Blend pot issue, need help!

[John Meyers]

I am building a new bass and I am wirring it up and noticed the Blend pot is not working like it should. It works when turned to one pup or the other but volume goes out when in the center position. What do I have wrong?

Also incase this makes a difference. I am using a Bartolini 3(that I am not a huge fan of) way electronics with a P Bass pup in the neck position and a MM style in the bridge/mid position. Did not know if two humbucking pups could cause an issue even though dual humbuckers a normal in the guitar world.

Ans on a sidenote, I had to use a stack pot for the Bass and trebel controls and it is the wrong type of pot. This one is about usless until it hits its limit in rotation before it does anything and when it does it is all or nothing. Which type do I have and which type do I need? Audio taper or what?

[Mark Swanson]

It sounds as though you may have some ground issues.
Also, if I could ask you to spell and type your posts a bit more clearly it would be a lot easier to understand.

[David King]

Blend pots are special. Only 1/2 of each trace's travel is resistive. The other half is connected directly to it's far terminal. This way when the pot is centered on the detent both pickups are on full. You need to switch your hot wire for your ground wire for it to work correctly. Once you make that switch you'll soon realize that the bridge pickup and the neck pickup will be reversed so you need to switch their input positions. The blend pots from different manufacturers will be wired opposite from one another. The easiest thing to do is check which way your pots work with a multimeter before you wire them. IIRC ALPS and Alpha brand work one way while Noble, CTS and Bourns work the other way. Once you figure it out make a note of it so you don't have to reinvent the wheel every time you wire one up.

Bartolini make domestic and offshore products (MK series made in Korea vs NMTB {"new mid treble bass"] made in Livermore, CA). They are different animals.
Generally Bartolini active controls use linear 50k ohm pots with center detents. It sounds like you are trying to use a stacked 250 or 500k audio pot which will sort of work but not really. Time to make some adjustments.

BTW getting 50k dual stacked pots with center detent is rather problematic. You might try bestbassgear.com. You can also plead with Aguilar or Bartolini or whoever else uses them and make up a cock and bull story that you had a pot go bad but that's a longshot. Perhaps Nordstrand will take pity. You can also use a dual concentric b100k pot with center detent which won't work as well but will work. These should be available from the kindly Audereaudio.com store in the parts section but don't tell them I sent you...

[Greg Robinson]

I think David has it right, you probably have the pot wired "backwards" - easy to do when there are 4 mirrored ways you could do it, and only one that will work. Doesn't help that they're not standardized either.

In regards to your stacked pot, sounds like you have a linear pot and an audio taper would suit you better, but it could be the opposite. Different people seem to hear different things from different tapers for passive tone controls, unlike volume controls, where people mostly agree. Do you have a multimeter? If so, turn the pot halfway, and measure the resistance from the middle lug to either side, if the values are around equal, you have a linear pot, if one side is significantly lower than the other, you have a log or anti-log, depending which side is which.

[John Meyers]

Blend pots are special. Only 1/2 of each trace's travel is resistive. The other half is connected directly to it's far terminal. This way when the pot is centered on the detent both pickups are on full. You need to switch your hot wire for your ground wire for it to work correctly. Once you make that switch you'll soon realize that the bridge pickup and the neck pickup will be reversed so you need to switch their input positions. The blend pots from different manufacturers will be wired opposite from one another. The easiest thing to do is check which way your pots work with a multimeter before you wire them. IIRC ALPS and Alpha brand work one way while Noble, CTS and Bourns work the other way. Once you figure it out make a note of it so you don't have to reinvent the wheel every time you wire one up.

Bartolini make domestic and offshore products (MK series made in Korea vs NMTB {"new mid treble bass"] made in Livermore, CA). They are different animals.
Generally Bartolini active controls use linear 50k ohm pots with center detents. It sounds like you are trying to use a stacked 250 or 500k audio pot which will sort of work but not really. Time to make some adjustments.

BTW getting 50k dual stacked pots with center detent is rather problematic. You might try bestbassgear.com. You can also plead with Aguilar or Bartolini or whoever else uses them and make up a cock and bull story that you had a pot go bad but that's a longshot. Perhaps Nordstrand will take pity. You can also use a dual concentric b100k pot with center detent which won't work as well but will work. These should be available from the kindly Audereaudio.com store in the parts section but don't tell them I sent you...

Thank you and I will give it a try. I am using the USA NTMB Bartolini electronics and yes I am using/trying to use stacked 50K pots with no center detents and they are not working. Time to do some searching for a new one.

[David King]

You have the brown wire going to the center lug on the tone pots? I'd be surprised if a log pot would cause that much delay in the pot's action but I've never tried it.

[John Meyers]

You have the brown wire going to the center lug on the tone pots? I'd be surprised if a log pot would cause that much delay in the pot's action but I've never tried it.

I will have to check and see. I know I have it wired the way the Bartolini sheet said to do it but I cannot remember off hand what that is.

