Convertible Volume 500k to 250k Push Pull

[frets]

Had a guy that wanted his volume to convert from 500k to 250k.  This is what I came up with.

[ashcatlt]

It'll definitely work. The taper will change, but who's gonna notice enough to complain?

[Yogi B]

May 23, 2022 19:27:40 GMT -5 ashcatlt said:

The taper will change, but who's gonna notice enough to complain?

Of the volume control? Barely.

It'll potentially have a more noticeable affect on a tone controls 'relative' taper (assuming one is present). At maximum tone switching to 250k should have a noticeable effect, but at some point (roughly midway) there'd be no difference whether the 510k resistor is engaged or not.

From a technical standpoint, the main disadvantage with the parallel resistor approach with a 'raw' volume (no treble bleed) you get the worst of both worlds in relation to loading. Usually the lower value of a 250k pot means that the effect of cable capacitance is lessened, such that in the upper ~50% of rotation (excepting max volume) you get a slightly brighter tone from a 250k pot than a 500k pot. Therefore, however, the addition of the resistor works as probably most people incorrectly imagine the switch between 250k and 500k goes (whereby the '250k' is always darker).

In order to switch between 250k & 500k properly you really need a dual-gang pot (any of 250k/250k, 250k/500k, or 500k/500k can be made to work).

[Deleted]

250K/500K

1) Dual 500K that have the Two Middle Lugs2 hooked together and the other (lug1/lug3) to the Switch that then go to the input and ground

2) just change the second wafer to 250K or 500K (easy as that)

[frets]

The dual gang pot is a better idea. I’ve got them but never cracked them open.

[newey]

Dual gangs with one 250K and one 500K gang are available, but hard to find. Mouser had them the last time I checked (which was a couple years ago). But would save the need to mod a pot.

[Deleted]

Push Pull is a easy one to do this
17mm

Just watch how you split a Push Pull
Pull it up and undo the claws
As you pull the case it will switch up

It's just sort of hooked up so as you pull up you push away (away from the solder tags)
Should be able to pull out the arm.

Need Dual Pot and change the lower wafer.

To put back together you have to watch how you do it. You need to hook the arm back in to the switch , need to watch this as once it clicks down it's hard to fix.. hook in and slowly aim for the claws to go in to the switch .

[frets]

I wonder if I can use a regular Bourns 250k wafer on the dual gang push pull? Otherwise, I’ll be scraping carbon.

I will look Newey on Mouser for the animal you speak of.

[newey]

I just checked Mouser and the only two-value dual-gang pot I found was a 1K/10KΩ, not suitable.

BUT!!!

There is a CTS version that has a Fender part number, these are available multiple places on the web, or, presumably, through any authorized Fender dealer (patronize your local music store, IOW)

www.amazon.com/Fender-0990847000-500K-SPLIT-SHAFT/dp/B01LWZ0IVU

[JohnH]

Couldn't you just use a dual-gang 2x 500k pot? One side is wired normally, the other has it's cold side grounded and a dpdt switch connects the wiper and hot outer directly to the respective lugs on the main side of the pot. Then it's two x 500k pots fully in parallel and equivalent to a 250k at all settings

[frets]

Thanks you guys.
For this guy, I’ll go with the pot Newey found. I already have the push pull with the resistor installed; but, I’m going to pull it in favor of the CTS pot. Nice find!😺😺😺. I did not know that pot existed.

[Yogi B]

May 24, 2022 10:17:23 GMT -5 @angellahash said:

1) Dual 500K that have the Two Middle Lugs2 hooked together and the other (lug1/lug3) to the Switch that then go to the input and ground

2) just change the second wafer to 250K or 500K (easy as that)

I'd do the first slightly differently, keeping lug 1 grounded and switching in lugs 2 & 3 (as JohnH described), but it's the same principle.

