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Originally Posted by aquin43
Here’s a long paper on the dielectric properties of wood vs. humidity that I doubt anyone here needs to read.
https://www.fpl.fs.fed.us/documnts/fplrp/fplrp245.pdf
Someone objected to ambiguity of the word “ground”, so I’ll make it clear I’m referring to the shield of the instrument cable.
I don’t know how useful this is to the OP, but it’s a difficult topic.
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08-20-2020 11:57 AM
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An example: I had an archtop with a built in humbucker with tone and volume. The output jack was on the lower bout as usual. All of the wiring was neatly done with shielded wire, the pots had metal cans which were grounded and the strings also were connected to ground (or common) The output jack was a simple Switchcraft type. Everything was as neat as could be.
When placed in playing position the guitar had a low level hum that got much louder as the volume control was turned down. It turned out that touching the wood of the guitar near the output jack induced enough voltage inside the guitar to couple into the contact spring of the jack, even though it was separated from the wood by the jack barrel and the ground contact on the jack. I made a very thin 3cm radius conducting disc from copper foil stuck to paper and placed it as a washer inside the guitar between the jack and the wood. With that in place and connected to the shield, the guitar became completely hum free, even without touching the strings.
To me that shows how easily hum can get into the system by capacitive coupling.
Another problem I have observed with some not very well constructed valve amps is that there can be hum coupled directly to the grid of the first valve that relies on the guitar to short it out. When the guitar volume is at full, the low impedance of the pickup does the job, but when the guitar volume is turned down the output impedance of the guitar rises and the hum appears.Last edited by aquin43; 08-21-2020 at 03:54 AM.
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Originally Posted by Spook410
Magnetic and electrostatic induction of hum are quite different and independent. A Rhythm Chief 1100 will be susceptible to magnetically induced hum but by itself, i.e. linked directly to an amp, it will not be open to electrostatic hum.
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Originally Posted by aquin43
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Originally Posted by KirkP
P.S. I have corrected 3cm 'wide' to 3cm 'radius' in the original post
Grant Green, What is This Thing
Yesterday, 01:59 PM in Ear Training, Transcribing & Reading