Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PsychicFuzz on August 10, 2024, 06:15:54 pm
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Hey all
Been a while but I'm looking at a 1987x that has had a MV mod done to it. I've never worked on one of these so figured I'd consult the wizards that reside in this forum.
What are your thoughts on the quality of the work/mod?
All I really know is that the presence has been moved to the back. Not sure what the push pull is used for.
Any insights are greatly appreciated, thanks!
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I don't see a push pull.
Show us the schematic.
And better picture where go theses blues and whites wires.
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https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Plexi-Superlead-Reissue-50W-1987X-Schematic.pdf
I think that has the schematic, there's 2 different ones, the one that works with your board is the 2 bottom pages.
Looks like a type B PPIMV was added. Also, for some reason the 2 dropping resistors to the PI power supply were replaced with 1 resistor, maybe of the same value? 2 resistors were added to the cathodes of the power tubes for measuring bias. Also, the balance resistors for the heater supply have been changed for some reason, and at least one bypasses the trace on the board.
Also, it looks like there was some work done at the input jacks, maybe shielded cables were added? Doesn't really look like Anita's work.
It would appear all the blue resistors are mods or repairs of some kind. It seems like everything was done from the top of the board, which may or may not be a good thing. I also don't see a push pull switch, but maybe I'm missing something.
Does it work as it is? If so it's probably ok.
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https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Plexi-Superlead-Reissue-50W-1987X-Schematic.pdf
I think that has the schematic, there's 2 different ones, the one that works with your board is the 2 bottom pages.
Looks like a type B PPIMV was added. Also, for some reason the 2 dropping resistors to the PI power supply were replaced with 1 resistor, maybe of the same value? 2 resistors were added to the cathodes of the power tubes for measuring bias. Also, the balance resistors for the heater supply have been changed for some reason, and at least one bypasses the trace on the board.
Also, it looks like there was some work done at the input jacks, maybe shielded cables were added? Doesn't really look like Anita's work.
It would appear all the blue resistors are mods or repairs of some kind. It seems like everything was done from the top of the board, which may or may not be a good thing. I also don't see a push pull switch, but maybe I'm missing something.
Does it work as it is? If so it's probably ok.
Yep My apologies, not a push pull.
It’s an amp I was considering purchasing, the seller sent me those pics so I haven’t had the chance to play it. I think I’ll be most likely passing on it tho.
I’ve never done a MV mod or worked on a Marshall and wasn’t really sure what to look for so thanks for all the feedback! Very interesting.
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It looks like a Type 2 or "LarMar" master volume which utilized a Dual Ganged or Stereo pot, not a push/pull. It's a pot with one control shaft turning two dials stacked on top one another.
Upon further investigation, the Grid Leak resistors still look like they're on the board and not on the pot, meaning this is either a Frondelli or a RichMod, which leaves them on the board. Same result, different approach.
The installer wasn't a dummy but they were definitely lazy. Other the the poorly wrapped wires (important if not using shielded wires) and them not being secured or ziptied to something, it looks perfectly serviceable and adds great versatility to the amp especially if there are no cascading gain stages.
If it were me, I'd wrap the wires tighter and ziptie them together in the middle so they're not flapping around inside. Wires that move or vibrate contribute to noise, especially when carrying signal halfway across the amp.
The presence control is fine where it is at as it tends to be a set-and-forget control that is adjusted depending on the room you're in. You can leave it at 2 o'clock for most scenarios. More important to have the master volume out front where you will be using it far more instead of awkwardly reaching behind the amp head.