1987x mods - PPIMV and one-wire cascade gain - please help!

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1987x mods - PPIMV and one-wire cascade gain - please help!

Postby Greg_L on Wed May 03, 2017 5:42 pm

Okay, I got this 2001 1987x here. No effects loop. Basic 50w Plexi reissue. Cool.
Image



I want to add a PPIMV so I can then try the "one wire" cascaded gain mod...and hopefully make it switchable somehow.

I will need help. I've worked on amps before with guidance. I know the safety protocols, how to solder, etc. I do not know the parts, names of parts, what they do, or why they work, but if you guys can show me the way, I can do it.

So first up, the PPIMV. I have a Triode PPIMV kit.
First question...what resistors do I need to remove? I think they're the one's I've circled in yellow. Please advise.
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Second question...in that same pic above ^^^^ I've circled in red the wires I need to remove from the power tube pins. I've researched to find that people install 5.6k "swamp resistors" in those spots and then the leads to the PPIMV pot. Is this correct? Are swamp resistors necessary? My kit does not include them and they are not on the amp stock. I can get some though, no problem. What do they do, and do I need them?

Please help me with these two starter questions so I can get this thing going. I've done my research and searched threads all over the internet and think I have it figured out, but some personalized guidance would be appreciated.

Thanks.
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Re: 1987x mods - PPIMV and one-wire cascade gain - please he

Postby pdf64 on Thu May 04, 2017 9:40 pm

Yes, those 2 resistors you circled (R25 and 26) are the bias feed resistors for the power tubes, which a Type 2 / LarMar master volume would replace, see p32 https://robrobinette.com/The_Trainwreck_Pages.htm
Swamp resistors / grid stoppers (mounted at the tube socket terminal) are good practice, as they increase the margin of stability, typically a weak point for Marshalls. They help to prevent the amp oscillating, especially at high control settings (oscillation may be above audible frequencies). The amp may (or may not) be fine without them at the moment, but even just a just a tube change might alter that. As could installing a master volume.
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Re: 1987x mods - PPIMV and one-wire cascade gain - please he

Postby Greg_L on Thu May 04, 2017 11:29 pm

pdf64 wrote:Yes, those 2 resistors you circled (R25 and 26) are the bias feed resistors for the power tubes, which a Type 2 / LarMar master volume would replace, see p32 https://robrobinette.com/The_Trainwreck_Pages.htm
Swamp resistors / grid stoppers (mounted at the tube socket terminal) are good practice, as they increase the margin of stability, typically a weak point for Marshalls. They help to prevent the amp oscillating, especially at high control settings (oscillation may be above audible frequencies). The amp may (or may not) be fine without them at the moment, but even just a just a tube change might alter that. As could installing a master volume.

Hey thanks for the reply.

I finished the PPIMV mod today - including swamp resistors - and it worked out great. And maybe it's just confirmation bias, but I do think it actually sounds better and maybe that's the swamp resistors helping out. They weren't in it before.

Anyway, now I wanna try the one-wire/cascade mod. I tried a version of it today and it worked, but it made the amp basically useless unless you want shredder gain all over the place.

If there's a way to make it switchable, I'm interested. Anyone got any ideas?
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