Hi everyone, I'm new to the forum and just got into instrument building.I've made a mountain banjo, a couple of stick dulcimers and I'm now near the end of completing 2 CBGs. So I've been looking at the diagrams that google throws up when i search for them. One thing that puzzles me is the wiring of the black wires on to the pot chassis and the 3rd tag. I have some electronic experience, and I can understand the connection of the red wires from the piezo to the pot tag 1 and tag 2 and out to the jack socket. Why doesn't the black wire go directly from the piezo to the jack socket?
The way the diagrams show it( with black soldered to pot chassis and tag 3) would put a short circuit or zero ohms across the socket output, when the volume pot is turned down. I would have thought putting a short circuit into an amp input, would harm the amp, or would it?
Maybe someone can explain this, thanks in advance, Mike
Replies
Jim, I think that answer to your original question may be that all of the components were earthed via a common point (maybe a volume pot body or a 3-way switch) in order to avoid earth loops, which can generate hum. In a simple one-pot diagram this may cause confusion but, with more complex schematics, the central earth point becomes clear....ish. The linked diagram earths to the 3-way switch and pot bodies.
http://www.seymourduncan.com/support/wiring-diagrams/schematics.php...
With respect to the follow-on point, yes, turning the volume to zero shorts the input positive to earth, but a huge amount of experience suggests that a guitar amplifier is not damaged by this.........even if theory says that it should be.
Perhaps 'Question 2' should be "what is the effect of shorting out a guitar amplifier input ?". I am not skilled in anything electrical but would have thought that if you short the input, there is no input signal on the positive terminal and, hence, nothing to amplify. I know that shorting the output can cause mischief and that amps have protection against it.
I have now run out of electrical knowledge :-)
thanks for your reply, much appreciated.
*it doesn't NEEd to be soldered to the pot chassis at all, its only a convention. If the ground from the disc and the jack met at lug three there in that diagram it would work exactly the same except for that the grounding on the chassis makes a faraday cage around the pot wafer, shielding it from electric fields / noise...
if you think back to the beginnings of electrifying guitars, Rickenbacker, Fender, Bigsby, all those guys.. they were probably among the firsts to use potentiometers in a wooden chassis like a guitar, pots had been around for radios and stuff for sure, but they were being mounted into a sheet metal chassis of some kind, inside an old bakelite radio, anything like that, take a look. Therefore they were already grounded just by screwing em in no? So I guess it seemed important to those guys (Rickenbacker, Bigsby, Fender etc) to solder to the rear of the pot, at least when it wasn't bolted to a steel plate like on a telecaster or stringmaster steel etc...
I personally think it unwise to join the ground from the pup and the jack on the rear of a potentiometer like in this drawing because these solder joins are so fragile. I prefer to join em on lug 3 there and just run a little jumper to the chassis, or (much better) skip it altogether because all my pots etc are grounded anyway by virtue of aluminium or copper tape or similar on the inside.
Shunting signal from a couple pickups to ground will have no harmful effect on your amplifier, don't worry about that, its a minuscule amount of current :)
Really the fact that it is grounded to the pot chassis doesn't make any difference to my point, because even if the black wires were joined at the tag 3 and not grounded, there would still be short circuit across the output jack socket, when the volume is turned down on the pot. It is this part of the diagram which puzzles me, and not the fact of being connected to the pot chassis. I'm sorry if I'm not making myself clear, but I don't know of any other way to describe this. Thanks again for the reply.
Been 30 years since my electronics school (Navy) the volume pot never zeros out there will always be resistance it may only be around 1K - so I don't see how you find a shorted circuit it is still protected via the tap in the pot.
Hi Jim, it's getting on 40 years for me since I studied, so that's why I seem a bit unsure. Things have changed a lot since then, but I've always assumed the pot zeros either side when the wiper(tag2) gets to the end of the track.So, as you've put doubt in my mind, I got the old meter out and measured one of the pots, and sure enough it pretty much zeros, maybe 1 or 2 ohms at most, certainly not as much as 1k. So am i to assume these modern amps will take a virtual short across the input?
Been a long time and I can't say, I get what you're asking I just can't provide you with the answer other than to say, it works ;)
Maybe this will help you chase it down - sorry I don't have more than that at this point
http://www.electricalfun.com/workbenchfun/basic_electric_guitar_cir...
Thanks Jim and sorry to labour the point, but I wanted to make sure before I blow up my amp.
Not sure what diagram you are looking at, I follow these schemes without issue
http://oldshop.cbgitty.com/?p=87
the second diagram on your link is the one I'm referring to. Thanks for the reply.
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