JCM900 footswitch

[vonFrenchie]

Hey everybody,

I just bought a JCM900 2500. Thats the 50 watt master volume model. I was planning on getting a footswitch for it but I realized one thing. The footswitch controls two master volumes. It sounds sort of useless to me.

My plan was to add another pot to the front of the amp (no I don't care about resale value). This pot would be a second preamp volume. This way I could adjust the preamp volume for channel a and channel b and adjust the master volume for channels a and b. That way I can have a good clean volume and switch right over to a high gain setting.

My question is, how would I go about doing this. I have the schematics but it looks like the footswitch almost goes nowhere. I know I would need to replace the switch with a dpdt and the footswitch jack with a stereo jack but beyond that I can't figure out how to get this done. Thanks in advance for the help.

Here is a link to the preamp schematic
www.drtube.com/schematics/marshall/cd0109-iss4.pdf

Edit: I tried putting a hyperlink to the schematic in but it came up with a 404 error, sorry.

[vonFrenchie]

I was thinking about cutting a lead from VR1 (see schematic) and connecting it to the switch along with a second potentiometer wired the same and in parallel. I realize that that isn't as complex as Marshall has the master volumes wired up (with a slew of resistors and an IC) but I don't think it would be a problem.

If I used this set up would I need a second footswitch jack and switch out the switch with a tpdt switch in order to get it to work?

[sumgai]

vF,

Yes, you can insert another pot in parallel with VR1, and control which one is active. You'll need to make/break both the upper and lower connections for each pot, but you can leave the wipers permanently attached on both units. However, in order to choose which pot you want in the circuit, you'll have to have at least two poles (one for the upper connection and one for the lower connection) and those will have to be double-throw, as you've surmised. A good ol' DPDT to the rescue. But.....

The kicker here is, finding a jack for the footswitch that will accomplish the same job..... not something to be found at your local Radio Shack, trust me on that one. In fact, that's why Marshall inserted "that slew of resistors and an IC" - they make up a logic circuit that needs only one pole and one throw in order to select which master volume to use. In effect, you'd need to duplicate that functionality for your intended scheme..... unless you can find a switching jack that has isolated contacts. newey might be able to help you out with that, I seem to recall that he found something like that, oh so many moons ago.

Oh, wait.... You say you wanted a single jack to control all the so-called channel switching? Sorry, that's gonna send you straight into logic-control territory, no other way around it.

But in either event, if you do make this project a go, you might consider using a concentric-shaft, dual-gang pot, instead of drilling a hole for a new pot. A dual 250KΩ unit will work fine here, and shouldn't be hard to find. Granted, it's a bit more expensive than a standard pot, but it sure makes for a neater-looking job.


HTH




sumgai

[vonFrenchie]

I read online that the P801 switch, the one for the JCM900 master volume, is just a spst. No led, or anything else. So I could effectively throw a 5pdt switch in and wire two poles for each preamp channel and the other one for the logic circuit, right?

I'm fine with running two cables from the amp to the pedal. They obviously would be a stereo cable for the preamp switching and the standard cable for the master volume switching to the pedal. One jack isn't necessary and I don't want to make it so complex that I futz it up. I realize that two cables running to a single footswitch looks silly, but until 3 channel trs jacks become common place, I'm out of options.

You're right about the concentric pot. It totally slipped my mind. Though I couldn't have the nice matching knobs, it would look a lot better than a pot randomly placed on the other side of the input jack.

Thanks for the help thus far

vF