Gizmo for testing pot and cap values

[John Clifford]

I built this little test box to allow me to try out the effects of different wiring options on a guitar with passive magnetic pickups. You connect the red lead to the hot wire from the pickups, the green lead to the output (tip) wire from the jack, and the black lead to ground. Then you can experiment with 250K vs 500K pots, 50's vs modern wiring, and a variety of cap values, all while playing the guitar. What I learned from this is that I really prefer a much smaller cap value than the factories typically use. I have a wiring diagram, if anyone's interested.

[Gordon Bellerose]

Is the diagram in "Electronic-eze?" or is it in plain ol' English with colored wires and clear connections?

The gizmo is very interesting, and useful for sure.

[Glenn Cummings]

Sure, I'd love to see the wiring diagram... and it would help round out this thread with the necessary info.
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It would also be nice to compare potentiometers of various tapers.
I guess it wouldn't be to hard to chase down an audio taper and a linear taper. maybe a reverse audio, or exponential..
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In the old radio days, we would have coils on jacks to easily swap out values. Something similar on this test equipment would be handy.
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[Gordon Bellerose]

This is just a thought; but would it be possible to have a simple 1/4 inch jack in, and another out?

[John Clifford]

Here's my rough wiring diagram. I'm interested in any thoughts y'all have on how to improve it. I thought about trying to incorporate linear vs audio taper options, but that would involve either another set of knobs (which I wanted to avoid), or 4-gang potentiometers with different tapers (which I don't know how to source or build). Unless you know of another way??

You could do 1/4" in/out jacks, but I don't see how that would help much. In order to use this, you have to disconnect the pickup and jack hot wires from the rest of the guitar circuit (assuming you're working with an existing instrument) and connect them to the box. That would mean you'd have to wire 1/4" plugs to the disconnected leads. Alligator clips seem simpler to me.

[John Clifford]

Just to round this out, here's a picture of the inside. The diagram is probably easier to follow.

[Gordon Bellerose]

Looks simple enough, thanks!
Are those DTDP switches? On/On/On?

Where did you find the box? Nice that is has screw holes for the back already.

[Joshua Levin-Epstein]

I like gizmos like this. Thanks for sharing.

1) If I understand correctly, the pots (250 or 500) are actuated by the same shaft. So you could not have a 500k volume all the way up with a 500k tone all the way down.

2) What might be useful is a way of removing the volume and/or tone control out of the circuit. This would test the value of a "blow" switch that sends a pick up right to the output (for maximum tone) or the value of those Fender tone controls that bypass the capacitance of the pot (for maximum tone).

I am skeptical of those bypass circuits.

Did you notice a difference between the 50's and modern wiring?

A box like this could also be used for comparing different caps of the same value ie oil and paper vs ceramic vs whatever. Once again I admit to being skeptical.

[John Clifford]

Looks simple enough, thanks!
Are those DTDP switches? On/On/On?

Where did you find the box? Nice that is has screw holes for the back already.

Gordon, they are DTDP on/on (2 position) switches. The way it ended up, I could have used SPDT switches for two out of three.

The box came from Amplified Parts:

https://www.amplifiedparts.com/products/enclosures

[John Clifford]

I like gizmos like this. Thanks for sharing.

1) If I understand correctly, the pots (250 or 500) are actuated by the same shaft. So you could not have a 500k volume all the way up with a 500k tone all the way down.

2) What might be useful is a way of removing the volume and/or tone control out of the circuit. This would test the value of a "blow" switch that sends a pick up right to the output (for maximum tone) or the value of those Fender tone controls that bypass the capacitance of the pot (for maximum tone).

I am skeptical of those bypass circuits.

Did you notice a difference between the 50's and modern wiring?

A box like this could also be used for comparing different caps of the same value ie oil and paper vs ceramic vs whatever. Once again I admit to being skeptical.

Joshua,

The pots are dual-gang like these:

https://www.amplifiedparts.com/products ... audio-dual

I used a 500k ohm dual gang for the volume pot. If you use one gang only, it's 500k. If you wire the two gangs in parallel, you get 250k. That's what the switch does. I used a 250k ohm dual gang for the tone pot. Similarly, if you use one it's 250k, and if you wire them in series, it's 500k, and that's what that switch does. So yes, you CAN have a 500k volume all the way up with a 500k tone all the way down.

The rotary knob as built actually has an empty slot with no capacitor (not shown on my diagram), which has the effect of taking the tone knob completely out of the circuit. That makes a handy reference.

There are big differences between 50's and modern wiring. With 50's wiring, the volume and tone knobs become highly interactive, and you don't lose treble when you turn the volume down - in fact it really sounds more trebly. It's kind of hard to describe the differences - you just have to play with it and decide which you like better. I think for most purposes I prefer modern wiring, because it's more straightforward.

I don't think the type of capacitor (oil and paper, ceramic, etc) makes any discernible difference by itself; HOWEVER, there is a big difference in ACCURACY of the capacitance rating between different makes. I've found the tiny little brown ceramic caps are usually way off compared to the orange drops, which is why I prefer the latter.

[Joshua Levin-Epstein]

John,

Thanks for the in depth tour. That is a very elegant way to compare the controls.

Now I really like this gizmo.