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Old 01-25-2013, 02:24 AM   #1
loop-de-luke
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Value of 90's Marshall 1959SLP "plexi" head? Should I get a Master Volume mod?

Had my Marshall 1959SLP "plexi" reissue for a couple of years now and I love the shit out of it. I took it in to get serviced and re-tubed as it was playing up a bit, while it was in getting fixed I borrow a mate's amp head. He got given it by one of his friends who had no idea what it was... infact, neither of them did. It was a half-bare wood box with no logo, most of the covering and typical Marshall parts torn off it. One look at the back of this haggard beast and it turned out to be a 1974 Marshall SLP!! Needless to say I'm offering the guy a couple of hundred bucks to "take it off his hands"

Anyway, so I played this thing and it sounds magnificent. It also has a Master Volume knob unlike mine... which proved very very useful. Will this kind of mod devalue my 90's model SLP? I'm wary of modding mine as the guy who totally ripped the '74 one to shreds did with his.

Also, what would a mint condition 90's model Marshall SLP be worth with no mods?
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Old 01-25-2013, 02:57 AM   #2
trashedlostfdup
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bottom line if you value the resale value of the head (in $$$) you wouldn't mod anything.

modding pretty much no matter what it is even if its a good mod lowers resale value.
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Old 01-25-2013, 03:04 AM   #3
loop-de-luke
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Quote:
Originally Posted by trashedlostfdup
bottom line if you value the resale value of the head (in $$$) you wouldn't mod anything.

modding pretty much no matter what it is even if its a good mod lowers resale value.

Yeah I thought that... I can't see myself ever selling the amp though, was just curious. I don't even know how much it is worth... A MV mod would be useful though
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Old 01-25-2013, 03:16 AM   #4
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Well, if you think a MV would benefit you, then do it. I wouldn't base my decisions off of the resale value of my amp if I wasn't planning on selling it. Why sacrifice the sound you want for an event that may not ever even happen? I mean, you'll have to realize when you sell it that it will lose a little value, but then again, you may find someone that would pay a little more for it just because they were going to have the same mod done. I've seen that stuff go both ways, especially for such a common amp mod.

At the end of the day, though, if you want it, get it.
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Old 01-25-2013, 03:24 AM   #5
trashedlostfdup
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Quote:
Originally Posted by loop-de-luke
Yeah I thought that... I can't see myself ever selling the amp though, was just curious. I don't even know how much it is worth... A MV mod would be useful though


its really comes down to making a decision. but i can definitely see the value in terms of usage of a MV.

i don't know much about attenuators overall, but this could be one situation where they could be beneficial. and if it worked that way, your amp would still be stock.

however even attentuated to really bake the tubes it would still be loud.
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Old 01-25-2013, 05:09 AM   #6
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Thanks for the advice guys! I was only really being cautious, just incase in 10 years time I go mental and decide to become an accountant and give up music forever... but you're right, I can imagine a lot of people would get this mod done. I jump the channels on it, which makes it a pain in the arse to get consistent tone at different volume levels, as I'm using both volume controls for different tone mix...
If I was to get a MV mod done, is that a straightforward process? Would any amp servicer/repairer be able to do it?

Does anyone know the average value at the moment of an early 90's model SLP in mint condition? I can't find much on eBay for a solid value.
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Old 01-25-2013, 05:29 AM   #7
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I'm not a fan of the Plexi sound but from modding articles I've read, it seems that such a mod will actually change your tone a little.
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Old 01-25-2013, 06:21 AM   #8
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If you sacrifice an input jack for the MV then it's a simple matter to return it to stock if you ever decide to sell it. This will be a PCB based reissue, no? In which case you can remove the coupling cap on the board following the wiper of the treble pot and use those two doughnuts as your break in point.
ie pot side donut to the top of the 1MA volume pot via a .02uF capacitor mounted in one hole with the other end to the top of the pot. Wiper to the other donut (ie the phase inverter input.) Bottom of pot to ground.
To change back to stock remove the pot, solder the cap back in place and refit the input plug.
As for the input jacks, I don't know how they mount on the reissue. The one to sacrifice would be the bottom right one. Hopefully they are chassis mount and then it's easy.

Not a big deal really.


You can even do a post phase inverter master volume (PPIMV) if you like. You just pull out the two 220K resistors and insert a stereo 220K log pot. You can be sneaky there too with the circuit board. You remove the two .1 uF caps and solder a new one in leaving one leg unconnected and use that as a terminal to wire in the top of the pot.
Wire the wiper of your PPIMV pot to the other donut. Make sure the coupling cap is connected to the phase inverter side on the board - otherwise you're extending almost 400V to the front panel. Bottom of pot to gnd.
Again too easy to revert back to stock. You just replace with the old components. You've cut nothing and drilled nothing.


Edit: So you can see what I'm talking about:
http://www.drtube.com/schematics/marshall/1959t-66.gif
The phase inverter is V4
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Old 01-25-2013, 05:18 PM   #9
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You could always get a power attenuator and use that as your master volume control. That way, you don't have to alter an expensive amplifier. The power attenuator would probably cost less than the modification, too.
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Old 01-25-2013, 05:52 PM   #10
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Plexis aren't good for much. I'd pass it off on some old fart and get a JCM800 or JMP2203 instead.
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Old 01-26-2013, 05:11 AM   #11
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Quote:
Originally Posted by FatalGear41
You could always get a power attenuator and use that as your master volume control. That way, you don't have to alter an expensive amplifier. The power attenuator would probably cost less than the modification, too.


Well, in short, no.

Any half-decent attenuator will cost more than having this done by someone, and any attenuator will cost more than doing it yourself.

Also, attenuators are known for cutting tone. The more you attenuate, the worse it gets. Even the best and most expensive will still do that.

If you go by the instructions Cath listed, then there is no risk. However, unless you know what you're doing and are comfortable opening your amp and doing the mod yourself, have a tech do it. There are lethal voltages stored in your amplifier, even when it's been unplugged. If you don't know what you're doing in there, stay out.
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