Marshall Internal Jumper Mod

calieng

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I was working on a JTM45 kit from Mojotone and thought I would post a quick photo of the super simple internal jumper mod so you no longer need the external patch cord to blend the channels on vintage Marshall style circuits.

Just add a jumper wire between pin 2 and pin 7 on the first preamp tube.

I also did this on my Suhr SL67 so it can work on other brand amps if they followed the original circuit.

I am sure most people know how to do this but for those who did not just passing it along.

As always have an experienced amplifier tech do any work on your amps unless you have some good experience already. The internal capacitors can give you a very bad shock if not careful.


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ARandall

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Is this a pic of your build???

I'd have done all the heater wires long before trying to wire any other parts of the sockets. In fact its best done as one of the first steps in the chassis fullstop. Gets a lot of the pesky PT wires out of the way as well as not having chassis mounted components in the way.
 

calieng

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Is this a pic of your build???

I'd have done all the heater wires long before trying to wire any other parts of the sockets. In fact its best done as one of the first steps in the chassis fullstop. Gets a lot of the pesky PT wires out of the way as well as not having chassis mounted components in the way.

I have gone either way on that..heaters first or heaters last. On this particular one the heaters will be up vertical above the sockets with an air gap from any other leads and not resting against the chassis floor. I do not have any trouble wiring them last. I am moving a bunch of components around to reduce leads and be more PTP so I did not want the heaters in the way. I probably could just take out the turret board and use terminal strips instead but I am already half way along. Maybe next time. But yes I leave the speaker jacks and impedance selector switch off until last to make the socket wiring easier.

The PT area with the two cap cans on the left inside of the chassis sure is a tight squeeze on this particular design (Mojotone offset JTM45). That wiring I did already. But I would actually mount those cap cans elsewhere in hindsight.

Thanks.
 

ARandall

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Does that kit come with cloth pushback wire??
I've got the brown vibroverb in the works at the moment, and of course that does have cloth pushback being vintage accurate for that amp. But also has the heaters being above the tubes rather than offset/parallel to the chassis floor.
I mean, its easy enough with the floor routing to keep it in place if the wire is plastic coated, but I'm hoping the 'bending memory' that the cloth wire has will keep it far enough away and staying as placed once I get there myself.
 

calieng

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Does that kit come with cloth pushback wire??
I've got the brown vibroverb in the works at the moment, and of course that does have cloth pushback being vintage accurate for that amp. But also has the heaters being above the tubes rather than offset/parallel to the chassis floor.
I mean, its easy enough with the floor routing to keep it in place if the wire is plastic coated, but I'm hoping the 'bending memory' that the cloth wire has will keep it far enough away and staying as placed once I get there myself.

No it came with modern wire but I am using some spools of 20AWG solid core that I have for the leads as you can form it in place how you like and it does not move around. Cloth wire may have been nice but the amp will not be vintage accurate anyway as I am doing a master volume mod and some other minor changes.
 

The Ballzz

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@calieng
If YOU like what that "jumper" mod does that's fine, but it is not the same as the multiple ways to use the four jack system. I will not bother with a long dissertation, as Rob Robinette has already done it in much greater depth and detail than I could ever provide. Sadly, his amazing site is currently down for some maintenance, otherwise I'd provide a link!
Just My $.02,
Gene
 
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The Ballzz

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Well @calieng ,
Robs site is back up and running! :cool: Here's a link that should be fairly enlightening. Bear in mind that even though the heading mentions Fender jacks, the Marshall 4-hole system is identical, except that many use Cliff UK jacks instead of Switchcraft style. Allother components and functions are identical:


Worth The Read,
Gene
 

calieng

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Well @calieng ,
Robs site is back up and running! :cool: Here's a link that should be fairly enlightening. Bear in mind that even though the heading mentions Fender jacks, the Marshall 4-hole system is identical, except that many use Cliff UK jacks instead of Switchcraft style. Allother components and functions are identical:


Worth The Read,
Gene

OK. Thanks for the link. I will check it out.
 

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