Rivera Era Fender Input Jack Question

[plumcrazyfx]

I'll try to get a schem up to show what I'm talking about but here is a link with a lot of good info: http://www.stratopastor.org.uk/strato/amps/twoseriesfenders/twoseriesfenders.html

All of the input jacks are 4 lug and appear to short to either the second grid (Bassman 20) or to the tone controls (Champ 2) when a guitar cord isn't plugged in the amp.

1 What are they doing here and why?

2 What jack do I need to replace the one on my Bassman 20?  There are so many to choose from.
Thanks

[sluckey]

1 What are they doing here and why

It's doing exactly what you said. It's a better way IMO. Down side is the jack is not available everywhere. Might need to go to Mouser or other big distributor.

2 What jack do I need to replace the one on my Bassman 20?  There are so many to choose from.

There's only one that does this...

     Switchraft 13E, 1/4" Mono 2 Conductor Jack w/Nut & Washer, Isolated Break Circuit

     http://www.switchcraft.com/Product.aspx?ID=2703

[plumcrazyfx]

What benefit does shorting to the 2nd grid or tone control provide when a plug isn't in the amp?

[sluckey]

Same as shorting the grid of the first preamp tube. Only better.