Universal / bathtub / swimming pool pickup cavity - issues to consider?

[ZachM]

Hello everyone! Long time poster here, new builder.

Preamble

I'm not a luthier and haven't really done anything to date beyond simple part swapping on existing instruments. However, I've wanted to get into the game for some years and I am considering doing a parts build P-bass, alá something like what @Dark Horse is always cooking up, probably also using MJT body and neck. Initially, I basically want a good no-frills p-bass, very much like this.

However... there is one custom option I've always coveted but never really seen: the ability to have different pickup sets within the same instrument that are easily swappable by changing out the whole pickguard. Basically, I'd like to have an instrument platform that remains the same, but be able to use a P pickup one day and a Thunderbird set the next day with a quick swap. (By quick swap, I mean something you could accomplish in 10 minutes, not between songs on a gig or anything like that)

I think this is something you could accomplish with a big universal route, at least for a few variations. However, there seems to be some possible issues with this:

Mounting

Most bass pickups are traditionally direct mount, i.e. screwed into the body rather than mounted to the pickguard. I'd need to figure out a good way to solidly attach direct mounted pickups to the pickguard for them, and the P pickup would be particularly complex due to its shape and the fact that it's two pieces. Basically for each pickup, I'd have to engineer a new mount for them.

Pickguard flex

Universal routing seems a bit more common on guitars than on basses and some of the super-strat type guitars have pretty big ones. However, my concern is that there would be a significant degree of flex with a bigger pickguard like a P bass guard. Indeed, if you search you can occasionally find guitarists complaining about the flexiness that is caused by swimming pool routes, but crucially it doesn't seem to matter all that much when they're actually playing. Most guitarists would be playing with a pick and not really striking or otherwise contacting the pickups. In contrast, many or even most bassists (including myself) tend to put their fingers on the pickup as an anchor when playing. I'm concerned that the degree of flex induced by a large rout + plastic pickguard + guard-mounted pickups might be enough to actually mess with string balance while you're playing, especially on the lower strings.

Tonal changes

This sort of gets into the realm of preference, but the resources I've found on this topic do generally seem to imply that there's a noticeable difference in tone between direct mounted pickups and pickguard mounted ones. I have not been able to test this myself on the same guitar, but it does make logical sense that a more stable platform for the magnets would translate into different tonal results. My T-40 has pickups mounted in the guard, and it's not what you'd call weak sounding - but it has a lot of wood behind the guard, and mechanically it's very solid.

In summary, has anyone here tried this before and ended encountering this stuff? Or am I making a mountain out of a molehill here?


TLDR: I want a bass with a big universal route for pickup swaps. How much pain am I in for?

 

[GretschWretch]

I'm no luthier but I have flirted with this idea from time to time: multiple pickup options on the same bass. If you are considering using P pickguards exclusively, you might consider gluing pickup mounting rails in the correct placement for the given pickup beneath its companion pickguard. Rails attach to underside of 'guard, pickup(s) screw into rails through pickguard, pickguard anchors the whole apparatus to the bass.

Gretsch and other manufacturers have used rail mounts for pickups, but in most of these cases the instrument is a hollow-body and the rails are in some way glued to the underside of the top. For your project, using rails instead of just mounting blocks beneath the pickups would add some strength and stability to the pickguard.

 

[ZachM]

Pickguard shape is another issue I'm still thinking about... ideally, I'd like for the pickguard to have exactly the same outline as the original, just routed differently. The pickup combinations I'm interested in being able to covering with this project are these single pickups:

• P bass
• Thunderbird II
• Serek B-90, as shown in their Midwestern bass

And these double pickup sets:

• Delano Times Square
• This Mike Lull TPT 4
• Thunderbird IV
• Travis Bean TB2000

The sets in the first list are pretty coverable without disturbing the original pickguard outline, but the double pickup list (aside from maybe the TB2000) requires routing that goes pretty far past that. So I'd have to design another pickguard that looks like a P, but extends pretty close to the bridge.

 

[ZachM]

I'm no luthier but I have flirted with this idea from time to time: multiple pickup options on the same bass. If you are considering using P pickguards exclusively, you might consider gluing pickup mounting rails in the correct placement for the given pickup beneath its companion pickguard. Rails attach to underside of 'guard, pickup(s) screw into rails through pickguard, pickguard anchors the whole apparatus to the bass.