---

So there're alternatives to using a pot with disparate wafers, both a dual 500k or a dual 250k can do the job (almost) equally as well (newey just wants you to use the option that leaves no lug unsoldered ). Here's what all three options would look like:

Note: using a duplicate of whatever dual-gang used for the volume, it's also possible to simultaneously switch a tone control from 500k to 250k using the same push-pull (optionally changing the tone cap value, too) — though a tiny amount of fudging may be required.

The main advantage an actual 500k/250k dual gang has is that differing tone controls and/or treble bleeds can be used without worrying about the constraints imposed by the other options.

May 24, 2022 14:44:01 GMT -5 frets said:

I wonder if I can use a regular Bourns 250k wafer on the dual gang push pull? Otherwise, I’ll be scraping carbon.

If you were to pull them apart you might as well get one dual 500k & one dual 250k and swap one of each of the wafers instead. Although that would slightly confusingly mean you'd end up with one that had 500k on top & 250k below and the other vice-versa.

[frets]

Excellent….thank you Yogi.😺😺😺
I have several dual gang A500k’s. I always use them for tone considerations but this added volume differentiation is neat😻.

[Deleted]

Jealous of the graphic Yogi

I gotta get my head around the 250K/250K one (it's late and I'm not well)
It's one I couldn't figure out

[JohnH]

im also not getting the 250/250 one. eg, it looks like the resistance between the signal hot and the jack hot is always that of one 250k pot, and doesn't double when in 500 mode?

[sumgai]

Even in my deteriorating mental state, I can see that in the 250/250 diagram, with the switch set to down, the path from signal hot to signal ground is runs through the lower deck (pins 3 to 1), is jumpered between the two decks at pin(s) 1, through the upper deck, and thence onward to shield ground via pin 2.  Presto, we have a 500K load.  It must be assumed, however, that signal ground (from the pup(s)) will never be grounded at any other point in the entire circuit - all such grounding must be accomplished here at the Volume control.  (This is because the shield ground will eventually become the output jack's ground.  Allowing signal ground to directly meet the jack's ground terminal will negate this particular control.)   Of some concern, as the knob is rotated downwards from maximum, the resistance between pins 1 and 2 of the upper deck will decrease, thus changing the actual observed load.  I suspect that this will not significantly affect the overall tone, particularly if a treble-bleed circuit is installed.

When switched up, the upper deck is shorted directly to ground on both pins 1 and 3, rendering that deck null and void. Thus the load presented will be the same 250K that we'd expect if we were looking at a normal control with no encumbrances.


All in all, nice job Yogi!   I'm sure the Beta Particle Bombarder is proud of you!!

HTH





sumgai

[MattB]

May 25, 2022 23:05:59 GMT -5 sumgai said:

Of some concern, as the knob is rotated downwards from maximum, the resistance between pins 1 and 2 of the upper deck will decrease, thus changing the actual observed load.

sumgai

As the knob is rotated downwards, the resistance between pins 2 and 3 of the upper deck increases, putting resistance between signal ground and shield ground.

If I'm understanding this right the top half of the voltage divider is split into two pieces, one on each side of the pickup. That means the load on the pickup remains constant and the taper doesn't change.

Edit: this is what the pickup sees with the knob at 5 (assuming a 10% log taper):

[Yogi B]

May 25, 2022 23:05:59 GMT -5 sumgai said:

This is because the shield ground will eventually become the output jack's ground

Yep, that's what I meant and it's the fault of my poor labelling if anyone thought otherwise (it should be better now though).

Of some concern, as the knob is rotated downwards from maximum, the resistance between pins 1 and 2 of the upper deck will decrease, thus changing the actual observed load.

(I'm also thinking I should've flipped the diagrams upside down, such that "up" & "down" corresponded to the direction of the push-pull mechanism)It's the same as a regular volume control, yes the resistance between the jack hot & ground decreases, but the load on the pickups (from just the volume itself) doesn't — because the other (increasing) half of the pot is in series with the pickups.

---

The three below circuits give equivalent results. I'm using linear pots such that the resistances can be directly found from "wiper_pos" (the percentage of wiper rotation from the direction of lug 1) without calculating the effects of a log taper.