Also about the Blend pot, which I have not had a chance to re-wire yet. The way I have it is like the Bartolini sheet says to do it, which is hot wires to center lugs from each pick up and then I used two short wires from the remaining two outside lugs and crossed them down to the other two outside lugs. Was this correct? I know you said I need to re-wire it differently and wanted to know based off this info which wires to move where? Thanks for your help on this.

[David King]

That's correct, the outer lugs are wired criss-cross. It's just that you need switch the wire going to ground with the wire to the hot lug of the volume pot.

[John Meyers]

Just so I follow you correctly. I need to switch only the two outer wires on the volume pot? The reason I ask is, the volume knob turns in the right direction to increase the volume. Also doing this could help the over all volume of the bass? It just seems the volume is not all there unless I max out the tone controls.

[David King]

John, It doesn't matter at which pot you switch the wires. it's just that right now you have the hot from the balance pot going to the ground of the volume pot and vice versa. The ground pot on the volume still goes to the ground lug of the output jack.

You can adjust the gain of the Bartolini preamp by turning the little blue and white trim pot CW.

[Mark Swanson]

The ground pot on the volume still goes to the ground lug of the output jack.

I think you must mean the wire soldered to the back of the volume pot? The way you word that seemed confusing to me David.

[John Meyers]

The ground pot on the volume still goes to the ground lug of the output jack.

I think you must mean the wire soldered to the back of the volume pot? The way you word that seemed confusing to me David.

Mark the crazy thing with these electronics, is that you do not ground wire the back of the pots. That is the way the diagram say to do it anyway. The diagram says to just run a ground wire from the ground lug to a grounding point (together with all other ground wires). If this is not correct please let me know?

Also David I will try and switch the ground and hot wires tonight and see what the results are. Also what I was talking about with the volume issue was. The volume is very weak when tone controls set to flat and volume know at max. I know about the trim pot and it is set at 3/4 percent. It has just been so long since I used these electronics that I cannot remember them being like this. So I was not sure if fixing this ground issue at the volume and blend pots would help over all volume of bass or if this could be caused by the stack pot for trebel and bass? Or if this is just the way they are?

Sorry for the hundred and one questions but I will tell you the biggest turn off of these electronics has always been the amount of wires this thing has. They could have come up with a better layout then this, one with a lot less wires or at least wiring that makes since. You have wires coming from the electronics that cross over the electronics boxes. Why not make it one row runs this and the other that and so on. Sorry for the rant but this is what drives me crazy about them. I see a lot of other companies that do the same with a lot less of a mess going on. Ok I am done now.

[John Meyers]

Do you think this will be a good stacked pot to use, for the tone controls?

http://www.allparts.com/EP-4486-000-500 ... _1442.html

[David King]

John, unfortunately that pot says 2x500K not 2x50k. Allparts used to have a tiny stash of Bartolini pots in a drawer. Call them and ask to speak to Joe.
Call around to the other places I mentioned. You can call Bartolini's sales agent, Tom Wagner, 386-517-6823 and see if he can get you the correct stacked pot.

Regarding wiring, these preamps almost always come prewired from Bartolini with all the pots attached and only input connections to make. Where did this unit come from?

There should be a lot of gain available to you and an extremely low noise floor. You haven't said anything about the pickups you are using or the blend and volume pot values you have. Please take a photo of what's connected now and perhaps we can help you further.

You are correct about not soldering to the backs of pots, that went out twenty years ago with the advent of minipots. What I wrote before should have read "The ground LUG on the volume pot still goes to the ground lug on the output jack".
Ideally you would establish a star grounding point to which you will connect the following:
The shield drain wire from each pickup.
The bridge ground,
The PU cavity shields
The control cavity shield
(Do not shield the battery compartment.)
The black wires from each Bartolini module
The grounds from the volume and blend pots
All of these then connect to the ground lug of the output jack.

Make sure you haven't switched the preamp grounds (black wires) with the battery terminal negative wire at the jack's ring and sleeve terminals. The battery always connects to the ring.

[John Meyers]

John, unfortunately that pot says 2x500K not 2x50k. Allparts used to have a tiny stash of Bartolini pots in a drawer. Call them and ask to speak to Joe.
Call around to the other places I mentioned. You can call Bartolini's sales agent, Tom Wagner, 386-517-6823 and see if he can get you the correct stacked pot.

Regarding wiring, these preamps almost always come prewired from Bartolini with all the pots attached and only input connections to make. Where did this unit come from? I got these electronics 14+ years ago and they came un connectted but with all the correct pots needed. At the time I had a local repair guy build me a custom bass with these in it. I baught them directly from Bartolini and I think I spoke to the guy you have in this post.

There should be a lot of gain available to you and an extremely low noise floor. You haven't said anything about the pickups you are using or the blend and volume pot values you have. Please take a photo of what's connected now and perhaps we can help you further. I am using for now a old 80's Fende P bass pickup and I have not check it with a meter and the other is a GFS MM style alinco pick that reads 8.35K (killer pickup). The pot values. Man I wish I had the bass here at work. The volum pot is the one that cam with it and it is 250K and the blend pot I think is 250K as well.

