube Screamer/Maxon 0D808-- It's got a little more gain and wasn't popularized by SRV, but those aren't glaring deficiencies. There are circuit mods to improve the SD-1's bass response and turn them into Tube Screamers, but I like the stock sound just fine: The SD-1 adds mid-range fullness with growl, preserves the highs, and at high volume, the Fletcher-Munson curves generally take care of the bass response.

By modern stompbox standards, this is about as bare-bones as they get: A Tone control, a Drive control, and a Level control. This matches the number of control pots in a standard Strat, but there's one small problem: You don't want to sacrifice any of the standard Strat's essentials. If the battery dies unexpectly, you still want your guitar to function like a normal guitar. At the very least, a guitar should have a volume control. A tone control is desirable, but not absolutely necessary.

THE LEVEL CONTROL After studying the circuit, I determined that the SD-1's Level control couldn't be shared with the Strat's standard volume control. They're radically different values (10K versus 250k) and the SD-1's Level control is situated before the final output transistor in the circuit. I didn't want to use up two of the three control spots for two volume controls, even though this would mean losing the ability to switch between presettable volume levels. Instead, I decided to install the 250K volume control on the output side of the circuit, before the output jack. In bypass mode, the pickup selector signal would be sent to the DPDT bypass switch, and then to the output volume pot. In "circuit on" mode, the pickup selector signal would be sent to the bypass switch, to the circuit, back to the switch, and out to the output volume pot. This does the same thing as the SD-1's Level pot-- it controls final output volume-- so the circuit's control isn't needed. It can be replaced with a trimmer, fixed resistor values, or left to bang around inside the controls cavity. As for the setting, full tilt works for me, but if you want to balance it for a single overdrive level/bypass, do that before you seal up the innards or install an externally adjustable trimmer.

This deviates from the way it would work in a stompbox setup, where the input signal could be attenuated at the guitar before entering the circuit, to "clean up" and control the amount of "bite". However, an onboard installation doesn't sacrifice that ability since there's a more effective way to do that-- use the Drive control. That's easy to do if the control is mounted on your guitar.

THE DRIVE CONTROL A natural place to mount the Drive control is at the Strat's upper Tone control. This gives you fairly quick pinky finger access to it so you can tweak it on-the-fly. The SD-1 uses a 1 meg linear pot for this control, which is clearly at odds with the usual 250K tone pot. Fortunately, there's a fairly common specialty pot which can help you solve several problems at once.

Fender's TBX ("Treble-Bass eXpander" -nb: It's actually "Treble-Bass X-over", or "Crossover") circuit is provided in some Strat models as an upgrade to a regular treble cut tone control. Despite its impressive name, it isn't a very impressive circuit: a resistor, a capacitor and a specialty pot. It's basically a standard tone control for half its travel and then a no-load pot for the other half. However, the pot is useful in this application because it's a ganged (stacked) pot made up of a 1 meg section and a 250K section with a knurled split shaft that fits a standard Strat knob. This means that you can retain the traditional Strat cosmetics and have the control serve as an overdrive knob in one mode, and a standard tone knob in bypass mode.

However, it's not a ready-made solution. The mounting hole may need to be enlarged but that's trivial. The biggest problem is that the pot is manufactured with a center detent, and it's a very stiff pot that requires grasping the knob to turn it. This wouldn't be a problem for a set & forget control, but isn't acceptable for a control that you want to tweak on the fly.

ALTERING THE TBX POT To fix this, the pot must be disassembled. The detent mechanism is an inward retangular stamping on bottom of the metal housing of the 1 meg pot which engages an open area of the nylon rotor. Flattening the stamping on the housing will allow the rotor to turn freely and reduce the notched feel (it isn't entirely eliminated because one of the carbon tracks is cut in the middle).

The stiff feel of the pot is due to the compressive pressure of the pot's assembly and the thick grease in which the rotors swim, in both sections of the pot. The grease is easy to clean out with a solvent, but doing so will also make the pot feel less smooth.

The stiffness from the compressive pressure comes mainly from the outer 250K section. The nylon rotor is pressed hard against the metal housing. You can lessen that force by sanding down the nylon contact area, but you should finish with fine polishing to both the inside of the housing and the nylon contact area. This will help the pot feel a little smoother, although the main goal here is just to make the pot turn easier so it can be operated by pinkie, without having to grasp the control knob.

It should be obvious how to wire the overdrive pot using the SD-1 as an example. The only decision about the tone pot is where to connect it in the signal path. I intended for the control to be modal (you don't control overdrive and tone at the same time since they're at opposite ends of the pot), so the tone pot was connected at the bypass switch, in the bypass mode. This gives global tone control for all pickups in bypass mode. You could connect it at the pickup selector switch to act on only a single pickup, or in the global output path and it would bleed treble off in both modes. However, you wouldn't be able to bleed off treble with the pot set at full overdrive because the pot can only be in one position at a time.

THE TONE CONTROL The easiest controls decision was to replace the Strat's lower tone pot with the SD-1's tone pot. I never use the lower tone pot anyway, and usually wire the upper one to be a global tone control for all pickups. The SD-1's active tone control has more pizzazz than the stock treble cut circuit. As with my Hexpander installation, I used a switched pot to turn on/off the battery power. Basically, it's easier than plugging/unplugging a guitar's 1/4" switched jack, which is a common way to manage battery consumption (and might present some mounting difficulties anyway--see below).

