Extra grunt for Marshall 50 watt plexi

[ED]

Hey guys,
I am in the process of building a Marhshall Plexi. The schematic is found at www.drtube.com in the Marshall section under JMP 50 watt lead plexi.

It has 3 12AX7s and 2 EL34 output tubes.

The old plexis are great amps, but i was wondering about putting a gain stage with a gain pot somewhere in the circuit that is able to be switched on and off, so it will drive the tubes more and get a drive suited to metal.

I am mainly a stompbox builder, but I have no idea what circuit I should wire into it and where in the amp circuit.  Any help would be greatly appreciated

cheers,
ED

[aron]

The easiest way is to find another similar schematic with the extra gain stage and copy it. Look at high-gain amp schematics; they should be readily available online. Let me know if you have problems finding them.

[Ansil]

well iam still trying to get the page to load but looking at the old 1987 plexi model amp at schematicheaven.com  under the marshal section

http://www1.korksoft.com/~schem/marshallamps/1987_plexi.pdf

basically it shows four inputs.  and the bright input going into the second tube.  but why not make it a two input one..  make it always use the three stages there.. and move the bright input to the output to the input of the normal channel and instead of the bright cap add a birght switch.. hmm that or just make the channel switch the bright stage into the normal stage. woudlnt' be that hard at all..

also iwould change the tone stack personally and go with the marshall guvnor tone stack with 25k pots as this to my ears is more responsive..

[Ammscray]

Hey guys,
I am in the process of building a Marhshall Plexi. The schematic is found at www.drtube.com in the Marshall section under JMP 50 watt lead plexi.

It has 3 12AX7s and 2 EL34 output tubes.

The old plexis are great amps, but i was wondering about putting a gain stage with a gain pot somewhere in the circuit that is able to be switched on and off, so it will drive the tubes more and get a drive suited to metal.

I am mainly a stompbox builder, but I have no idea what circuit I should wire into it and where in the amp circuit.  Any help would be greatly appreciated

cheers,
ED

I have lots of great mods for plexi and metal panel 4 input tops that will make it sound like you've added an extra tube, but you didn't... and I would be glad to share them with you...

If you want the extra tube, the easiest way is to simply cascade the two gain stages of both channels together (which is NOT the same as jumping the channels on the outside, as you probably knew), with split cathhodes,

and then putting in a good dual-pot post PI master volume, of which I have several different circuits...there's LOTS of places to go from there!

[Johan]

just take the input of the channel you dont use and link it to the top of the volumepot in the channel you do use....when turned down, the amp sounds stock but when you turn it up you get a lot of extra gain. its out of phase, so around "1" it cansels itself but after that its fine....look at Mesa Boogie MK2b to see that very same sircuit...and its also easy to do and easily revercible...

Johan

[Ammscray]

just take the input of the channel you dont use and link it to the top of the volumepot in the channel you do use....when turned down, the amp sounds stock but when you turn it up you get a lot of extra gain. its out of phase, so around "1" it cansels itself but after that its fine....look at Mesa Boogie MK2b to see that very same sircuit...and its also easy to do and easily revercible...

Johan

Just to be clear, this is called "jumping" the channels and doesn't really add gain per se, it makes the sound a little thicker but in the case of 50 watt Marshalls IME they become muddy...

also the channels are not out of phase, you are putting them in parallel, which is why you don't get the extra gain that you do get when you put the two gain stages in series, or "cascading" them together

the first Marshall master volume amps (from 1976 and onward) did this and then of course Boogie followed, Soldano, etc...the trick is to voice each stage right to keep the sound tight and not mushy (like boogies)

[aron]

Ammscray,

If he really wanted to hack, I guess he could use Ken's utility gain stage from the Trainwreck pages...

[Johan]

sorry if I was unclear...the extra stage that is dailed in this way becomes parrallel to the 470k coming from the first channel, but cascaded with that channels tubestage...not just jumpered like most people used to do with a cable between the inputjacks. when the volume of the extra channel is turned down, its completly out of the circuit. the inputs goes only to one of the channels. the other one is fed from after the first stage in the first channel...I think only MK2b of all Mesas has it this way..its different from other Mesas, even other MK2's...and like I said, when the volume of the other stage is up about 1 ( or around unity ) it basicly cansels all sound...sorry I cant supply a link to a schematic...

Johan

[Ammscray]

sorry if I was unclear...the extra stage that is dailed in this way becomes parrallel to the 470k coming from the first channel, but cascaded with that channels tubestage...not just jumpered like most people used to do with a cable between the inputjacks. when the volume of the extra channel is turned down, its completly out of the circuit. the inputs goes only to one of the channels. the other one is fed from after the first stage in the first channel...I think only MK2b of all Mesas has it this way..its different from other Mesas, even other MK2's...and like I said, when the volume of the other stage is up about 1 ( or around unity ) it basicly cansels all sound...sorry I cant supply a link to a schematic...

Johan

I still don't think I follow...maybe you could draw it out and post it?

Yes Aron, that extra gain stage is a good place to start if you want to proceed with that kind of modification...

[Ansil]

uh so far it looks liek we are just saying the same thing already about utilizing the other tube stage and making it selectable.  or am i reading htis wrong

[Johan]

I made a really sketchy drawing in paint...give me a mailaddres and I'll send it...perhaps you can clean it up so others can see...( I still havnt figured out hove to post pictures here...)

Johan[/img]

[Peter Snowberg]

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-Peter