You are correct about not soldering to the backs of pots, that went out twenty years ago with the advent of minipots(reply)Great now you tell me this! But for real when that local teck that built the bass these were put in originally, he did ground all the pots and it worked some how. There is still a ground wire from the ground lug to the back of the volume pot. Let me know if i need to remove this?(end reply). What I wrote before should have read "The ground LUG on the volume pot still goes to the ground lug on the output jack".
Ideally you would establish a star grounding point to which you will connect the following:
The shield drain wire from each pickup.
The bridge ground,
The PU cavity shields
The control cavity shield
(Do not shield the battery compartment.)
The black wires from each Bartolini module
The grounds from the volume and blend pots
All of these then connect to the ground lug of the output jack.

This is the way I have it currently, as long as I have hooked up the barrel jack properly. Those things always throw me for a loop because you cannot follow the lug to a ring but I think I have it correct.

Make sure you haven't switched the preamp grounds (black wires) with the battery terminal negative wire at the jack's ring and sleeve terminals. The battery always connects to the ring.

As above I hope i have the barrel jack hooked up properly. Thanks for all your help and I will continue to work at it.

[David King]

Barrel jacks not only suck, they are also really easy to mix up the ring and the tip connections on if you don't test with a meter.

A 14 year-old NTMB may well be the previous version which is the NTMB-18 as opposed to the NTMB-918 which is the current iteration.
The battery connections are different for the 18 as it uses a true bipolar supply instead of the internal floating ground, voltage dividing gizmo.
There are 3 batt connections instead of 2. The series link of the two 9Vs (red and black) goes to the jack ring. There's still a red +9V wire and a separate, black -9V wire.

A 250K Volume would go between the Blend and the preamp input wire (Yellow). These often came with a 10 or 25K volume pot that went post eq, between the green wire and the output jack instead of the 250K. A 250K will work post eq but not ideal. The 10-25k volume pot will not be satisfactory at the preamp's input.

The reason not to solder grounds to the backs of pots is that the heat will sometimes cause the carbon paths to fail prematurely. It will still work but not recommended. Old habits die hard.

[John Meyers]

Barrel jacks not only suck, they are also really easy to mix up the ring and the tip connections on if you don't test with a meter.

A 14 year-old NTMB may well be the previous version which is the NTMB-18 as opposed to the NTMB-918 which is the current iteration.
The battery connections are different for the 18 as it uses a true bipolar supply instead of the internal floating ground, voltage dividing gizmo.
There are 3 batt connections instead of 2. The series link of the two 9Vs (red and black) goes to the jack ring. There's still a red +9V wire and a separate, black -9V wire.

A 250K Volume would go between the Blend and the preamp input wire (Yellow). These often came with a 10 or 25K volume pot that went post eq, between the green wire and the output jack instead of the 250K. A 250K will work post eq but not ideal. The 10-25k volume pot will not be satisfactory at the preamp's input.

The reason not to solder grounds to the backs of pots is that the heat will sometimes cause the carbon paths to fail prematurely. It will still work but not recommended. Old habits die hard.

Based off how you described the pre-amp you are correct this must be the prior version. It does have the battery wires wired like you have described. I just looked at the volume pot and it looks to be a 25K and the blend pot is a 250K.

You have lost me when talking about pre and post? To me it sounds like you mean, I would switch the green from the volume to the blend and take the yellow from the blend and put it on the volume? You know way to much about these electronics, this crap boggles my brain. That is why I will order only pre-wired electronics from here on out.

[John Meyers]

I forgot to ask about the barrel jack. If you switch two lugs will it still work or will you know it is not working?

[David King]

The active will come on if the ground and sleeve are reversed but it will act up in strange ways later on (I can't remember how just now). I sent a bass out like that once and it cost me $90 to have a local tech track it down and do the fix. Never again...

Most barrel jacks have the ground sleeve connection longer and thicker than the other two. Often the tip connection will be the shortest of the three lug. If you switched the ring and tip connections you would have shorted out the output and the preamp would not have gotten power.

[John Meyers]

Well last night I went back and took out the stacked pot and put back in the original 50k pots and I also noticed the blend pot had a bad connection. So I re-soldered the blend pot and what do you know the bass sounds just about perfect now. After doing this I did notice some ground issues when toughing the string or bridge (static when not being touched), that is when I noticed I had forgot to ground the blend pot. I thought that would fix it but then again when I try to connect the ground to the blend pot it would kill the volume 100% when in the center detent. So I am stuck as to why or how this is happening? I have attached a drawing to try and show how I am running the ground to see if that is the issue. Also I took the ground wire and try touching it from different lugs on the blend pot but still have either a static or volume loss.

In the drawing I am not sure if I could be causing a ground loop. I am running the ground wire from the blend pot to the ground lug on the volume pot and then a new wire continues on from that same ground lug on the volume pot to the star ground. Let me know what you think.