HSS with spin a split - need help with the pot values!

Littlejonny

I'm excited to be installing an Iron Gear Rolling Mill HB pickup in the bridge of my USA Strat.

Neck and middle are Fender Hot Noiseless and the Iron Gear HB will replace the bridge pup that I find too spiky.

For the bridge HB, I've decided on using the no-load pot as a gradual coil split so I can try and find some in-between sounds as will as a pure single coil. There will be a master vol, and a tone control for the two HNs. The bridge pup won't see a tone control.

My question is this - all the pots are 250k. Do I need to buy a 500k vol pot and do the special resistor thingy, or does the no-load (gradual coil split) pot not count as a load on the HB?

Dave_Mc

Hopefully someone else who knows better can confirm, but I don't think the no-load spin-a-split should load down the HB, and if you like the sound of a humbucker with a 500k volume and tone pot, then a 250k volume with no tone pot will probably be pretty similar, if not identical. Only reason to get a 500k and do the resistor thing would be if you like the tone of a humbucker with a 500k volume pot and no tone knob, or if you feel that the slightly brighter/more sparkly tone you get from that might make it match the singles in tone slightly better.

Littlejonny

Thanks @Dave_Mc thats really helpful. Turns out I ordered the wrong pick guard so I have to wait now. I will try 250k pots first, and I have a 500k on order. I’ll update this thread when it’s done. 

Dave_Mc

Littlejonny said:

Thanks @Dave_Mc thats really helpful. Turns out I ordered the wrong pick guard so I have to wait now. I will try 250k pots first, and I have a 500k on order. I’ll update this thread when it’s done. 

No worries. Most of what I wrote there is extrapolation from what I have tried, rather than actually having tried that exact thing! I've never actually tried no tone pot on the bridge pickup of a superstrat but with the tone pot connected to the singles... I'm sort of comparing in my head different guitars lol. Another option I just thought of would be a no-load for the tone knob too- that would mean you could connect it to the humbucker too (if both vol and tone are 250k) and have it sound more or less right, but also more or less right with the singles too... though would require a lot more fiddling on the fly. I think I prefer the resistors which kick in automatically.

CasperCaster

The spin-a-split won't load the bridge pickup in the way you are thinking since it isn't used like a conventional pot. Variable coil splits are odd, as it takes relatively little resistance between the coil junction and earth for the pickup to sound fully humbucking. It's been a while, and describing sounds is always a mine field, but from memory most split sounds where the resistance between the coil junction and earth is below about 6K - 7K tend to sound single coil like (warmer with higher resistance; most like a true single coil in the 1K - 3K area). Above around 10K things tend to sound more humbucker like, but with some single coil character. Obviously as resistance rises above 10K things sound more and more like a regular humbucker, and by the time that resistance between the coil junction and earth is 40K - 50K it will just sound like a full humbucker.

With a typical 250K log pot very little will happen from '10' until somewhere around '6' which is where the pot will get to the 40K - 50K region. If for example you like the sounds in the 2K - 5K region you may have trouble repeatedly dialling them in as you won't have enough resolution with a 250K pot (you've effectively wasted the range between '10' and '6'). A 50K or 100K log pot is actually a better choice than 250K, and the steeper the log curve the better (ideally something like a 5% taper if you can find one, but the typical 10% taper of a modern CTS pot isn't bad). If you want the reassurance that your humbucker is categorically fully humbucking at '10', you can convert a regular pot to no-load yourself (loads of videos online, but briefly it involves cutting the resistance track). In the case of a spin-a-split the no-load aspect ensures the connection between the coil junction and earth is completely broken when the pot is at '10'. Also, you may find you don't use sounds below a certain resistance (for arguments sake, you may find the split sounds below 1.5K too thin). In this case you could wire a 1.5K fixed resistor in series with the pot such that the lowest setting of the pot is always 1.5K and easy to find.

In your situation I'd definitely start with your existing 250K pot - it will work but with the aforementioned caveats. If you find you do really like the spin-a-split and you want more precise control then maybe try converting a 50K log pot (e.g. a Clapton volume pot) to no-load, or install a fixed resistor, or both.

It's been many years since I played with a spin-a-split. Ultimately I wanted a more conventional control setup, but playing with it helped me determine what values of fixed resistor worked best for me with partial coil splits. There is also the Peavey T60 rotary split and tone control circuit which seems like a good idea in principal, but is less useful in practice, and works best with a 'S' taper pot (you can substitute a linear pot, but some of the sounds will be bunched together).

Littlejonny

CasperCaster said:

The spin-a-split won't load the bridge pickup in the way you are thinking since it isn't used like a conventional pot. Variable coil splits are odd, as it takes relatively little resistance between the coil junction and earth for the pickup to sound fully humbucking. It's been a while, and describing sounds is always a mine field, but from memory most split sounds where the resistance between the coil junction and earth is below about 6K - 7K tend to sound single coil like (warmer with higher resistance; most like a true single coil in the 1K - 3K area). Above around 10K things tend to sound more humbucker like, but with some single coil character. Obviously as resistance rises above 10K things sound more and more like a regular humbucker, and by the time that resistance between the coil junction and earth is 40K - 50K it will just sound like a full humbucker.

