Diagram of Fender polarity ground switch with capacitor

Keith Hilton · Post by **Keith Hilton** » 12 Nov 2002 8:17 am

I am not good at posting diagrams, so I wish someone would post the following: A diagram of the polarity switch on the back of the old Fender amps, with capacitor and transformer. Purpose of the diagram is to show people how this switch is connected to a capacitor and transformer. If the capacitor is old and leaking this could cause the metal case that holds the electronics to have voltage. Most of the ground plugs have been cut off these old Fender amp cords. Many,many old Fender amps have this problem. Yes, voltage of 0 to 120 and even 600 volts is possible. With no ground on the old Fender power cord, this can easily allow 120 to 600 volts to come down the guitar cord. Most of these old amps work so their owners are not aware of the dangers involved. Maybe looking at the diagram will prompt them to replace the old capacitors and install a ground on the power cord. Making sure the ground is absolutly attached to the metal that holds the electronics of these old Fender amps.

Ken Fox · Post by **Ken Fox** » 13 Nov 2002 9:34 am

Keith, I looked at old two-wire Fenders, three wire grounded Fender amps and even some Peavey amp schematics. Pretty much all the same scheme, a cap of about .047 ufd is connected on one end to ground and the other ended is attached to the either the "hot" or "neutral" of the incoming 120VAC line via the two position ground switch. In a nut shell, if the cap is leaking, you can put up to 120VAC on the chassis of a two wire amp, once this happens, and the guitar or you get grounded out, a path for this current will exist. That of course would result in a nice shock or possible equipment damage (anything in the path of the guitar signal will see the excess voltage). A three wire amp would blow the fuse and or the circuit breaker if the cap was leaking excess current to the grounded chassis. Another good reason to convert any two wire amp to a grounded plug.

Peter · Post by **Peter** » 13 Nov 2002 11:21 am

Keith, please check your email.

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Peter den Hartogh-Fender Artist S10-Remington U12-Hilton Volume Pedal-Gibson BR4 lapsteel-Guya "Stringmaster" Copy-MusicMan112RP-Peavy Rage158- - My Animation College in South Africa

Stephen Gambrell · Post by **Stephen Gambrell** » 13 Nov 2002 1:24 pm

Just to reinforce what Ken and Keith have already said, PLEASE do not remove the ground, or even use athree-prong adapter unless it's been properly grounded! So what if you blow a fuse! Get your amp fixed! And if you have an older, two prong cord, then CHANGE IT to a grounded cord. Far too many clubs have "extra" outlets that were wired by somebody's brother-in-law, with the black wire where the white wire ought to go. Better shot by a jealous husband, than fried by a fault in the wiring.

Peter · Post by **Peter** » 13 Nov 2002 9:48 pm

Years ago I bought a little tester and a long power extension cable. The tester was a 3 prong plug with LEDs. It would tell you if there was a ground connected to the wall outlet. If there was no ground, I ran the long cable from the kitchen, where there usually is ground, due to the machinery.

THERE IS NO POINT HAVING A GROUNDED AMP IF THE OUTLET IS FAULTY.

I dont have the tester anymore, but I am sure Keith is planning to make one ??

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Peter den Hartogh-Fender Artist S10-Remington U12-Hilton Volume Pedal-Gibson BR4 lapsteel-Guya "Stringmaster" Copy-MusicMan112RP-Peavy Rage158- - My Animation College in South Africa

Keith Hilton · Post by **Keith Hilton** » 14 Nov 2002 8:48 am

Actually Peter,you can buy one of these testers for under $10.00 at any place that sells electric stuff for a home. In the old days there were just two prongs on an electric cord. Musicians used to reverse the order of the 2 prongs in the plug, to find the least hum. When three hole recepticals were required this prevented reversing the polarity by turning the plug over. This is really what the polarity switch on the back of the amp does. In a 120 volt AC line there are two conductors, a hot wire and a neutral wire. Usually,one of these wires tends to induce a voltage, hence hum, in the audio more than the other. That is the reason for the .047uf capacitor to ground. If that capacitor is leaking, it is easy to see how a voltage could get to your guitar cord.