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PostPosted: Tue, Nov 05, 2019 4:16am 
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Plank Cranker Wanker
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Hi guys

How much is needed to be done to mod a marshall stock type amp to JJ specs. Only the JJ channel or BE channel?

Was thinking about building another 2204 amp and turning it into a JJ.

I dont have much expirience with schematics but have build a ceriatone chupa 50 and 3 2203 amps.

:rock: :rock:

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PostPosted: Tue, Nov 05, 2019 1:37pm 
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That should be a pretty easy mod. Tone stack and PI/poweramp would pretty much stay stock.
The "BE" channel is just a modified JCM800 preamp.
The JBE channel is the BE channel with the MPSa06 diodes engaged. You can put the diodes on a switch or use a relay for BE/JBE switching. Just be aware of the volume drop when you engage the diodes. A second master vol is pretty much mandatory if you are going to make this foot-switchable.

The BE channel on the JJ has a few slightly different values in a few places vs the BE100.
The other difference vs the BE100 is the .0068uf depth cap value ( .0047uf parallel with a .0022uf ) on the JJ vs just the .0047uf on the BE100
I would recommend you do a depth pot vs the fixed 220k that Dave puts in his amps, and the NFB resistor should be changed from the 2204 values to 47k off the 4 ohm tap.

So if you have a stock 2203 type amp, the only things you have to figure out how to physically add into the chassis / turret board are the resistors, cap, and diodes off the cathode follower for the clipping, and the depth circuit. The rest of the changes can all physically go into the existing locations.

I just hope you are going to do this into a 2204 clone build on a turret board. I would not attempt this into a real Marshall 2203 with a PCB.
If you use Synergy coupling caps, silver mica for the snubbers and tone stack, and those little Nichicon .68uf electrolytics for the cathode bypass caps, it will sound :thumbsup:
Don't be tempted to "improve" the design by using "better" film caps on the cathodes, or high voltage ceramic for the pf value caps. It won't sound like a Friedman.

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PostPosted: Tue, Nov 05, 2019 5:05pm 
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PostPosted: Tue, Nov 05, 2019 8:41pm 
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Whoops...bad habit of hitting the wrong button...carry on.

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PostPosted: Wed, Nov 06, 2019 3:28pm 
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fusedbrain wrote:
That should be a pretty easy mod. Tone stack and PI/poweramp would pretty much stay stock.
The "BE" channel is just a modified JCM800 preamp.
The JBE channel is the BE channel with the MPSa06 diodes engaged. You can put the diodes on a switch or use a relay for BE/JBE switching. Just be aware of the volume drop when you engage the diodes. A second master vol is pretty much mandatory if you are going to make this foot-switchable.

The BE channel on the JJ has a few slightly different values in a few places vs the BE100.
The other difference vs the BE100 is the .0068uf depth cap value ( .0047uf parallel with a .0022uf ) on the JJ vs just the .0047uf on the BE100
I would recommend you do a depth pot vs the fixed 220k that Dave puts in his amps, and the NFB resistor should be changed from the 2204 values to 47k off the 4 ohm tap.

So if you have a stock 2203 type amp, the only things you have to figure out how to physically add into the chassis / turret board are the resistors, cap, and diodes off the cathode follower for the clipping, and the depth circuit. The rest of the changes can all physically go into the existing locations.

I just hope you are going to do this into a 2204 clone build on a turret board. I would not attempt this into a real Marshall 2203 with a PCB.
If you use Synergy coupling caps, silver mica for the snubbers and tone stack, and those little Nichicon .68uf electrolytics for the cathode bypass caps, it will sound :thumbsup:
Don't be tempted to "improve" the design by using "better" film caps on the cathodes, or high voltage ceramic for the pf value caps. It won't sound like a Friedman.


Nice post.

I’d only add that I have never liked the 500pf snubbers Friedman uses. To me it makes the high end weird. I’ve used 47pf caps to just shave off some high.

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PostPosted: Thu, Nov 07, 2019 11:11am 
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psychodave wrote:
I’d only add that I have never liked the 500pf snubbers Friedman uses. To me it makes the high end weird. I’ve used 47pf caps to just shave off some high.


Do you mean 47p anode bypass cap across a 100k resistor (in the Friedman design)?

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PostPosted: Fri, Nov 08, 2019 6:39am 
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Don't overlook that on a stock 2204, the gain pot is after the first stage and on a BE/JJ, it's after the second stage.

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PostPosted: Tue, Nov 12, 2019 11:37pm 
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I have a JJ jr. and I just sold a modded Marshall Superlead a few weeks ago due to needing some cash for some unexpected dental and car bills. The modded Superlead I had could do the JJ tones all day. I regret selling it but it was a few weeks of everything that could go wrong did. I have a JJ jr inspired PCB on the way from a guy in Australia. If you are interested I can sell you the board plus the build docs for what I have in it. Honestly though if I did sell it I would just turn around and send the money to V2.

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PostPosted: Wed, Nov 13, 2019 8:12pm 
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fusedbrain wrote:
I would recommend you do a depth pot vs the fixed 220k that Dave puts in his amps, and the NFB resistor should be changed from the 2204 values to 47k off the 4 ohm tap.


I know the BE was updated to use the 4 ohm tap, but was the JJ? The one I had was before the 2016 BE revision came out, but it was using the 16 ohm tap.


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PostPosted: Thu, Nov 14, 2019 5:12am 
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technomancer wrote:
fusedbrain wrote:
I would recommend you do a depth pot vs the fixed 220k that Dave puts in his amps, and the NFB resistor should be changed from the 2204 values to 47k off the 4 ohm tap.


I know the BE was updated to use the 4 ohm tap, but was the JJ? The one I had was before the 2016 BE revision came out, but it was using the 16 ohm tap.

Not sure. The stock JJ could very well still be on the speaker jack like the old BE100's.
A stock 2203 has 100k off the 4ohm and that's not enough NFB for a Friedman build.
I'd stick with the Friedman 47k, and it's easy enough to just move the wire around on the impedance selector.
The JJ clone I built sounded best with the 47k NFB off the 4ohm, but YMMV.

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Marshall 1987xl (modded with my take on a Cali / CCV )
Mesa Triple Rec ( rev G )
Orange Rocker 30
Peters Vega-FSM
Vox AC30HW
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