What's this setup (BL Q Filter and pots) doing to my 55 Pbass?

dagrev · #1 07-05-2009, 04:43 PM

Okay, I have an odd question that I'd like for some of you who know your stuff to help me with.

I have a '55 Pbass that has the following modifications:
Bourne pots- 250k vol; 500k tone; Bill Lawrence Q Filter. The tone pot seems to function as normal, so I'm thinking it's wired as the tone cap. (I've have this since about 1982 and the bass was this way then.)

Another odd thing is the there looks to be a tone cap attached to the vol pot (which can be seen in the pic below)!

I asked George Gruhn 20+ years ago to give his thoughts on the bass. He immediately recognized the bass and said it had been in Europe for about ten years. That's all the history I know.

My question is, what is this doing to the tone from what a standard configuration would do? IOW, how is this making it sound different than it normally would?

I get great complements on the sound whenever I pull it out, but can't help but wonder what it would sound like in a more original configuration--without having to undo it all.

Any thoughts?

Here's a pic.

Thanks!

Below is from a sight describing what it does (supposedly from BL himself).
.From the Man himself:

The Q-Filter is an LCR network - a 1 henry, low Q noisefree inductor in series with a .02 micro farad cap in parallel with an 8 kilo ohm resistor. Wired like a cap to a tone pot, you can gradually decrease the impedance of the circuit but maintaining a slightly higher impedance below 1100 Hertz. Also available without the cap and the resistor to gradually decrease the impedance of the pickup.

The EQ system is the most versatile system to alter the sound of a guitar and can be used for multiple functions. The basic components are a 900mh low Q inductor in series with a capacitor and a variable resistor (tone pot). We used to ship it as a complete pre-tuned unit to replace the cap at the tone pot, limited for one basic function. However, in conjunction with different pickups in different positions, it's much more versatile when the inductor is separated from the cap, like [shipped now]. In the past, this caused severe problems when players used the wrong caps and resistors, especially when the EQ was wired to a switch only. The system must be wired to a control or you totally limit its functions, as follows:

Function A - the L Filter - the inductor, wired like a cap to the tone pot.

When you turn the pot from 10 to 0, you slowly reduce the impedance of the pickup. This is equal to taking turns off the pickup or making a sweet clean pickup out of a high power, overdrive pickup. This works fine for the neck pickup. With the control set below 3, the impedance is much too low so we block the control with a fixed 10 kilo ohm resistor or don't turn the control below 3 or 4.

Function B - the EQ Filter
On the bridge pickup, we want to maintain more lows so we block the inductor with a resistor in parallel with a cap. Now, when you turn the control to zero and you use hi-fi speakers, you can duplicate the sound of an acoustic guitar. With the controls on 3, you can get out of the bridge pickup, the #4 position sound of a Strat.

dagrev · #2 07-07-2009, 07:44 PM

Have I stumped everyone, or is it a boring question?

ironrat · #3 07-07-2009, 08:07 PM

I don't know what it is but it looks :

dagrev · #4 07-11-2009, 11:26 AM

Still looking for a few good answers! One would actually do!

Derek Kiernan · #5 07-11-2009, 03:50 PM

I can't really tell what's going on in the picture

You most likely have it wired up as the EQ network. It's pretty interesting! The network drops the impedance above the crossover point by reducing inductive reactance. This will do two things. The cable from your bass to amp and the inductance of the pickup interact to form a resonance peak, which is usually in the upper midrange - from 2 - 5 kHz, somewhere. You have to know the values of the cable capacitance and the pickup inductance to figure this out, although there often are other anomalies involved, such as shorts in the pickup that cause eddy currents, adding capacitance.

This mid peak goes upward as the inductive reactance drops. If you originally have a warm resonance, this will give you a brighter sound. If the resonance is in a bright frequency range, the shifting will potentially smooth out the sound and give you more sparkling or shimmering type highs. In the context of the Q-filter network, since the lows are unaltered, the highs (really upper mids) feel attenuated as you go down the taper. You'll notice the change in emphasis/color provided by the resonance. The other effect, since highs are dropped off after the resonance and before due to inductive reactance, is that you'll have more extension and a flatter response above the crossover.

A normal tone pot, with the traditionally used values, will establish a low resonance below 1000 Hz (depends on setup). At the bottom of the pot, it creates a massive peak with little content above. This has the effect of a substantial boost around the resonance, with some boost to the content below. This can sound overly colored and blanketed depending on the situation. There's a range in the tone pot towards the bottom where you can get the effect of a combination of a mild boost/low end emphasis while defeating the often exaggerated upper mid peak, while keeping your highs (to an extent). In the upper half of the tone pot, the general effect is attenuating the resonance's height. The Q-filter network has the crossover at a higher frequency than the resonance established by a traditional tone pot, generally, and won't give you that "covered" sound. Depending on pickup position/inductance, cable capacitance, and your amplification setup, you can emulate a number of positions and sounds by turning the knob.