THE BATTERY BOX The easiest onboard battery solution for a Strat is simply to locate the battery in the 1/4" jackplate cavity. The jackplate is flipped so that it protrudes on the body surface and the jack is moved inside the hump. In order to use the jack to manage battery consumption, you would need to find a very small ring-tip-sleeve (stereo) jack, or wrestle with getting a common one to fit inside the plate without shorting or defeating the purpose of the switch. Small switch/stereo jacks do exist, but they aren't common.

 

GETTING DOWN TO IT

So that covers the planning phase... It's a good idea to mentally solve problems before actually getting down to it, but it's always a mix of both because some ideas need to be tested. The circuitboard is probably the first thing you actually work on, since if it doesn't work, there's no point bothering with anything else. Planning how the controls and everything else is going to work is a necessary preliminary exercise to help you decide if the job is feasible. If it isn't, there's no point... etc., etc.

TRUE BYPASS SWITCH With the circuitboard, probably the very first thing you should work on is adapting it for a true bypass double-pole, double throw (DPDT) switch. That's a useful thing even if you don't plan on mounting the circuit onboard. The SD-1 circuit (and Tube Screamer too, if I'm not mistaken) use a momentary contact switch which triggers a flip-flop circuit to internally engage or bypass the effect. I had to deal with this issue a long time ago when the momentary contact switch became maddeningly unreliable and I wanted to replace it with a simple mechanical switch that I could understand.

To use the circuit in a true bypass switch, the circuit needs to be permanently stuck in the "on" mode. I didn't have a schematic but managed to figure out where to jumper the circuit by following traces and poking around. Eventually, between all the squeals and squalks, I found a pair of points which when jumpered, forced the effect on, accompanied by the LED confirmation. (If it weren't such a hassle to disassemble everything, I'd show you where-- but this is a freebie, so don't bitch.)

Wiring a DPDT true bypass switch is easy and covered at other websites, so I won't go into that. I will point out that if you wanted to include an LED indicator in a footswitch, you'd use a triple-pole, double throw (TPDT) switch. For this application though, an LED would only bleed the battery faster without doing anything useful-- the position of the DPDT paddle switch is your effect state indicator.

FOLDING THE CIRCUITBOARD Now, for the thorniest part of the conversion: Fitting the circuitboard in the guitar. Staring at the SD-1 circuitboard, you'd probably conclude that it's too long to fit into a Strat's controls cavity. This is correct. However, if the circuitboard were folded/cut in half and stacked, could it fit? There was at least a good chance of that working-- at least by my eyeballing. I hadn't actually opened up the guitar at this point to measure anything since I didn't want to have to go through that work and reassemble it without accomplishing anything. Besides, it might be discouraging: I had a certain amount of faith that I could make it work somehow, because I really wanted it to work. Instead of wasting my time on that task, I decided to see if I could cut the circuitboard in half, jumper the severed connections, fold it and have the thing still work. If I couldn't do that, there was no point in opening the guitar. Even if it didn't fit in the guitar, I'd still have a working SD-1 circuit. Smart, huh?

FOLDING THE CIRCUIT BOARD, ILLUSTRATED (06/08)

The good news was that the circuitboard worked and fit within the controls cavity. The bad news was that the circuitboard sandwich was thick and its thickness combined with the selector switch above it prevented the scratchplate from seating on the guitar... by about half an inch. The trouble area was awfully small, but there was no way to eliminate that area of the circuitboard or switch. Hmmm... After you've done that much work you're not easily deterred and are open to more radical solutions.

IF IT DON'T FIT... POWERTOOL SHOEHORN IT I really didn't want to route a cavity in the guitar because it's gruelling work using a hobby rotary tool. However, I was willing to in this case because I really wanted it to work. So a few hours later, I had a rectangular cavity under the pickguard, about as deep as the pickups' routing, just forward of the selector switch. I bagged the circuitboard with shielding, packed it in there, soldered connections, tested it, and sealed it up.

Stratocaster onboard overdrive active electronics

THE RESULTS

I like it when stuff works. It's still the same Strat, but flipping the toggle switch kicks in the SD-1 overdrive. There's a switch pop the first time the effect is switched in after power is applied, but after that the switching is quiet. At a middle setting on the overdrive pot, the level and tonality are similar to the regular Strat sound, minus a little of the bass response. At the full drive settings, there's quite a bit of gain. The switched change in level is useful for kicking into solo, but may not always be appropriate. The guitar's output can be quickly adjusted or preadjusted with the volume pot, but it takes some getting used to so you know how much to raise or lower the volume. The switch is oriented so that the SD-1 circuit is engaged in the same direction as lowering the volume, so it can be done in one motion. Disengaging it can be done with the third finger while the pinkie cradles the volume knob. It's reasonably intuitive. Adjusting the SD-1 tone control down also cuts a bit of the output level. Running the guitar through a compressor or another effect that compresses a hot input signal will equalize the volume difference.

There may be some theoretical benefit in having the gain boost circuit close to the pickups (instead of at the end of a long cord), but it's not something that I've noticed. The circuit and guitar are well-shielded and as quiet as can be expected. However, it's the nature of high-gain circuits to amplify whatever EMF noise that happens to enter the pickups-- Kinman's are very quiet, but they're not advertised as being noiseless. (The only totally noiseless pickup is a broken one.) --09/16/03

 

PART 2- FOLDING AN SD-1 CIRCUIT BOARD

SHOEHORNING AN IBANEZ RG-2550Z

 

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