I gots a question!

We are fortunate to count Paul Gagon as a member here, so it seemed natural to give him a home to stretch out in and relax a little. Maybe exercise the grey matter and present a little history of the guitar and bass (amps too!) from his perspective as one of the eminent designers of our time.

Got questions? Great! Good questions might be, "What are the EQ mappings for the M Series preamps?" Or, "What was the thought process behind the MJ-4?" Troubleshooting questions should remain in the regular forums (he reads those too). Finally, please be mindful of how I feel about tech questions via PM or email.

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I gots a question!

Postby Kevin Baker » Sat Jul 30, 2016 11:17 am

Hey, Paul.

In a conversation with Dad about my bass project, the subject of metal covers for pickups came up. I know they exist, but was specifically wondering what an aluminum pickup cover would do to the sound of an MFD if said cover was included in the ground circuit. Dad guesses that it would darken the tone significantly, and I'm inclined to agree based on what shielding in general does to the sound of a bass, but I wanted to ask an expert.

Thanks!
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Re: I gots a question!

Postby Ken Baker » Sat Jul 30, 2016 12:57 pm

Kevin Baker wrote:Dad guesses that it would darken the tone significantly....


Notably.

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Re: I gots a question!

Postby Kevin Baker » Sat Jul 30, 2016 2:23 pm

I feel like if it changes the tone notably on a bass, it's pretty significant.

Jus' sayin'.
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Re: I gots a question!

Postby Paul Gagon » Sat Jul 30, 2016 5:49 pm

Hi Kevin,

Thanks for posting a question to my little part of the world. First off, let me just say how cool that is that you can have conversations with your Dad about stuff like this. Pickup covers is one of those topics that can cause a bit of confusion because you have several different types of materials that they can be made of. Plastic, wood, steel (in all its various formulation and coatings) and aluminum.

Let me assure you that I'm no expert in this area but I have done a fair amount of experimentation over the years. What I have found is that you have three issues to deal with when putting a cover over a pickup. Here they are in no particular order.

1. Are you adding inductance to the pickup due to ferrous materials in the cover and its proximity to the pickup coil.
2. Are you adding parallel capacitance to the pickup due to close proximity between a conductive cover and the pickup.
3. Are you disturbing the magnetic field of the pickup magnets due to the ferrous material in the cover.

These 3 issues aren't present in plastic or wood covers but they sure are in the metal ones. However, the aluminum ones do not behave the same as the steel variety. I did an experiment on a pickup where I measured its resonant frequency without any cover on it. When I placed a standard chrome cover on the pickup (BTW this was an inexpensive cover with a fair amount of iron in the steel) the resonant frequency dropped a few hundred cycles. When I exchanged the aluminum cover for the chrome cover, the resonant frequency actually went up around a hundred cycles. This was unexpected so I measured the inductance of the pickup with and without the covers and what I found was that while the chrome cover raised the inductance of the pickup, lowering its resonant frequency, the aluminum cover actually lowered the pickups inductance.....not a lot but it did go down.

That changing of the inductance followed what I was reading on the resonant frequency plots. I wish I had a super intelligent explanation for this but I really don't. The inductance is not the only thing that effects a pickups sound. Any additional capacitance across the the pickup will lower its resonant frequency. There's not a lot of capacitance added from pickup covers but it's there and its effect is the lowering of the pickups resonant frequency. The reputation that pickup covers have, of darkening up a pickup, is real and measurable.......when you're talking about metal covers with ferrous material in them. Aluminum is a different animal and I for one would love to see you experiment with this on your bass and see what you and your Dad find.

The final issue of magnetic fields and their ability to be disturbed by pickup covers is perhaps something that could be discussed in a new post. I have been toying around with digging up old drawings and measurements I did on pickup magnetic fields and how they contribute to a pickups "Voice" I don't know.....it could be fun.

Anyway, thanks again Kevin for the question. You and Ken are welcome to swing by anytime and do some testing on my Audio Precision setup.
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Re: I gots a question!