I prefer the Q-filter approach for traditional pickups. The concept of the traditional tone pot would be an invaluable tool in context of a low inductance setup, to introduce color where you please and balance high end. To my knowledge, there are no passive lower inductance pickups on the market today. Hopefully this helps

dagrev · #6 07-11-2009, 04:56 PM

Yes, indeed. It helps clarify what might be going on. Thanks for your insight and taking the time to share your knowledge in this matter. In fact, you shared more than I can use--as I'm no electronics person by any stretch!

I guess my question has always been (in layman's terms), is it making the tone brighter, mid pronounced, or in the other direction warmer. I gather from your second paragraph that it could actually go either way. I keep wondering what it would sound like without this wiring configuration.

Thanks again Derek!

koobie · #7 07-11-2009, 07:25 PM

Since the pots & filter are not original, there'd be no harm in having original value pots & cap installed to test which you prefer. The setup you've got sounds very interesting, I'll have to see if Bill Lawrence still sells anything similar.

And how about a full pic of your 55 P? Inquiring minds and all...

Derek Kiernan · #8 07-11-2009, 08:10 PM

You're welcome dagrev. I tried to explain it as simply as possible, but the effect is really quite complex. It's a little bit easier to hear what's going on when you know what you're listening for!

Koobie, Bill sells a modern version that's about the same height, width and depth as the standard CTS pots Fender uses. There's a bass and guitar version. I've used both in a variety of instruments with great results.

GlennW · #9 07-11-2009, 08:45 PM

I looked into one of those a few years ago for a Tele (guitar), but never got it. IIRC they're made to work better with "hot" pickups and have the ability to make the pickup sound less overwound. If your pickup is stock it should read about 7K, + or - .5K, and I'd think the Q-filter would be fairly worthless with that pickup. Then again, I've never heard one.

GeoffByrne · #10 07-12-2009, 03:00 AM

Of course, you could actually get in touch with Bill Lawrence - his tel. no.is around somewhere................

SGD Lutherie · #11 07-12-2009, 09:44 AM

The Q-Filter works like a LC notch filter. Bill gets fancy with his description, but it's really like one setting on a Gibson Varitone switch.

Bill used the same Q-Filter idea on the Gibson L-6S and the L-9S Ripper bass, which he designed. On the Ripper he used a .1 cap.

What the control does is to mildly notch the upper mids, so you get a mellower tone, but with some highs intact, unlike a conventional tone control which removes all the highs above the cutoff frequency.

I had a Q-Filter in my Fender Mustang guitar, along with two L-250 pickups, back in the 80's. I thought it was pretty cool.

Now the L-250s are in my Tele/Les Paul hybrid guitar. But I lost the Q-Filter at some point.

Bill makes nice stuff.

GlennW · #12 07-12-2009, 10:41 AM

Very nice guitar, David. What's the LP influence, scale length and/or finish?

SGD Lutherie · #13 07-12-2009, 11:08 AM

Quote:

Originally Posted by GlennW

Very nice guitar, David. What's the LP influence, scale length and/or finish?

Thanks!

The set neck, thick body, and carved maple top. It's 25.5 scale.

dagrev · #14 07-12-2009, 06:30 PM

Thanks for the information!

I would describe the tone as clear but not twangy (as one might consider given the single coil PUP and the maple FB). Possibly it's smoothing out some of the harshness.

Anyone heard of Bourne pots? Junk, good, or somewhere in between?

koobie- here's a couple of pics. And no I didn't put that pickguard on it. Sure wish it had the original.

SGD Lutherie · #15 07-12-2009, 06:44 PM

Sweet bass! Lucky you!

Bourns pots are very high quality, and were in rage in the late 70's when everyone was bitten by the Alembic high end bug. Brass nuts, and brass everything else, was also in vogue at the time.

GlennW · #16 07-12-2009, 06:51 PM

That's a very cool bass. It looks like the headstock has been refinished and the wrong decal added (not nit-picking, just an observation). Is the pickup original, with a short ground wire that would go to the cover?

dagrev · #17 07-12-2009, 07:10 PM

Quote:

Originally Posted by GlennW

That's a very cool bass. It looks like the headstock has been refinished and the wrong decal added (not nit-picking, just an observation). Is the pickup original, with a short ground wire that would go to the cover?

Good eye Glen. I never noticed it. In fact, years ago when I took it to a world renowned vintage guy--he never noticed it either!

The whole thing is rough from a lot of use so I never though of it being refinished. Must have been early on in it's life. The
decal looks to be 64 or later. I bought it about 1981. If I remember correctly I've got about $600in it, so I still happy with it. I think I remember hearing that Seymour Duncan (himself) rewound the pickup at some point. Not sure about the ground wire. I know the wiring has been fooled with long ago. I try not to move much around, and have never had the PU out. It does look like there is something of wire nub in the middle of the PU on the bridge side.