On the left is the standard volume wiring; in the centre we have 'upside down' wiring where the series resistance (the wiper to lug 3) has been moved from the hot side of the circuit (as it was in the previous) over to the ground side, N.B. the orientation of the pot; the final circuit combines the previous two circuits, splitting the total 500k into the two 250k gangs.

[sumgai]

Matt,

May 26, 2022 2:20:51 GMT -5 MattB said:

As the knob is rotated downwards, the resistance between pins 2 and 3 of the upper deck increases, putting resistance between signal ground and shield ground.
If I'm understanding this right the top half of the voltage divider is split into two pieces, one on each side of the pickup. That means the load on the pickup remains constant and the taper doesn't change.

In essence, you've over-analyzed - the load to the pup remains the same at all times (in either switch position), regardless of the wiper position.  What you caught me out on was a mistake on my part - I said "observed load".  That was a hold-over from a previous sentence that I edited out, and somehow overlooked that portion.  At the time, it seemed to me to "be correct" when I proof-read it, but obviously I was pretty far off-base.  That constitutes an "oh crap!", thereby resetting all of my attaboys.

I think what I meant to say was that because the signal ground must go through an increased resistance to get to shield ground, there is now a "floating factor" in the equation..  Your diagram properly denotes this, and clearly shows that the load presented to the pup will not change.  However, in order for the "entire" signal to be seen and treated by the amplifier/effects boxes, the actual impedance presented to them will be somewhat different than the simple one seen by the pickup.  Still, as I look at it further, I think (take that loosely!) that even with some resistive element between signal ground and amplifier ground, we've already take steps to reduce the volume.... by merely rotating the shaft to a less-than-full-strength signal hot level.  At this point, the amp is seeing a resistance between both legs of the signal, but for all of that, it doesn't care - signal is signal, and the amp will be quite happy to carry on as usual, TYVM.

I think we're done here.  frets, you should give careful consideration to using any of these diagrams to give your customers what they want.  (We'll leave off of what I think about their desires.....)  The overall cost may be some non-trivial fraction above that of the circuit in your original post, but we seem to agree that they will result in the most satisfactory performance.

HTH





sumgai

[frets]

Sumgai,

I bought the CTS pot A500k/A250k that Newey found; but, in the future, I’ll use Yogi’s dual gang push pull method.

The customer has an HHH. He now wants 3 5 way rotaries, 1 to each pickup, the A500k/A250k Convertible Volume, a tone switch and a 6-Way. One of the stranger ones I’ve made. I’m going to diagram it out; so, I’ll probably post it.

[JohnH]

Hi frets, is this the same customer who was involved with your Black Beauty thread? I hope he's going to pay you well!

I get the 250/250 version now. V clever and Matt's diagram helped. So the ground at the jack is then not the same as the cold side of the pickup circuitry, instead there is a pair of mirror-imaged voltage dividers in play when in 500 mode on hot and ground side. I think it should all work fine so long as signal cold is kept separate from any shielding grounds on pots, braids and switch cases etc. Another interesting consequence is that if a treble-bleed circuit was wanted, there may be two needed, one on hot and one on cold.

Still, the other 2-gang options may be more straightforward

[frets]

John,

No, this is a different guy. The Black Beauty guy is still in the hopper, but this is a completely different fella. He keeps on adding stuff. Ha!
Turns out I have to use the dual gang push pull as he wants the option to select the A250k when the Humbuckers are in single coil mode.
Thanks to you guys for helping me with this. It’s an odd one but should be a fun build. And he wants it Solderless!!
I have yet to figure how much to charge. He’ll have to pay for the 5-ways and the 6-Way Freeway upfront. I had heard (it may have been here) that Freeway has an “Ultra” switch. I need to check that out as I have no idea what it can do more than the regular 6-Way.
He also wants a tone switch that as he put it, “does bass, midrange and flat.” I’m trying to get him to explain the construct of “flat.”
Like I said, I will post the diagram as I will need some other eyes on it. It is a cool build.