gelsmander
New Member
- Joined
- Oct 26, 2022
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First time poster here. I recently got a 1974 JMP 50 lead and I have some questions about a mod done to it. It’s a European model with no voltage selection and EL34s. I just retubed and biased it with TAD in the preamp and JJ power tubes. The amp is mostly in original shape with a few changes I was able to identify.
The power cord has been replaced with a socket and the impedance selector has been changed to a rotating (and slightly flimsy feeling one). Other than these, I see two changes on the pcb: the leftmost cap (blue vertical one) in the picture doesn’t look original and the 500pF cap is missing altogether (edit: next to the ’Marshall’ text on the pcb). The 500pF one is the one I’m interested in. What is the function of this cap and why would someone remove it? I’m wondering if I should replace it or not. Any insight on this?
The amp sounds pretty great, but I’d like to know how ”normal” my description seems. Bright input is definitely bright and gainy, already breaking up quite a bit with the volume on 2 or so (Les Paul with Tim Shaw & T-top). After that the volume doesn’t increase much but the gain does. Input 2 on the other hand is everything but bright: no high end whatsoever and an insane amount of low end on tap. The volume is significantly lower than input 1 on the same setting. Input 2 is totally useless on its own, but can be used to add low end when bridging the inputs. I’ve understood that this is somewhat how these work normally.
I’ve been thinking of playing with bright cap values on both inputs. Now they are original, so no cap in input 2 and whatever came in input 1 originally. I’d like to make input one slightly less bright and gainy, and input 2 a bit more bright, or just keep them as is. That’s why I’m wondering about the missing 500pF cap and its possible effect on the preamp. Should I try replacing it? Thanks in advance!
The power cord has been replaced with a socket and the impedance selector has been changed to a rotating (and slightly flimsy feeling one). Other than these, I see two changes on the pcb: the leftmost cap (blue vertical one) in the picture doesn’t look original and the 500pF cap is missing altogether (edit: next to the ’Marshall’ text on the pcb). The 500pF one is the one I’m interested in. What is the function of this cap and why would someone remove it? I’m wondering if I should replace it or not. Any insight on this?
The amp sounds pretty great, but I’d like to know how ”normal” my description seems. Bright input is definitely bright and gainy, already breaking up quite a bit with the volume on 2 or so (Les Paul with Tim Shaw & T-top). After that the volume doesn’t increase much but the gain does. Input 2 on the other hand is everything but bright: no high end whatsoever and an insane amount of low end on tap. The volume is significantly lower than input 1 on the same setting. Input 2 is totally useless on its own, but can be used to add low end when bridging the inputs. I’ve understood that this is somewhat how these work normally.
I’ve been thinking of playing with bright cap values on both inputs. Now they are original, so no cap in input 2 and whatever came in input 1 originally. I’d like to make input one slightly less bright and gainy, and input 2 a bit more bright, or just keep them as is. That’s why I’m wondering about the missing 500pF cap and its possible effect on the preamp. Should I try replacing it? Thanks in advance!
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