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Schem is attached below
What I'm doing basicaly is I want a Marshall 2204 (JCM800 50W single channel) sort of clone, with PPIMV and Resonance controls added.
The "Plexi/800" switch basically, in "800" position, we get the regular cascaded channels of the 800, with the "low-input" feeding into the gain control. In Plexi position, it parallels the two triodes (call it a serie/parallel switch if you will), for a "jumpered" plexi sort of vibe.
I'm using a dual-ganged gain pot, instead of a fixed voltage divider after the 2nd triode (which wouldn't work too well in parallel!).
The bigger decisions are related to the mixing resistor. On a Plexi, the two channels are mixed via 470k resistors. The 800 on the other hand drives the cathode-follower right from the voltage-divider after the 2nd gain stage. The voltage divider is a pair of 470k resistors, the first one paralleled with a 470p cap, a high-pass filter basically.
For the first triode, the solution is actually simple, when its output is switched to go straight to the cathode follower, the 470k resistor is added. However, before the gain pot is also a 470k/470p filter. That makes a lot of resistance before finally hitting the cathode follower. I've thought about maybe removing that filter altogether, or maybe using a smaller value for the resistor. I've put 220k on the schem, as this seem a common mod on 800s.
For the 2nd triode, what I tried is relocating the 470k/470p filter AFTER the gain pot. This is actually how its done on the Plexi (1987) so I guess it's ok.
I'm guessing a bit of tweaking will be used, but I'm hoping some more knowledgable people here can chime in and point me in the right direction.
NB: After drawing the schem, I've noticed that in "Plexi" mode, the low-input goes straight to the Cathode Follower (going through a gain pot first), wonder how that will sound
Thanks!
What I'm doing basicaly is I want a Marshall 2204 (JCM800 50W single channel) sort of clone, with PPIMV and Resonance controls added.
The "Plexi/800" switch basically, in "800" position, we get the regular cascaded channels of the 800, with the "low-input" feeding into the gain control. In Plexi position, it parallels the two triodes (call it a serie/parallel switch if you will), for a "jumpered" plexi sort of vibe.
I'm using a dual-ganged gain pot, instead of a fixed voltage divider after the 2nd triode (which wouldn't work too well in parallel!).
The bigger decisions are related to the mixing resistor. On a Plexi, the two channels are mixed via 470k resistors. The 800 on the other hand drives the cathode-follower right from the voltage-divider after the 2nd gain stage. The voltage divider is a pair of 470k resistors, the first one paralleled with a 470p cap, a high-pass filter basically.
For the first triode, the solution is actually simple, when its output is switched to go straight to the cathode follower, the 470k resistor is added. However, before the gain pot is also a 470k/470p filter. That makes a lot of resistance before finally hitting the cathode follower. I've thought about maybe removing that filter altogether, or maybe using a smaller value for the resistor. I've put 220k on the schem, as this seem a common mod on 800s.
For the 2nd triode, what I tried is relocating the 470k/470p filter AFTER the gain pot. This is actually how its done on the Plexi (1987) so I guess it's ok.
I'm guessing a bit of tweaking will be used, but I'm hoping some more knowledgable people here can chime in and point me in the right direction.
NB: After drawing the schem, I've noticed that in "Plexi" mode, the low-input goes straight to the Cathode Follower (going through a gain pot first), wonder how that will sound
Thanks!
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