Including channel switching in a Marshall Plexi 50w project

[Curbdog]

What are some options for including channel switching in a 1987 model Plexi project that I am considering? I would like to be able to switch a lead tone in and out. I see channel switching mods and add-on boards available from various vendors. Any of these worth installing for this purpose? I have some real estate in the chassis, and if an add-on PC board would make it easier to add the channel switching, I am all for it.

On anothe Plexi I built, I tried an A-B switch, but when I was switched to the non-lead channel, the noise from the cranked lead channel was still very evident. Wondering if I would have the same problem installing one of these channel switching boards on a single- input Plexi, to switch the guitar signal input from one grid to the other.


I am sure someone out there has a much better way to do this than I can think of right now. Thanks !

[Ripthorn]

I haven't built mine yet (still need another couple parts) but I am going to be using relays to both switch on an onboard booster (unused triode) and to switch from clean to dirty channels (two relays, one for each function). The relays were like a buck or two each.

[Jana]

Another option for plexi channel switching,

http://www.procosound.com/?page=viewprod&id=741

[Curbdog]

Thanks for your replies.

I did a search on channel switching, and got some great information from previous posts. I think I have an idea how I would like to proceed with my Plexi-channel switching, and I would like some opinions.

Say I were to build a regular 2-channel, 4-input 50w Plexi, and located a channel switching relay at the mixing resistors that feed into V2. If then I were to plug the guitar into channel 1, bridge to channel 2 with a patch-cord, and adjust the volumes independently, I am thinking that I will be able to switch between the channels with a footswitch, but not mix them.

Am I correct, and how well will this work? Thank you.

[ampdoc1]

That should work fine. It's about the same as implemented on the D*mble amps,.... switching output of either of two gain stages to the next stage.

If you're interested, I still have some of the 3-channel relay boards available. Pretty quiet, and easy to install.

ampdoc1

[Stanz]

I am thinking about a similar approach with an extra twist. One button to toggle between the two channels, and a second button to allow both channels through. Basically just ground the signal before the mixing resistor, like mentioned above. The footswitch will ground the signal when power is applied to the relay, that way without the footswitch, or a failure, both signals will be at V2. The second button will turn on both channels and when hit again, it will revert to the setting on the first switch.

[solderstain]

On anothe Plexi I built, I tried an A-B switch, but when I was switched to the non-lead channel, the noise from the cranked lead channel was still very evident. ...I am sure someone out there has a much better way to do this than I can think of right now. Thanks !

I'm going to assume that your mention of using an A-B switch with a Plexi you built implied that you were using an external A-B switcher, such as a floor switch?

If my interpretation is correct, then I can relate to what you experienced. (If I misinterpreted your statement, then ignore the rest of this post... )

I've been building my own A-B-Y switch boxes (with a built-in clean boost) for a number of years. I use them to switch channels, or combine channels, in any number of amps I've had over the years. One of the amps I still use it on is my metal-panel (four hole) Super Lead (which is slightly tweaked to make Channel II more useable - 500pf cap around the Channel II mix resistor).

My early versions of the switch box produced the same result you mentioned - channel bleed to the non-selected side. I finally traced it to the physical mechanical switch itself. It leaks. I use both the common 3PDT switch and the DPDT switches.

I also had a problem with one amp where I'd get feedback in the non-selected channel of the amp if I had it set to a really high level of gain.

I solved it by modifying the design of my A-B-Y switcher. The first version left the signal section of the non-selected channel ungrounded. So the bleed from the switch went to an active, 'listening' amp input. I fixed that by altering the design of my switcher such that the non-selected channel gets grounded, silencing it when the other channel is selected. The "Y" portion of the switcher accomplishes its task partially by lifting the ground on the non-selected side as well as putting both channels directly in parallel.

WORKS GREAT!

Perhaps, rather than trying to engineer a complicated internal switching scheme for your Plexi project, fix your external A-B switcher so it grounds the non-selected channel to keep it quiet.

But again, if I misinterpreted your original statement, all of this is pretty much useless to you...