1974 JMP 50 with removed 500pF cap

gelsmander

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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.

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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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Pete Farrington

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What is the function of this cap and why would someone remove it?
Its action is to exacerbate the characteristics you describe below.
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.
 

gelsmander

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Its action is to exacerbate the characteristics you describe below.
Just to be clear, you’re saying that putting the cap back will be a step in the wrong direction based on my description? Making the two inputs even more uneven?
 

gelsmander

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The amp seems to be missing control grid stoppers and screen grid resistors, on its output valves.
I suppose these too were removed afterwards and should have originally been in place? Is it a common mod to remove these and why?
It’s starting to look like I might want to take the amp to a qualified tech to have it returned to original specs and start from there.
 
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Pete Farrington

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The amp seems to be missing control grid stoppers and screen grid resistors, on its output valves.
That’s the stock arrangement with earlier 50 watters.
The lack of screen grid resistors puts significant stress on the output valves’ screen grids when overdriven. Mullards could take it, later valves not so much.
The lack of control grid stoppers on the output valves will make the amp more liable to high frequency oscillation.

 

Spanngitter

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Please have the Tech also check where the Bias Supply branches off, you want to have it preferrably in front of the Standby so Bias Voltage is still supplied, even with Standby active.
 

gelsmander

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those can caps look out of place too
Yeah forgot to mention, these are replaced for sure.

Please have the Tech also check where the Bias Supply branches off, you want to have it preferrably in front of the Standby so Bias Voltage is still supplied, even with Standby active.
Is the bias supply the third (brown) wire coming from the OT (the other two going to power tubes' pin 3)? If so, that third one is split before the standby switch so that one end goes to the switch and the other end to the pcb (or at least under it, can't check at the moment).

That’s the stock arrangement with earlier 50 watters.
The lack of screen grid resistors puts significant stress on the output valves’ screen grids when overdriven. Mullards could take it, later valves not so much.
The lack of control grid stoppers on the output valves will make the amp more liable to high frequency oscillation.
Stock arrangement did not have these in place, is what you're saying? Either way, sounds like I might want to have them there. I did already encounter some high-pitched oscillation on input 1.

I guess as the amp is not 100% stock anyways and will never be, I'd rather make it the best sounding and functional amp rather than go for full original specs. Keeping it as original as possible, but fixing some of these more apparent things. What's the consensus: leave 500pF cap out, add screen grid resistors and grid stoppers, check the bias supply thing, maybe play with different bright caps on the inputs? Thanks for all the help so far!
 

gelsmander

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Btw these are the current can caps. Haven’t seen ones like this before, any insight on these?
 

AndyD

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Hi, are you sure this is a ‘74? It seems odd that it has a 60s style power transformer When a stand up transformer would be right for the year. Is there a serial number on the rear faceplate?
 

Spanngitter

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Is the bias supply the third (brown) wire coming from the OT (the other two going to power tubes' pin 3)? If so, that third one is split before the standby switch so that one end goes to the switch and the other end to the pcb (or at least under it, can't check at the moment).
Nope, that's the OT Center Tap carrying full B+....the Bias Supply is normally a white wire from the PCB to the Standby and it needs to be on the side wich is the feed and therefore unswitched. This way it prevents that the Powertubes go full tilt when Standby is flicked, possibly ruining the power transformer....
For the Powersupply:
These are F&Ts from ´91, a quite high quality product, however I would consider them worth a replacement anyway due to age.
Since F&T has also the blue 50+50uF now in production you can revert the amp to a more original look (just need to buy Cap Clamps as well) or you remain with the scre in Version, those are also still made and available by F&T.
 

gelsmander

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These are F&Ts from ´91, a quite high quality product, however I would consider them worth a replacement anyway due to age.
Since F&T has also the blue 50+50uF now in production you can revert the amp to a more original look (just need to buy Cap Clamps as well) or you remain with the scre in Version, those are also still made and available by F&T.
Thanks for the insight, maybe it's best to update the caps while at it. I still have to check the bias supply situation.
 

gelsmander

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Small update: what I will do is replace the 500pF, new can caps (F&T) + clamps, new bias caps, remove the bright cap (and try other values if I feel like it), replace the impedance selector (which did work but was feeling super flimsy), clean all jacks, pots and tube sockets etc. I already started and am still waiting for some of the components, but everything seems doable. I built my first filter cap drain tool (à la Mojotone) and drained the old caps, which were indeed still carrying significant volts.

I checked the transformer codes and they are definitely all original Drakes (1202-113 PT, 784-139 OT, C2542 choke). It seems that this PT is common in the Scandinavian models of ’74.
 

gelsmander

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Done! I replaced the can caps and bias caps (all F&T), put the 500pF back on, replaced the impedance switch, installed screen grid resistors and grid stoppers, and rebiased. I also replaced the ~5000pF bright cap with 120pF and put a 470pF on input 2.

Overall there’s a good improvement in tone. Reduced hum and no more squealing feedback. The inputs are more even now with still having their own characteristics. I still might try a bigger bright cap on input 1, but this is much better already. The amp also takes pedals much better.

Thanks for all the help and input. I’m glad I ended up doing this on my own as I got a lot of understanding of the amp and its circuit.

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Pete Farrington

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put a 470pF on input 2
If you don’t want that channel to sound dull, it seems perverse to revert to fitting the 500pF cap across channel 1’s 470k mixing resistor.
The channels will inherently sound different whatever, due to the differing arrangements at their cathodes.
 

gelsmander

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If you don’t want that channel to sound dull, it seems perverse to revert to fitting the 500pF cap across channel 1’s 470k mixing resistor.
I guess so. Don’t get me wrong, input 2 is still dull and bassy and pretty much unusable on its own, and I’d rather have the circuit as close to original as possible, doing necessary tweaks with the bright caps.

Before, I had the treble and presence knobs all the way down and now they’re maxed, but I feel like being more in control of the tone now. We’ll see if I go back to the original bright cap values, but those are easy and quick fixes with significant impact on tone.
 

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