Gretsch and other manufacturers have used rail mounts for pickups, but in most of these cases the instrument is a hollow-body and the rails are in some way glued to the underside of the top. For your project, using rails instead of just mounting blocks beneath the pickups would add some strength and stability to the pickguard.

Interesting idea to use rails! That would give a relatively convenient way to attach the pickups and give some rigidity at the same time.

 

[ZachM]

Another approach for the pickup attachment occurred to me after the original post: instead of having a bunch of empty space in the bathtub when the whole thing is assembled, why not also build a simple "plug" for the route you made and mount all the stuff to that?

I'm not concerned with weight relief here, just providing different routing options without compromising structural stability. So, maybe something like this would be the approach:

• Bass body
  • make a big universal route, maybe 1.25" deep in a 1.75" body and extending pretty far to the neck pocket and bridge
  • drill holes in the back of the body, intended to take threaded inserts (same thing you might do for neck attachment)
  • only wiring permanently attached to the guitar is the bridge ground (some sort of harness to connect it to the output jack)
• Pickup "plug"
  • intended to fill the space left by the route, minus any needed removal for wiring, etc
  • top of plug is routed for pickups, and has some screw holes for the pickguard
  • pickups are mounted to wood
  • back of plug attaches to body with threaded bolts
• Pickguard
  • oversized compared to normal bass guard, extends much closer to the bridge to cover the bathtub
  • screws partially to the plug, partially to the body with the existing screws

So, I now have more questions. FIrst, how close can you route the bathtub to the neck pocket before you start compromising its structural integrity? I'm thinking of the TB2000 bass in the other post, which has the neck pickup basically right against the fretboard. This works on the Travis bean because the neck beam goes all the way into the body and the pickups screw into it, but that won't be happening if I retrofit a normal bolt on. Are you okay leaving ~1/8" of wood there or is that asking for trouble?

Second, how much wood would you need left in the back of the body (the thickness between the bottom of the route and the back face) to adequately secure the plug to the body, if you were using threaded inserts?

 

[fhm555]

The quick and dirty solution would be to build a module that would accommodate the largest pickup you wanted to use, then route your body to accept that tub. If you wanted different positions between neck and bridge, carve out a trough the length required to fit your tubs where you wanted them. Once you have that done, build a tub to accept each pickup combo you want, the cut your guard for that pickup/position and mount your tub to that guard. Once you had all your loaded guards done, figure out if you can use a universal plug or if you need an adapter plug for each unique wiring configuration.
I’m sure this may sound confusing to read but I’ve got it all in my minds eye and it would allow you to basically have an instrument you could swap pickups by removing the guard, plugging in the correct wiring adapter and screwing it down. If you were to go that route, I’d also suggest you set threaded brass inserts for your guard screws and use machine screws to mount your guards. IME, the more you remove/replace wood screws, the punkier the threads in the wood becomes, but threaded inserts and machine screws would solve that problem.
To save weight you carrier tub could be made from a thin plastic shell full of foam carved to accept whatever pickup you wanted in it.

 

[notabene]

Somehow... the "double bass" forum ends up in electric land for me. without any conscious shifting on my part.

 

[ZachM]

The quick and dirty solution would be to build a module that would accommodate the largest pickup you wanted to use, then route your body to accept that tub. If you wanted different positions between neck and bridge, carve out a trough the length required to fit your tubs where you wanted them. Once you have that done, build a tub to accept each pickup combo you want, the cut your guard for that pickup/position and mount your tub to that guard. Once you had all your loaded guards done, figure out if you can use a universal plug or if you need an adapter plug for each unique wiring configuration.
I’m sure this may sound confusing to read but I’ve got it all in my minds eye and it would allow you to basically have an instrument you could swap pickups by removing the guard, plugging in the correct wiring adapter and screwing it down. If you were to go that route, I’d also suggest you set threaded brass inserts for your guard screws and use machine screws to mount your guards. IME, the more you remove/replace wood screws, the punkier the threads in the wood becomes, but threaded inserts and machine screws would solve that problem.
To save weight you carrier tub could be made from a thin plastic shell full of foam carved to accept whatever pickup you wanted in it.

Great idea to replace the original pickguard screws with machine screws and do inserts. I did not think about that originally, but it would definitely be an issue at some point down the road. Overall this definitely seems like the approach to take, it would let the pickups be mounted however they normally would be and also let the wiring loom be nicely routed for.

One question - do you suppose that ~13 machine screws + inserts would be enough of a mechanical connection that the back of the plug/tub wouldn't need to also be affixed to the rest of the body? That would further simplify this whole setup, but I don't know if that's enough.

