sourcing potentiometers - another electronics query

[ik9000]

Trying to find the right pot to swap into a guitar, and having trouble finding the specs to begin to make sure it will fit. I could also do with suggestions on reliable places to order from. Any tips?

I need a "15A 500k pot" with additional push-pull switching. simple right? Turns out no.

The original part no from Yamaha is QC764500. After much hunting and frustration I've found out that Yamaha no longer supply this part and it is now suggested to use EG110100. A few US sites say they have it, as do Yamaha US (who are willing to sell direct to consumers) unlike Yamaha UK/EU who insist you go through a service center. The uk apparently has one place only, and it's miles away.

I don't want a direct swap, what I need is a matching pot, with the same diameter, stem length, performance etc, but additional push-pull switching on the shaft (which presumably makes it SPDT? instead of SPST - as much as a pot can be ST that is.)

I'm reluctant to dissassemble the thing until I have all the parts ready to swap in otherwise I'd just measure the existing one for physical specs. If anyone has a good way or magic table etc that can confirm the specs of an EG110100 I'd be most grateful to know what they are!

Then there is its electrical spec too:

The service manual doesn't explicitly say whether it's linear or log, though it's a Yamaha, so Japan, possibly, or maybe Taiwan made (it is a cheaper one). I found a table and apparently Asia use A for log, which would make sense for a volume pot from what I've read on various places. However the tone pot is listed as 250K 20SL (and it does have push-pull switching already). Does that suggest they're using Vishay references (http://www.resistorguide.com/potentiometer/)? If so then could I actually need a linear pot for the volume, if L is being used for log pots?

To counter that the service manual for the sister model (similar but without the coil-tap switching) lists a "05A 250K pot" for the tone, and a similar 15A 500k pot for the volume, albeit a different part number (presumably due to needing a shorter shaft on that model). To me that suggests the volume and tone pots should need the same taper behaviour. So perhaps they've just used parts from two different places using different reference systems on my guitar, and both are actually log pots after all?

Is there a way to determine this by testing the output eg using a multimeter? Not sure how I'd achieve a constant input when it's still wired inside the guitar but presumably there is a way of working this out?

edit: for anyone who finds this via google and is having the same problem: What you're after is a metric Alpha medium-shaft A500k (log) pot with push-pull such as can be ordered from here: https://www.axesrus.co.uk/Medium-Sha...hpullalpha.htm

[matts-uk]

Long time since I had to mess with analogue audio (analogue anything really) but IIRC

Push / pull, is dual throw (DPDT). Tone controls are typically the R in an RC filter and Linear. Be careful with part numbers as designators may not relate to anything (useful). SL, stereo linear maybe, maybe not ? To be sure, you read the data sheet.

You could start here https://www.ebay.co.uk/bhp/push-pull-pot

[ik9000]

ok, so I've given up and ripped the thing apart. Physical sizes confirmed, but still no wiser re log or linear. I might just order both - it will be quicker.

It's an M8 thread, knurled spline knob protruding 13mm from the body, so probably 16-18mm total (the knob cover is 16mm internal clear depth). The knob is plain inside so uses a grub screw so the splined aspect is not as essential as it otherwise might be. The pot body is approx 17.5mm wide with "A500K" stamped on the body.

There is loads of space in the cavity for a wider and deeper pot. The neighbouring tone pot is indeed DPDT just with unused connections which makes sense.

[matts-uk]

ok, so I've given up and ripped the thing apart. Physical sizes confirmed, but still no wiser re log or linear.

I did some digging and it turns out I was about right.

The A in A500K is for 'audio taper' otherwise known as logarithmic or log. Audio signals conform to the inverse square rule (double the distance = half the power) and are measured in dbm (decibels per meter). One db being roughly equivalent to a doubling of the power. Volume controls are normally a log pot so that a linear adjustment (turning up a notch) corresponds to an apparently linear response in perceived loudness. Electrically the signal change (Delta) from say '1' to '2' is much smaller than say '9' to '10'

The tone control is a simple band pass filter, formed by the pot (R) connected to a capacitor (C). Adjusting the pot changes the rate the capacitor charges, clipping more or less of the wave from the output signal, so modifying the effective frequency. The position of the pot corresponds directly to a time constant (t) with Delta-t being linear, hence a linear pot.

You can test for log or lin using a multi-meter set to Ohms. A small adjustment creating a large change of resistance near the extent indicates log.

Having said that, guitarists are weird, arty types who think nothing of eschewing perfectly good electronic design for the sake of 'the right feel'

[Millennium]

There is an electronics workshop coming up in March in Crystal Palace, they may be able to help with your questionablitly

[g8ina]

Just back from hols will read and reply asap

[ik9000]

no rush + thanks in advance

another question - do you know if it's possible to get a rotary switch (or 3-way on-off-on) around the base of a continuous pot? Like a 2-in-1 double shaft pot?

For the immediate need this would be better than a push-pull 2-way pot. The ideal would be a push-pull pot to allow coil splitting, with a rotary around the base to allow switching between standard wiring, and two treble-bleed circuit variants. I could do the latter via a new switch in the scratch plate, but it would be nice to not have to do that level of intervention.

final query as I think you're a bit clued up on muso electronics. Is it worth foil isolation in the cavities? It sure does hum and it would be nice to reduce it. Youtube is awash with this suggestion to shield things better - I'm willing to give it a go, but am cynical that kitchen foil would be sufficient on its tod. Surely copper foil would give a better result?

[g8ina]

as long as you can electrically attach to it properly copper or ali foil, or mu metal will do the same (ish) job. Its just that you can solder to copper

As far as the pot goes, tricky.... Custom pots for guitars are pricey. Volume pots will be LOG. Getting a pot to fit is easy enough, its the switching that makes it really awkward. You may have to go with an equiv pot electrically, but add a new switch as well.

[ik9000]

well I've sorted the wiring diagram and ordered all the bits from 4-5 different shops. Found some self-adhesive copper on Amazon. Just hope it's thick enough. You're right about cost: £5 per pot with push-pull. Not too bad individually but when you need a few it adds up! It was at least from a proper guitar shop so should be what I need though, and the dims matched.

First batch arrived this morning, other switches from an electronics supplier arriving later this week (annoyingly after what they charged for shipping (£12) vs the guitar shop (£2).

Quick Q: for a rotary switch to toggle between tone-pot bypass caps should I use break-before-make or vice versa? I've ordered both as cost was peanuts vs the shipping but not sure of the merits each way. I'm minded to use the former to hopefully avoid any momentary stages where no cap is acting, but it will mean temporary parrallel caps. Any thoughts which is the lesser of the two evils?

I can share the wiring schematic if that would help.

Having just looked at my new 5-way super switch I can see I'm going to have to rout a bit more of the body. This could get awkward as not much pickguard to hide it under.

[g8ina]

Make before break would be my recommendation.