Quick question about bassman 20 schematic

[slowferrari]

I was looking through The Tube Amp Book, browsing for something I might build with some transformer sets I inherited from my late father. I came across the schematic for the 80's Fender Bassman 20, and it looked like a prime candidate, but I'm confused by one thing on the schematic. There's a connection between one pin of the input jack and the grid of V1b. I'm thinking I'm not clear on which pin of the input jack is connected there, but I can't see an obvious reason for it?

Can anyone explain what's going on here?

[sluckey]

It's a special jack with isolated switch. With nothing plugged in the switch contacts are connected. This shorts to grid of V1B (pin 7) to ground so the amp will be quiet and not buzz. Insert a plug and the switch contacts open to remove the short on pin 7, thus allowing the instrument signal to pass.

This may help...

     https://www.tubesandmore.com/products/14-jack-switchcraft-mono-isolated-break-circuit

[plumcrazyfx]

I have 2 of the originals.   They are great amps.  Look at the schem and then look at the 6G3 Deluxe.   I think it's based on a single channel Deluxe, minus the trem, adding a TMB tone control, and silicon rectified.

[PRR]

As said, it makes the amp quieter when unplugged.

Maybe in a very quiet showroom, this would matter. Or if you were playing for a room of silent-vow nuns -AND- unplugged when done so as not to break the mood. I'm not sure the plug really works, for me, as a mute switch. And I'm sure that #14 jack will not be in stock when you go to order parts.