Postby Ken Baker » Sat Jul 30, 2016 7:00 pm

Paul Gagon wrote:When I placed a standard chrome cover on the pickup (BTW this was an inexpensive cover with a fair amount of iron in the steel) the resonant frequency dropped a few hundred cycles. When I exchanged the aluminum cover for the chrome cover, the resonant frequency actually went up around a hundred cycles. This was unexpected so I measured the inductance of the pickup with and without the covers and what I found was that while the chrome cover raised the inductance of the pickup, lowering its resonant frequency, the aluminum cover actually lowered the pickups inductance.....not a lot but it did go down.


What did you expect from the aluminum cover? I would not have expected a whole lot of difference from wood or plastic so long as the cover remained ungrounded. Grounded, it would act as a shield and I would expect a change in resonance.

Anyway, thanks again Kevin for the question. You and Ken are welcome to swing by anytime and do some testing on my Audio Precision setup.


I don't know about using his bass build as an experimental platform, but picking up a bass pickup for playtime might be a worthwhile thing. A little sheet aluminum, brass, & thin steel could be fun.

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Re: I gots a question!

Postby Paul Gagon » Sat Jul 30, 2016 10:12 pm

Ken Baker wrote:What did you expect from the aluminum cover? I would not have expected a whole lot of difference from wood or plastic so long as the cover remained ungrounded. Grounded, it would act as a shield and I would expect a change in resonance.


I figured the aluminum cover would have little or no effect on the pickups sound but was pretty surprised when I saw the pickup inductance go down and the resonant frequency rise.......granted is was a small amount. Oh, and this effect was in both the grounded and non grounded mode.

I don't know about using his bass build as an experimental platform, but picking up a bass pickup for playtime might be a worthwhile thing. A little sheet aluminum, brass, & thin steel could be fun.


Yeah......I guess asking you guys to use your new bass project as a test jig is a bit much. Sorry about that. Anyway, I'm up for trying any experiments you would like. I miss having my lab at G&L where adventures like this was easy but I think we can figure out something.
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Re: I gots a question!

Postby GeorgeB » Sun Jul 31, 2016 12:24 pm

Aluminum is a very good conductor and acts as an efficient shorting turn (sort of a shorted secondary of a transformer). That's why inductance is dropping, and resonance frequency raises but the peak is less pronounced as the shorting turn also act as resistive damping.
Brass is not a as good a conductor, and german silver even less so, that's why it is used for quality covers and base plates that don't affect tone but do provide electrostatic shielding.
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Re: I gots a question!

Postby GeorgeB » Sun Jul 31, 2016 12:35 pm

Paul Gagon wrote:Anyway, I'm up for trying any experiments you would like.
Time to power up your(?) AP 8-)
Your might need a high-Z buffer as the classic AP's high-Z impedance is only 100k.
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Re: I gots a question!

Postby deltafred » Sun Jul 31, 2016 2:12 pm

Paul Gagon wrote:I figured the aluminum cover would have little or no effect on the pickups

Thinking back to my old days playing with RF if you want to raise the inductance of a coil you used a core of compressed iron dust or ferrite, and brass to lower it. I seem to recall it was because of the eddy currents getting shorted out acting as a partially shorted turn (on a virtual secondary winding within the core). IIRC any non ferrous metal will lower the inductance by an amount depending upon it's conductivity.

Aluminium was used as the drag cup in mechanical speedometers, the rotating magnet caused eddy currents in the cup (which increased with speed) which then acted as weak magnets and dragged the cup against a return spring. The needle was attached to the cup to indicate the speed. (Hope all this makes sense.)
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Re: I gots a question!

Postby Paul Gagon » Sun Jul 31, 2016 3:13 pm

GeorgeB wrote:Time to power up your(?) AP 8-)
Your might need a high-Z buffer as the classic AP's high-Z impedance is only 100k.


Thanks George. I actually have a really nice buffer that I built a few years back. Works great for doing pickup measurements on the AP.
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