 

[Bruce Johnson]

Great idea to replace the original pickguard screws with machine screws and do inserts. I did not think about that originally, but it would definitely be an issue at some point down the road. Overall this definitely seems like the approach to take, it would let the pickups be mounted however they normally would be and also let the wiring loom be nicely routed for.

One question - do you suppose that ~13 machine screws + inserts would be enough of a mechanical connection that the back of the plug/tub wouldn't need to also be affixed to the rest of the body? That would further simplify this whole setup, but I don't know if that's enough.

Another variation on fhm555's idea is to make the pickguard itself stronger and use it as part of the structure. For example, make the pickguard(s) from aluminum. I usually use 6061 aluminum plate, 0,060" thick. It's not very hard to make aluminum pickguards. I have a thread on here somewhere showing my process for making them.

The point is, the aluminum plate, when fastened down by 13 machine screws and inserts, becomes a part of the structure of the body. So you can rout out a big bathtub underneath it and not worry about structural issues with the body.

Then, mount the pickups, the controls, output jack, and all the wiring to the pickguard. Mount the pickups directly, or into a wooden sub-block that's screwed to the aluminum, like what fhm555 described. All the components just hang down freely into the big rout. The structure is in the aluminum plate. Make up multiple pickguards, each with its own combination of pickups, controls and wiring. Swapping the configuration of the bass is just the 13 screws, no wiring disconnects.

As a bonus, the aluminum plate makes a great part of the shielding, covering the whole top of the cavity. Use shielding paint or tape in the cavity, with some good connection to the underside of the pickguard.

If you don't like the look of the aluminum pickguards, you can decorate the front face by epoxying on something like black vinyl, thin tortoise shell vinyl, wood veneer, etc.

 

[ZachM]

Another variation on fhm555's idea is to make the pickguard itself stronger and use it as part of the structure. For example, make the pickguard(s) from aluminum. I usually use 6061 aluminum plate, 0,060" thick. It's not very hard to make aluminum pickguards. I have a thread on here somewhere showing my process for making them.

The point is, the aluminum plate, when fastened down by 13 machine screws and inserts, becomes a part of the structure of the body. So you can rout out a big bathtub underneath it and not worry about structural issues with the body.

Then, mount the pickups, the controls, output jack, and all the wiring to the pickguard. Mount the pickups directly, or into a wooden sub-block that's screwed to the aluminum, like what fhm555 described. All the components just hang down freely into the big rout. The structure is in the aluminum plate. Make up multiple pickguards, each with its own combination of pickups, controls and wiring. Swapping the configuration of the bass is just the 13 screws, no wiring disconnects.

As a bonus, the aluminum plate makes a great part of the shielding, covering the whole top of the cavity. Use shielding paint or tape in the cavity, with some good connection to the underside of the pickguard.

If you don't like the look of the aluminum pickguards, you can decorate the front face by epoxying on something like black vinyl, thin tortoise shell vinyl, wood veneer, etc.

Thank you very much Bruce for this detailed answer! The pickguard itself becoming structural is a good idea. If you don't mind I have another question for you (and anyone else that might know): For a typical Fender-style bolt on like the P-bass, how much wood depth will be necessary between the vertical face of the neck (i.e. the part that is perpendicular to the fretboard) and the first pickup route?

On the T-40, there is a little more than an inch of wood thickness between the end of the neck pocket and the beginning of the first pickup route. On my Jazzmaster guitar, it's right around 0.5" but that still seems pretty solid. I'm just not sure how much strain gets put on that part of the body via string tension. I circled the part I'm referring to in the pic below:



The reason I'm asking is that one of the setups I'm most interested in is the Travis Bean TB2000 configuration, which has the neck pickup touching the end of the fretboard. That instrument can get away with it because the entire neck is a single piece of aluminum that goes all the way through the body, and all the hardware is mounted to it, including the bridge.



The Gibson EB-3 is another bass which has the neck pickup butted right against the fretboard, but since it's a set neck the stresses might be applied differently. Basically, can I pull off the TB2000 pickup configuration with a bolt-on neck instrument?

 

[Bruce Johnson]

Thank you very much Bruce for this detailed answer! The pickguard itself becoming structural is a good idea. If you don't mind I have another question for you (and anyone else that might know): For a typical Fender-style bolt on like the P-bass, how much wood depth will be necessary between the vertical face of the neck (i.e. the part that is perpendicular to the fretboard) and the first pickup route?

