Waz's MMB-4

[wazkelly]

Thanks Ozzie, good advice and will give that a go after work tonight.

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[wazkelly]

Wiring up GFS MMB pickup to Belcat Pre Amp

Having lots of fun trying to work out best way to wire up on blend pot that comes with the eBay 'Belcat' pre amp.

Kit supplied PUP had 5 wires where one was outer shield and theoretically should have allowed for front coil, rear coil isolation via the blend pot, and when centered it was in series as a humbucker. Played around with that PUP over a few days and could never get that to happen plus the wires were so thin they broke off at solder joints way to easy.

GFS replacement arrived a couple of days ago and this one is 4 wire but technically 3 as the outer shield does not share a signal with the 3 inner Red, Black or White wires. This probably means I can only coil split in one direction. When measuring things the rear coil lines up pretty close to same position as bridge J Bass PUP (10 mm further forward) whilst front coil is about 34 mm further towards neck. At the moment it seems I have wired in series as nothing is happening on either left or right side of center detent. Still sounds quite meaty and happy with that sound but seeking some guidance on which wires to swap around as my patience is almost at an end. At this stage I am leaning towards splitting to front coil as hopefully that should still retain more lower mids and maybe sound a bit like P Bass, and if I want J Bass sound I already have one of those.

Here are a couple of photos and diagrams and happy to wait for some feedback and guidance before heating up the soldering iron again. Last handwritten diagram is where things are at...



Thanking you in advance.

Cheers, Waz

[WeirdBits]

I wish GFS were a bit more explicit with how their bass pickups are configured and can be wired etc., it would save a lot of hassle.

MM pickups are traditionally wired with their coils in parallel, which makes it easy to connect them to blend pots etc. However, the GFS MM Pro actually has its coils wired in series, like a standard humbucker, which makes using a blend pot trickier... but not impossible.

With parallel coils, or two pickups in parallel, a blend pot controls how much signal from each goes to ground and how much goes to output. With coils in series, like the GFS MM pup, the blend needs to work more as a fade/bypass and control how much signal goes through a coil and how much bypasses it. This changes the way things work a bit and how you need to wire the blend pot, but it’s doable. If you want I can sketch up how you’ll need to configure it (which should work) but it may take me a day or two to get time.

If you just want to cut one coil then that’s easier, it just means you’ll only use half the blend pot.

[wazkelly]

Hi Scott, thank you for your reply.

This build has been on the go for past 15 months so another few days isn't going to be a problem. If it can be configured to blend front to rear coils, that was my initial intention but if it is not going to make too much sound difference and just add to the degree of difficulty then maybe just coil split to front (neck) coil will do.

Appreciate any guidance and happy for you to take as long as you need as I am back to work tomorrow and will only have evenings or next weekend to tinker with it.

Cheers, Waz

[WeirdBits]

Ok, bear with me. It's a bit difficult to tell from your photos etc., but the original configuration of the Belcat blend/balance pot should have been something like this:

The black ground would be connected to either the top left lug or bottom right (with respect the the above orientation), and each of those lugs has a link to the pot case. The red hot from the volume would either be the bottom left lug or top right and have a coated wire linking between those two lugs. Normally, your parallel pickups/coils would then connect to the middle lugs.

However, because your pickup coils are in series rather than parallel we need to connect each coil across the resistance side of each half of the pot, which requires some re-wiring. Essentially, when at the centre detent the signal will then go through each coil in turn rather than through the resistance (mostly) and turning the knob away from the centre will bypass one coil or the other.

First step is to disconnect the pot case link from the lower right lug, otherwise it will short your signal to ground. Next, disconnect the wire link from between the bottom left lug and top right lug.

Then, connect the pickup and volume wires like this:

That *should* get you what you want. The sound of the pickup may be slightly different in the centre position compared to what you're currently hearing, due to the change to the way the coils are loaded. And, you'll probably find the balance works a bit more... switchy. That is, you may not hear much change until it's almost fully turned one way or the other and then you'll hear the coil drop out quite quickly. Hopefully

Another option would be to just remove the blend/balance pot entirely and replace it with an On/Off/On mini-switch to select the coils as one/series/other.

[wazkelly]

Thanks Scott, I will give that a try.

A switch was also running through my mind and wondered whether a traditional Gibson/Gotoh style 3 way would work? The pot holes in the boomerang control plate are quite a large diameter to fill if using a mini DPDT switch but then one of them would need to be drilled out to fit a Gotoh 3 way?

Modifying blend pot for now and see how that sounds.

Cheers, Waz

[RalphH]

Thanks Scott, I will give that a try.

A switch was also running through my mind and wondered whether a traditional Gibson/Gotoh style 3 way would work? The pot holes in the boomerang control plate are quite a large diameter to fill if using a mini DPDT switch but then one of them would need to be drilled out to fit a Gotoh 3 way?

Modifying blend pot for now and see how that sounds.

Cheers, Waz

If you want to keep the MM 4 X control knob look, using a rotary switch with the same knob as the pots is the go. I used a 4-pole / 3 position to achieve the individual coils/parallel switching config as the EB era Stingrays do with a blade switch. I prefer this to the blend pot as there is more signal in the parallel position, and I found blends (other than full parallel) did not offer much tone variation.

Another little trick with the Belcat EQ-B2T preamp is that the bass boost/cut can be excessive, with a limited useful range of the pot. I "corrected" this by changing a few components around to get the travel of the pot to provide the best tonal variation. Hope this helps.

[Simon Barden]

Unfortunately RalphH, that switch wiring won't work as-is with Waz's pickup as it's only 3-wire. The pickup coils are internally wired in series, with the 'third' wire connected to the centre connections between the two coils. So it can be tapped but cant be connected in parallel unless Waz takes the pickup apart and manages to wire it as a 4-wire pickup so there's fully independent access to both coils.

[wazkelly]

Hi Ralph & Simon, Rotary switch had me intrigued enough to go searching for what was available and not much out there that would either fit in the cavity or easily do required job.

Been a bit busy with work and other chores to have a clear enough mind to get stuck into this again. Without too much effort it can be configured in a simple coil split mode, just need to experiment to work out which wires go where to split to the better sounding coil. My guess is the coil towards neck will give a more lower mid to deeper sound. Will probably muck around with this before tackling Weirdy's suggestion just to get a base reference for how things sound and ultimately which is most pleasing to the ear.

[WeirdBits]

If you did want to go down the rotary switch route then this switch is probably the most available option. Just trim the shaft length and set the number of positions, depending on the knobs you want to use and if it’ll fit in the cavity.

If you want use a full sized toggle switch then a standard 3-way won’t work, it will need to be an On/Off/On like a Danelectro toggle switch.

But I’d try the balance pot option first, it’s not hard to wire up.