Marshall JCM 2000 DSL 50 mods

Marshall Amp Discussion

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dpin
Posts: 21
Joined: Wed Sep 12, 2012 7:51 pm
Location: Pa.

Re: Marshall JCM 2000 DSL 50 mods

Post by dpin » Tue Sep 05, 2017 12:41 pm

If you plan on keeping the AU7 in the PI you might want to change out the shared cathode resistor to approx. 2.7k to keep it close to center biased. With stock values it's biased very hot. By center biasing you will allow the biggest input signal before clipping.

You can experiment with the mid cap or slope resistor values in the tonestack to move the mids around a bit.

I'm not following the schematic that pompeiisneaks posted as being accurate...It shows C12 as being a first stage bypass cap.
(Your description - Removed C12 on the "front board", which served as a bright-cap across the Red Channel's Gain pot
)
Things would be easier with an accurate schematic posted on the thread.

dpin
Posts: 21
Joined: Wed Sep 12, 2012 7:51 pm
Location: Pa.

Re: Marshall JCM 2000 DSL 50 mods

Post by dpin » Tue Sep 05, 2017 7:14 pm

I see now that the same designations appear on the 2 different pages. (i.e. C12 page 7 = Rk, C12 page 1 = bright cap)
And, since there is a separate tone stack for the red channel, you should be able to dial in the mids there, and not effect the green

dpin
Posts: 21
Joined: Wed Sep 12, 2012 7:51 pm
Location: Pa.

Re: Marshall JCM 2000 DSL 50 mods

Post by dpin » Tue Sep 05, 2017 7:56 pm

I brought this over for the sake of discussion.
If this is the actual schematic you should be able to dial in the mids here.
I chose this snippet because you mentioned working the C12 -bright cap off the red channel, so I am assuming this is the red channel
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dehughes
Posts: 1142
Joined: Thu Mar 23, 2006 2:29 pm
Location: Portland, OR

Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes » Wed Sep 06, 2017 6:27 pm

dpin, yep, that's the schematic and it correlates to my generation of PCB. Thanks.

Yeah, as I understand it, the "left area" on that section of the schematic is for the Green Channel whereas the right side (right of the relay....C10 on over) is for the Red Channel. If that's correct, then there appears to be a ton of circuitry on the Green Channel compared to the amps it's trying to emulate (1959, JCM 800, et al).

Regardless though, the Red Channel is what I'm needing to focus on. I did the Joey Mods but again think I need to move back toward stock, especially as I changed V1B's plate resistor, cathode resistor, and cathode cap to more "Marshally" values (see prior posts). I'm thinking those changes will manifest downstream, as V1B is shared by both channels, so a return to a more stock version of the schematic you posted might do the trick. I'll mess more this week/weekend as time allows, and post back.

That said, this really is a very good amp, and a great platform for simple, gradual modifications to one's taste.
Tempus edax rerum

dehughes
Posts: 1142
Joined: Thu Mar 23, 2006 2:29 pm
Location: Portland, OR

Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes » Wed Sep 13, 2017 3:37 am

I figured it out! The Red Channel is MUCH better now...prefer it over the Joey Mods by far.

Lemme figure out how to post a PDF and I'll share. Key was removing components just as much as it was changing the value of others....less is more. Only thing left is to increase the value of one of the voltage dividers and increase the value of the Red Channel filtering (thinking of going from 10uf to 22-ish uf).
Tempus edax rerum

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