Second Master Volume

[GtrScott]

Hello Everybody,

I'm a newby but learning fast so pardon me if I'm not as sharp as most you who've posted here.

I have a home built combo amp (50w marshall plexi master volume type circuit). I would like to add a second foot switched master volume so that I can have a boost or cut in volume without changing the gain etc.

I have a 100w Marshall Plexi head with the exact mod that I want to duplicate but I don't know much about switching and such.

Any help via partial schematic or referal to good reference material would be greatly appreciated.

Thanks in advance!

Scott

[Jana]

One way to do it, but this would only give a boost, would be to put a second pot, probably about 250k, between the ground leg of the current master and ground. Wire the pot using only 2 of the terminals so it is like a variable resistor. Then, your relay or switch would either short out this pot, across the two terminals, or not. With the pot shorted out you would set your regular volume, then pressing the footswitch to "unshort" the pot, set the boost volume with that pot.

[GtrScott]

Thanks for the response. That's a good alternative. What is the switching portion consist of? I see a small IC chip (looks like anyway) and a very small resistor in the 100w head that I mentioned. I think I need to read up on switching methods.

Any recommendations?

Thanks!

[Jana]

If you're talking about the IC that comes in Marshalls, it is some kind of dual op-amp that allows you to switch one on while the other is off. That would take a regulated +-15 volt power supply. It seems like a lot of *stuff* to just switch a pot in and out.

I would try something a bit simpler first.

Just wire it up like I said and then use an insulated jack and connect the sleeve connector on the jack to the ground point on the boost master pot and the tip to the other terminal of the pot that is going to the other master. Then, use a footswitch that is either shorting the terminals or not. Use a good shielded cable to connect to the footswitch, no more than 15 feet long. The shorter the better. I have tried something similar to this in a different part of the circuit and it worked. The key will be the attention to ground point and the shielded cable. Try it and see if it works, it will be a lot simpler than shoehorning in another power supply to run that IC. If it don't work, well nothing lost, you need a jack for the eventual footswitch anyway.

[Jana]

Those that are reading and thinking outside the box (chassis?) may ask, "so, why can't I hook up a pot in that old wah wah pedal shell I have in the junk box and plug this into the footswitch jack and have a variable boost? Or, if I turn the original master all the way down, have a volume control?" Well, you can, I have done it.

[GtrScott]

Thanks for the idea Jana although not quite what I'm trying to reproduce.

My layout is like the attached schematic. The problem, I think, is that signal is bleeding back through the pot that is supposed to be off.

What's the simplest way to switch and not get the bleed? A different type of switch on both sides of the pots?

Thanks again!

Scott

[doctord02]

yep. a DPDT switch that was wired on both sides would completely switch the pots out of the circuit. I suspect it's gonna pop tho, so one of the bigger brains will have to tell you how to avoid that...

[skyboltone]

yep. a DPDT switch that was wired on both sides would completely switch the pots out of the circuit. I suspect it's gonna pop tho, so one of the bigger brains will have to tell you how to avoid that...

10 meg resistor across the DPDT contacts.

[Cygnus X1]

A clean overdrive works best for me.

Sorry.

[GtrScott]

All Hail the Wise Ones!!

I guess I just needed to ask the right question

To simplify, can I put one 470k after the switch instead of one on each pot?

Now part two:

Now that I have the signal circuit, I still don't really understand the switch part of the circuit. I'd like to have an LED on a footswitch but I don't know how to wire it up. Any advice?

Thanks!!!

Scott

[Jana]

You said this is a 50w plexi with a master volume, this is after the tone stack, right? The plexi's didn't have a master volume but it was common to add one after the tone stack. The thought just crossed my mind that these 1meg pots with the mix resistors might be in the pre-amp section. Is that where you are trying to do the switching?

[GtrScott]

The thought just crossed my mind that these 1meg pots with the mix resistors might be in the pre-amp section. Is that where you are trying to do the switching?

Yes. These pots are the pre-amp section.

To clarify, this is a home built amp, not an actual marshall.

Thanks for the help!!!!!

[Jana]

Ah, okay. this isn't the actual master volume then but the volumes in the pre-amp.

Try this...

(my first time at trying to put a jpeg in here)

sorry for the low quality and the rotation, lol, I are a doofus with this stuff.

[GtrScott]

Thanks Jana! Will this work or do I need both 470k's?

Where do I put the 10 ohm resistor that skyboltone mentioned?

I went back and looked and these pots are after the tone stack. Is that going to make a difference in the switching?

[Jana]

theoretically that should work. You don't need any 470K with this arrangement. The resistors are not 10 ohm, they are 10 MEG ohm. put them across each place that you are switching to help reduce pops, although I think this is still going to make noise when switched. So, there would be a 10M from the realy common to the relay NC, a 10M from the relay common to the relay NO. Repeat on the other half/relay.

I am still curious as to why you need to have a full zero volume to max volume with each 1 meg pot.