Marshall Input Switching 3P4T w/ High Jump Tweak

[ioplex]

Here are some variations on an input switching scheme for Normal / Bright / High / Low style amps. The idea is to use one of the jack holes for a rotary 3P4T rotary switch to select between Normal High, Bright Low, Bright High and Bright High Jumpered through Bright Low to Normal High.

This first arrangement is almost exactly equivalent to switching using the jacks:



But this can be greatly simplified and maintain the same impedance on the input and grids (resistors can all be little 1/4 watt types):



But as you can see, in the Jump postion, there is some significant high frequency loss because of the high series resistance reacting with the typical tube input capacitance of ~200pF. So the Jump position can be simplified further and in the process "fix" the high frequencies (and reduce noise):



This should work equally well with other amps that have multiple inputs like Supros.

The 2 open holes can be used for anything such as parallel inputs for recording dry tracks or splitting to multiple amps or a master volume.

Just posting for posterity but if anyone sees an issue by all means let me know. I'm planning on putting this into a new build but I haven't actually tried it yet.

[ioplex]

Implemented "Simplified Jump High":



Works great so far but I've only been playing with the amp for about 12 hours.

More build pics here:

http://www.marshallforum.com/workbench/79773-weber-6m45p-head-kit-build.html#post1338099