On the T-40, there is a little more than an inch of wood thickness between the end of the neck pocket and the beginning of the first pickup route. On my Jazzmaster guitar, it's right around 0.5" but that still seems pretty solid. I'm just not sure how much strain gets put on that part of the body via string tension. I circled the part I'm referring to in the pic below:

View attachment 7029034

The reason I'm asking is that one of the setups I'm most interested in is the Travis Bean TB2000 configuration, which has the neck pickup touching the end of the fretboard. That instrument can get away with it because the entire neck is a single piece of aluminum that goes all the way through the body, and all the hardware is mounted to it, including the bridge.

View attachment 7029040View attachment 7029044

The Gibson EB-3 is another bass which has the neck pickup butted right against the fretboard, but since it's a set neck the stresses might be applied differently. Basically, can I pull off the TB2000 pickup configuration with a bolt-on neck instrument?

I see what you are getting at. That strip of wood at the end of the neck pocket isn't going to do much structurally if it's narrower than an inch.

If you hogged out a big trench of a cavity, and took that strip out completely, the main load would be carried by the long strip of wood down the center of the back of the body. It starts as the bottom of the neck pocket and extends all the way back. The width will be whatever the width of your trench is.

That long strip will be loaded in bending as a beam by the neck and the string tension. The thickness of that strip is critical to make it strong enough and stiff enough to withstand that bending load. The body will need to be quite thick in the center. A quick guess is that you'd need that center strip to be at least 3/4" thick. That is, from the bottom of the trench to the back surface of the body, including the thickness under the neck pocket. That's what the strip would need to be if it wasn't getting much reinforcement help from anything else.

There are ways to reinforce it, so it doesn't need to be that thick. You could rout a pair of slots on the back and embed some rectangular carbon fiber bars, like are used in necks. Or attach a long rectangular aluminum plate on the back, resembling that Travis Bean. Or do some details in attaching the body wings to the center strip, so they carry some of the load. And the structural pickguard on the top surface will help too.

You need to do some engineering if you want to carve a big cavity in the center of a bass body.

 

[ZachM]

I see what you are getting at. That strip of wood at the end of the neck pocket isn't going to do much structurally if it's narrower than an inch.

If you hogged out a big trench of a cavity, and took that strip out completely, the main load would be carried by the long strip of wood down the center of the back of the body. It starts as the bottom of the neck pocket and extends all the way back. The width will be whatever the width of your trench is.

That long strip will be loaded in bending as a beam by the neck and the string tension. The thickness of that strip is critical to make it strong enough and stiff enough to withstand that bending load. The body will need to be quite thick in the center. A quick guess is that you'd need that center strip to be at least 3/4" thick. That is, from the bottom of the trench to the back surface of the body, including the thickness under the neck pocket. That's what the strip would need to be if it wasn't getting much reinforcement help from anything else.

There are ways to reinforce it, so it doesn't need to be that thick. You could rout a pair of slots on the back and embed some rectangular carbon fiber bars, like are used in necks. Or attach a long rectangular aluminum plate on the back, resembling that Travis Bean. Or do some details in attaching the body wings to the center strip, so they carry some of the load. And the structural pickguard on the top surface will help too.

You need to do some engineering if you want to carve a big cavity in the center of a bass body.

Ah okay, I think I'm beginning to understand now. I had mistakenly thought that the pressure was applied to the butt of the neck pocket, i.e. applied along the string path, but the bulk of the force pretty much gets transferred to the body along the neck line. So it seems like two approaches could work then:

Method	Pros	Cons
Thick body and/or heavier wood. 1.75" body possibly made from maple	Solid structure Doesn't need carbon fiber reinforcement Less complexity in attaching to the body because you don't need carbon fiber rods pickup plug can be lighter wood	Heavy may not be necessary
Softer body + carbon fiber rod reinforcement	lighter weight overall solid structure	Additional complexity with hardware attachment since there's CF rods to work around

I'm leaning towards the heavier wood solution especially for a POC like this. Even though the deepest pickup I'm planning to route for will be the P pickup (which seems to usually get a 3/4" pocket in normal mounting situations), I'll need to route more than 3/4" if I wanted to be able to mount the P pickup in the plug rather than the body, which I would since quick(ish) swapping of the whole system is a key part of the requirements.

However, if I did pursue the CF rod route - how would I need to install them? I'm not sure whether they would have the same strengthening effect inset into the bottom of the trench or the back. Not only that, but how much wood do they need to be covered by?