# Marshall Plexi Power Amp Input Mod



## taskforcestudios (Dec 21, 2007)

Hey guys,

I have a question for all you amp techs out there. I'm looking at putting in a power amp input mod on my custom built Marshall Plexi 100 clone. I understand that this would involve interrupting the circuit at the end of the preamp stage but I'm just wondering how I would go about doing this. I've hacked around building and modding tube amps, but haven't attempted this kind of thing yet. I want to have the ability to run different preamps through the 100W power amp section of this head. I know some of the more modern Marshall amps have this feature, but it would be pretty handy to have it on this Plexi style amp. Since this amp isn't a real vintage Marshall or anything, I don't mind messing around with it.

Any advice on where to start with this or links would be great!

Thanks!


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## LydianGuitars (Apr 18, 2013)

It looks like what you want is an effects loop. The MetroAmp loop is probably your best bet and it comes with easy to follow instructions. 
http://metroamp.com/store/index.php...ducts_id=499&zenid=b08945m68otgcejlu6gmvd3771


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## nonreverb (Sep 19, 2006)

Just some clarification, you are looking to slave the power amp section of your amp, not install an effects loop correct?


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## taskforcestudios (Dec 21, 2007)

Yes you are correct, looking to slave the power amp section. The amp already has an effects loop.


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## ezcomes (Jul 28, 2008)

my understanding is...if it has an fx loop...

the fx loop is after the pre-amp and goes directly into the power section...so if you ran the other amp with a line from the FX loop out to the in of the Plexi...you would be using the plexi power amp with the pre of the other...

no modding required...


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## taskforcestudios (Dec 21, 2007)

The effects loop works a lot differently. It's where the loop is placed in the circuit. The send comes out after the preamp section and the return comes in before the tone stack on the treble pot.

What I need is an input that goes right in front of the output stage of the amp bypassing the entire preamp and tone stack.

I would also like to have a preamp out as well. Kind of like what the old Ampeg V4 heads or even the Fender Hot Rod Deville has. Preamp out/power amp in.


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## taskforcestudios (Dec 21, 2007)

This is also known as a Slave in and out.


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## bolero (Oct 11, 2006)

have you looked at an ampeg schematic? may have some clues

I had a marshall 4203 combo a while ago I used to do this with...a different preamp into the FX loop...worked great, and the master volume on the amp was functional


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## taskforcestudios (Dec 21, 2007)

Yeah I was looking at the Ampeg schematic and it looks like the output jacks are right before the phase inverter tube. It's weird though because the Ampeg just has 2 jacks in parallel. Not sure how one can act as a power amp in... So maybe that's different than what I want.


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## nonreverb (Sep 19, 2006)

The effects loop can indeed be different. Some have a buffer in the return which you don't want...


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## jb welder (Sep 14, 2010)

Can you post a schematic or be more specific about the actual model this is a clone of? Do you have a schematic of the FX loop you have?


taskforcestudios said:


> The effects loop works a lot differently. It's where the loop is placed in the circuit. The send comes out after the preamp section and the return comes in before the tone stack on the treble pot.


Are you sure it is not between the wiper of the treble pot and the .022 cap that feeds the phase splitter? This is fairly standard and is post tonestack, not pre tonestack.
Ceriatone does their's that way: http://ceriatone.com/images/mods/Plexi50-with-interrupt-styl.jpg



taskforcestudios said:


> Yeah I was looking at the Ampeg schematic and it looks like the output jacks are right before the phase inverter tube. It's weird though because the Ampeg just has 2 jacks in parallel. Not sure how one can act as a power amp in...


 The original V4 had the 2 "ext.amp" jacks that could be used as inputs or outputs. You could connect several V4's together and plug into the input of one and the others would be slaves. Later they switched over to a series type loop with "pre-out" and "pwr amp in" that had a switching jack to break the circuit between pre and pwr amp.
The V4 loop jacks were not right before the phase inverter, but one tube stage before that.


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## taskforcestudios (Dec 21, 2007)

Hey JB, yes you are correct and actually my clone is a Ceriatone.

So basically my amp already has the feature that I'm talking about?

I assumed that this was just a regular effects loop. So I can literally just plug a different preamp output into the return and it will work? Will the volume controls on my amp effect this preamp in?


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## taskforcestudios (Dec 21, 2007)

I was hoping to have an input that would break the path between the pre and power amp. Is that doable or can I modify my effects loop to have that feature? Is it even necessary? Also, if I take the send output that will be my preamp output right? I could use that as a DI or for recording?

I like the idea of just being able the use the power amp section only without the volume controls affecting the input coming in from the external pre.


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## nonreverb (Sep 19, 2006)

It is indeed. According to the schematic JB provided, the signal is broken at the phase inverter. You can use that to slave the power amp. 
As for using the preamp, it should work as long as the input impedance of whatever you're plugging into isn't way off.


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## taskforcestudios (Dec 21, 2007)

That's great news! Thanks for the responses guys, really appreciate it!

I still might add a simple line out with a pot to control the level just for proper stage or recording levels.


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