Marshall 2204 channel switcher mod?

[rubyfocus]

Does anybody know of a good way to build channel switching into a 50 watt JCM 800? Right now, I've always got to walk over to it and switch inputs by hand, and then of course deal with volume change which results. I need an elegant way to switch it remotely.... Any ideas?

[loverocker]

Marshall did work out how to do a 100% passive A/B footswitch for the High/Low cascaded design. But they never put it into a Marshall. Have a look at the Park 1210 schematic (it's at http://www.prowessamplifiers.com/sch...hp?sFileID=194 if you don't have the Doyle book). The key is in using a stereo socket and cable. Should be easy to experiment for a few bucks.

[MitchK]

this is a really good question, I'm curious about this schematic but can't really see whats happening in it???

[Enzo]

It is pretty straightforward. Like LR says, the high gain input needs to be a TRS jack, and the wire to the AB switch needs to be two wire shielded. It works like this.

Keep in mind that in this amp there is an additional gain stage between the low and high gain jacks.

Guitar plugs into the AB switch, and the AB switch cable plugs into the high gain jack. This opens the normal connections inside that jack, so the AB sw is now connected to the tip and ring contacts.

In the as drawn condition, from the AB input, the signal goes through the switch to the tip contact, which goes right to the high gain grid. (And after that stage, the signal routes through a 470k resistor up to the low gain grid.)

Also the other half of the switch grounds the ring contact, which grounds the low gain jack. The 68k resistor there works as a voltage divider to tame the signal from the first stage somewhat.

NOw flip the switch - or stomp on it - whatever.

Now the AB input is routed to the ring contact, which goes right to the low gain jack, and then on through the 68k resistor into the low gain stage. The other half of the switch grounds the tip contact which then grounds th grid of the high gain stage to prevent it amplifying noise and stuff while it is not in use.

[drewl]

Anyone know if this arrangement yields more noise than if you switched the inputs after the firts gain stage with a relay which is common in alot of amps?

[jaysg]

There are some un-subtle and un-readable changes from the 2203/2204 circuit -- the Hi input volume control cap + resistor or two caps? and the stuff after V1b...there's no cathode follower. Still, it shows a sincere attempt on Marshall's part.

The KOC circuit works pretty well, though it eliminates one of the high frequency pass caps. FETs are used to switch in more gain for the Lo input which makes them closer in volume.