With a typical 250K log pot very little will happen from '10' until somewhere around '6' which is where the pot will get to the 40K - 50K region. If for example you like the sounds in the 2K - 5K region you may have trouble repeatedly dialling them in as you won't have enough resolution with a 250K pot (you've effectively wasted the range between '10' and '6'). A 50K or 100K log pot is actually a better choice than 250K, and the steeper the log curve the better (ideally something like a 5% taper if you can find one, but the typical 10% taper of a modern CTS pot isn't bad). If you want the reassurance that your humbucker is categorically fully humbucking at '10', you can convert a regular pot to no-load yourself (loads of videos online, but briefly it involves cutting the resistance track). In the case of a spin-a-split the no-load aspect ensures the connection between the coil junction and earth is completely broken when the pot is at '10'. Also, you may find you don't use sounds below a certain resistance (for arguments sake, you may find the split sounds below 1.5K too thin). In this case you could wire a 1.5K fixed resistor in series with the pot such that the lowest setting of the pot is always 1.5K and easy to find.

In your situation I'd definitely start with your existing 250K pot - it will work but with the aforementioned caveats. If you find you do really like the spin-a-split and you want more precise control then maybe try converting a 50K log pot (e.g. a Clapton volume pot) to no-load, or install a fixed resistor, or both.

It's been many years since I played with a spin-a-split. Ultimately I wanted a more conventional control setup, but playing with it helped me determine what values of fixed resistor worked best for me with partial coil splits. There is also the Peavey T60 rotary split and tone control circuit which seems like a good idea in principal, but is less useful in practice, and works best with a 'S' taper pot (you can substitute a linear pot, but some of the sounds will be bunched together).

Happy Easter and Thank you so much for taking the time to write this…I do have 50k log pots in my pedal bits box. I will try one of those and maybe send the 500k back as it £8 and every penny helps!

Littlejonny

https://imgur.com/a/bUq1xWr

Littlejonny

I think I’m having trouble with this gradual coil split mod. I’ve drawn a pic of what I’ve done, and was hoping someone could debug it. I haven’t strung up, I’m just tapping with a screwdriver…I can hear that all pickups work on the correct switch positions, the tone control is active on the neck and mid, the bridge has no tone control. The only thing is, the gradual coil tap doesn’t seem to be doing anything at all - I would expect a thinning of the sound or at least a reduction in volume. Can anyone help?

Littlejonny

Littlejonny

Littlejonny

 

Littlejonny

I'm definitely having trouble embedding an image!

Littlejonny

[img]https://i.imgur.com/eN9altO.jpg[/img]

BillDL

Your First Image Link

Your Second Image Link was of the same diagram.

Littlejonny

Thank you - I was just struggling to upload

Littlejonny

In this diagram are the tone and coil split pots actually grounded? Shouldn't I connect the tone pot to ground on the back of the vol pot?

CasperCaster

The Imgur link worked fine for me - as long as there is a valid link to a picture I don't think embedding the image is critical.

On to the wiring - a few things to check/amend:

Firstly, the final lug of your volume pot needs to be connected to earth for that pot to function as a voltage divider, and it isn't connected in your drawing. And where is the earth connection for the output? Personally I usually solder the earth side of the output to the back of the volume pot, then the third lug of the volume pot, then on to the output jack - saves bending the third lug onto the back of the pot, which if not done with care can break the connection between the lug and the end of the track ( it's only a crimped connection).

Secondly, the spin-a-split does need to be connected to earth. Via the back of the tone control is fine assuming that is also earthed. The tone control pot 'should' be earthed via the aluminium foil or metal shield plate on the back of the pickguard, but there often isn't good continuity, so a soldered earth connection between the back of the volume and tone controls is good practice/ belt and braces. Check the earth continuity with a meter if you have one, or just add the extra earth whilst you correct the volume control wiring (you.could go from back of tone, to third lug of volume pot, to back if volume pot making a soldered connection at each of those three locations).

Did you convert the spin-a -split pot to no-load? If so, did you cut the trace at the correct end of the track?

Finally, just general checking of connections, are all soldered joints good etc, and are they actually as.drawn? Also, for your switch, are the commons in the locations as you have drawn them - I've seen the odd switch where the commons are at the other end of the switch - Stew-Mac used to sell a switch with the lugs transposed like that.

Hopefully you'll have it sorted.soon.

Littlejonny

Thanks so much @CasperCaster yes the 3rd lug of the volume is actually soldered onto the back of the pot (not shown in drawing). I plugged it in again (still no string a on) and did notice a subtle change in tone with the spin a split, but my prediction is that it’s not dumping enough to ground and may be the lack of grounding you mentioned. Although wouldn’t that tone pot not work at all if not grounded?

CasperCaster

You are correct r.e. the tone control, so the spin-a-split should be seeing earth. When it's turned down to zero the wiper will be in contact with earth, so your humbucker 'must' be fully split at that point. The only other variable is the value of the pot, and as we discussed above 50K between the coil junction and earth should be enough to ensure the humbucker is fully humbucking, whilst still allowing decent resolution across the range of the pot. If the issue is that the humbucker isn't going fully humbucking, then it requires a larger value pot - if that's the case, my apologies and I'll have to re-write my notes. 

Littlejonny

Argh…I’ve added the ground wires and it’s exactly the same. Also, disappointingly, I wound a string on and discovered there is a HUGE volume discrepancy between the Hot Noiseless and the Irongear Rolling Mill. The HNs sound tiny against the humbucker! I think there’s about 12db difference (at a guess).