Creating two channels from one

[MrBassett]

Hi everyone!

I'm new to the amp garage, but I plan on being here often now, as I love tube amps and looking forward to having some modifying my old Marshall.

What I have is a 79 "Master Model 50w Lead" I'm really happy with this amp but the problem is that it only has one channel, and this is sometime a hindrance in live playing. I've attached a picture of the board in hope someone might have a good schematic to work off of. The board says "Marshall ST1 iss. 202" I hope this helps and can try to answer any questions.

Do any of you know of any previous used methods or have any good ideas in creating a two channel amp out of my mine? I'm happy with the eq not changing, I just need to switch between to master volume settings and would really like if possible a preamp setting. Basically all I'm looking to do is take my amps lovely crunch, then get it slightly more saturated in the preamp if possible and louder.

I'm thinking of using a relays to switch between the two master volume settings and preamp settings. But I'm concerned with popping, especially in the preamp.

Let me know what you folks think of this and thanks in advance for any advice. Cheers!

(edit) Also, any good reading material on tuning my amp up and technical workings of tube amps? I just want to get more educated with them.

[Structo]

No picture.

Can you give more info about which model this is?

Is it a JMP? Like a JMP 2104 MV Lead 50w?

[MrBassett]

Sorry about the lack of info, I figured I might have left something out.

It is a JMP MV 50w Lead. Since the serial number declares it as 1979, it is a 2204.

Here is a picture of the pcb board and chassis.


[img:604:453]http://photos-c.ak.fbcdn.net/photos-ak- ... 2_3526.jpg[/img]

[img:604:453]http://photos-c.ak.fbcdn.net/photos-ak- ... 0_3552.jpg[/img]

[rfgordon]

I perfectly understand your desire to improve/mod your Marshall. However, to be perfectly frank, that PCB is gonna be real tough to deal with. That amp isn't even designed to be easily repaired, much less modded for what you want. The traces on the board may not survive direct soldering.

That having been said, the iron, chassis, sockets, etc. make for great raw material. With 3 peanuts to work with, plus a fairly big chassis, you could use the Dumble ODS topology and make a nice channel switcher in that box.

Or you could build a Trainwreck Express in there, and as long as your guitar has a volume knob, you don't need to switch channels to go from clean to scream. If you don't believe me, watch/listen to Glen Kuykendall's vid on youtube!

Good luck.

[MrBassett]

I have seen the train wreck footage and WOW! However the main reason for the two channels is for dynamic issues in a loud two guitar band . The clean to scream with the volume knob as been something I've been doing for a while, although my JMP doesn't quite scream like those 'wrecks'. But when the bands going full tilt and I'm up for some lead a substantial volume boost is needed to be hear and pedals just won't cut it.

As I gig on a fairly regular basis at the, and being a poor college student I can't afford another amp, any kind build requiring a gutting of my chassis can't happen for a while.

Could you expand a little on the Dumble ODS?

Since I currently have some of some relays that can handle the power coming out of the preamp. I'm am thinking of using it switch between two master volume pots. What would be the implications of this and would I get less "pops" wiring both pots up to the line coming out of the preamp and switching the outputs, or switching the inputs into the pots. I'm assuming before because the noise would have a resistor to run though. I also found a decent schematic for reference.

[img:604:445]http://photos-d.ll.facebook.com/photos- ... 5_1791.jpg[/img]

[10thTx]

Unless I am missing something ....... you have two channels prior to the second gain stage? Why not use a simple A/B box to switch between those. That seems like a very easy solution if I am understanding this correctly? The schematic you posted is your amp?

You can always modify one of the channels on the PCB if you feel like it needs tweaking.

With respect, 10thtx

[MrBassett]

Its not a bad idea, but currently one is cascaded through the first tube, and the other isn't. This is the low and high inputs there is quite a gain difference. However, some mods to the pcb board (or rather by passing the pcb to a new piece of circuitry) would open up quite a few possibilities, but i just wouldn't be ready for that yet. Perhaps later....turn it into something more analogous to a twreak.

Any kind of switching with a physical switch causes some nasty pops, as I soldered a circuit in for giggles last night. So that is out of the question. But then I realized I'm not really needing to "real channels" out of my amp. I just need my one tone, then that just quieter.

So i came up with a really simple solution. I taped a wire into the input pole on my master volume, used a pot as a variable resistor, then put that to ground with a switch in between. This way I get no poping even when loud. It does what i need and its nice and simple. But getting the live mix right might take a little longer, but i can deal. This should also work for the pre-amp as well.

I'm gonna bypass the low sens. input jack and use that for the line operating the relay. Then install a dual pot to keep the amp looking clean. Any good places to tap into on the amp for that power? I'm thinking the heater circuit.

[MrBassett]

10thTX, to clarify cascaded I mean that there is a High and a Low input. The High input runs through one half of the first preamp tube, then into the low input before headed to the second half of the tube. But you probably knew that. The problem with the two channel idea is that it would require something radical to get both channels gained up to what I would want them to be as the low is very very clean and I don't want the high to get too much more juiced up.

But I do have to say, you've got me thinking of some ways to possibly get this amp to really rip in the future

[jamdigs]

Unless you have modified you amp already; the schematic shows the two halves of the first tube as not being inter-connected. As you have pointed out, the Hi & Low inputs have different gain structure. 10th TX suggested an AB switch to select between the clean low input and the driven high input. If you like the individual sounds of the two channels then the AB switch is the way to go.

[10thTx]

I did not realize your amp was not the same as the schematic you provided?

Here is a link to show how to use a relay on a Marshall amp in just the fashion you are describing. You can always use a trim pot in the amp to control the difference in volume IF needed.

http://www.el34world.com/projects/relay_switch.htm

And the hot switch mod which is similar to what you have without the switch.

http://www.el34world.com/projects/hotswitch.htm

With respect, 10thtx

[ampdoc1]

I love the Trainwrecks I've built, but the change from sweet and clean to nasty is down at the very lowest end of the guitar volume pot.

I've found a very easy way to get a switchable clean sound is to go through an overdrive pedal and set it to the cleanest tones at a low volume. Plus, if you want a little grit you can dial that in too. I've used my $35 Bad Monkey very successfully like this.

a'doc

[MrBassett]

10thTx - I must have mixed my words up early, but we we thinking the same about the two inputs. The although the low sounds nice, it just doesn't have the crunch sometimes needed, and I can get a very good facsimile by turning down the preamp, so its gone now. And thank you for the relay circuit. I was at my desk getting a little stumped on how to wire in a on board switch as well as the foot stomp.

So tomorrow I will finish wiring up the relays and my amp will be a two channel amp of sorts. Thanks to all who've helped! I'll post some pictures of the jobs when I'm done.