JTM-type jack socket
Classic ‘Cliff’-style design, 4 solder lugs for switching
Made popular in ’60s British amps, this chrome nosed jack socket is still in production. This one is made by Neutrik – I’ve tried others, but always come back to this. This is the 2-pole switched version: the four solder lugs are in two pairs – one pair for the tip and one pair for the sleeve of the jack plug. With nothing inserted into the socket, the pairs are connected: 1 to 2 and 3 to 4. When a plug is inserted, the connections are broken: the tip of the plug is connected to lug 1, and the sleeve (i.e. the shield) of the plug is connected to lug 4.
This is very useful for guitar input jacks; for example, in a typical Marshall-style input, the input to the first valve is grounded when there’s no lead plugged into the socket. You can see a typical wiring scheme in the third photo on the right: the blue conductor carries the guitar signal to the first valve, and the black wire runs to ground. At all times, the four solder lugs of this jack socket are isolated from the chassis. This avoids the possibility of an unwanted ground loop causing hum.
Dimensions & info
The jack socket requires a 11.2mm hole in the control panel, to seat the black outer ring. It comes with a single washer to use for mounting (two washers are shown in the photo on the right and some are supplied with two from the manufacturer!)