68 Plexi Refurbish

[plexified]

Phil , your playing has always been inspiring and the tones always in the zone . Here I feel you might want to think about the bass response . I need to hear a good solid dry 'THUMP' with a palm mute . I hear a gloss over of this without grabbing it with authority . The highs are right there and the mids are too , even to debate the mid pot is foolish without the thump . A Marshall cab has that thump and if you say you are up to volume , move that mike because it is not catching it . Back that mike up , way back that way you can hear it . I always like using different mikes , screw that sm-57 crap . Its like an ear . Think of it that way and everyone has different ears , that allows you to use different mikes in different positions . That's where all the magic happens away from your hands . Whats in the room ? a close mike says 'I don't know' , why ? because its strapped to the source volume . Got it . Now get it maestro . Cost for troubleshooting , free . Yeah baby , we are back on track to affording the vh tone to all . Now we can focus on technique, style and bringing the sunshine or the new inspiration into everyones arsenal . Then we all have better music to listen to . Not like the junk we have to bear . My point ends with this note . Collaboration . Its a concept to share artists workboards . Leading to even more unbelievable results . Imagine If I could jam with Kanye or who knows like Rob Zombie or be on That Metal Show as a guest . NOW we are talking .Eddie Trunk I am sending you clips , I am main stage , although its only like six feet . Better six feet over than under , right? I'm ON FIRE!

[Strat78]

I grabbed a couple of good thumps on the E string during the bridge, the E string tone and feel is my favorite part of that clip. I could adjust the bass, move mics around, spend more time recording but you can hear what the amp is doing without all that. This amps has got it. The original plexi tranny's just can't be duplicated. To me this is a perfect example of what distinguishes a 12xxx from other amps: clarity, with a nice compression and sag when you dig in, aggressive with good note separation and sweet and tight low end without sounding too metal. That is the PT doing what it's supposed to do there. The NOS 50k pot I put in effects the bass pot more than anything else, but the 50k has a nice sweet spot that a 25k won't deliver. No screechy pre-amp mod type tone or polystyrene fatiguing high gain 5150 sandblasters, just a beautiful organic gain with a nice woody response on the wound strings.

[guitar007]

Are you using a CTS or a PEC?

[Strat78]

Are you using a CTS or a PEC?

The amp has all original CTS type RS pots. I switched out the original 25k mid pot for a 50k that is a NOS CTS that looks pretty much identical to the old RS pots. Not sure I like the 50k PEC pots now after trying these:
http://www.ebay.com/itm/1-50k-Audio-NOS ... 43d4f4d190

[Tazin]

Interesting that you went with a audio (log) pot for the 50K. Perhaps the taper (audio, log) is what's creating the sweet spot your talking about....That coupled with the fact that it is a 50K. Marshall did use both audio (log) and linear taper 25K pots for the Middle control....The same holds true for the 250K Treble pot.

[guitar007]

Gotcha. I picked up a few. Thanks for the tip, Strat.

[Strat78]

Here are some pics of the amp restored.

[Tazin]

I've been waiting for someone to notice this screw up from the factory regarding this particular amp and some of the others built just like this (I've mentioned them before). If you look closely you'll notice the H.T. (B+) wiring for the preamp is incorrect. The B+ voltage is not even going through the last 10K/1w dropping resistor to feed the 100K plate resistors for V1. And as a result of this screwed up wiring some of the amps have 1/2 of the preamp filter cap wired wrong. Obviously when the new board layout was adopted someone at the factory didn't pay attention in class and wired a handful of these Super Leads incorrectly. The link wire from the V1 100K plate resistors should go to the other end of the 10K/1w B+ dropping resistor (near the pots) and where applicable the red wire that feeds the 1/2 of the preamp filter cap should be moved over to where the new link wire connects (move it over to the other 10K/1w resistor). This way the H.T. voltage goes through the last 10K/1w resistor and the filtering is in the correct spot.

EDIT: damn grammer!

[neikeel]

I've been waiting for someone to notice this screw up from the factory regarding this particular amp and some of the others built just like this (I've mentioned them before). If you look closely you'll notice the H.T. (B+) wiring for the preamp is incorrect. The B+ voltage is not even going through the last 10K/1w dropping resistor to feed the 100K plate resistors for V1. And as a result of this screwed up wiring some of the amps have 1/2 of the preamp filter cap wired wrong. Obviously when the new board layout was adopted someone at the factory didn't pay attention in class a wired a handful of these Super Leads incorrectly. The link wire from the V1 100K plate resistors should go to the other end of the 10K/1w B+ dropping resistor (near the pots) and where applicable the red wire that feeds the 1/2 of the preamp filter cap should be moved over to where the new link wire connects (move it over to the other 10K/1w resistor). This way the H.T. voltage goes through the last 10K/1w resistor and the filtering is in the correct spot.

So this begs the question, wire it to how it should be or keep is as and example of mistake (like the stamp collectors with the error worth thousands......... )

p.s. well spotted

[guitar007]

Pick the one that sounds better.

[Strat78]

Yep, I was scratching my head over that one. You can see what it looked like before:
http://forum.metroamp.com/viewtopic.php?f=4&t=43768
The filter cap is wired correct but the jumper is on the wrong side, but both sides of the filter cap are still being used, it's just V1 that is not getting that second 10k dropper. I wired the filter the same way as it was originally but under the board and forgot all about it. Below that pic there is another pic of a transitional plexi but the filter is wired over the first 10k. Who knows whats on the other end of that second 10k, probably nothing. Somehow I guess it all works, but I'll check with Steven and see if he would like to changed this. Still, like Neil said, probably should keep it historical. I should run the hook up wire back through the board as well just to keep original. Butt it looks so good with everything underneath.

[Tazin]

The way these amps are hooked up they will still work fine....Just not quite as intented circuit wise. The result is a higher preamp voltage to the plates of V1, and in some cases a little different filtering of the H.T. line. If it was my amp, I'd leave it the way it was from the factory (with the factory goof up).

[Tazin]

I did a little checking and it looks like the vast majority of these transistional style plexi amps; the ones with the two filter caps mounted top side; have the H.T. wired up incorrectly. This includes the Super Lead, Super Bass, Super Tremolo, and Super P.A. models. There are also some 50 watt amps wired up this way too.

[Strat78]

I did a little checking and it looks like the vast majority of these transistional style plexi amps; the ones with the two filter caps mounted top side; have the H.T. wired up incorrectly. This includes the Super Lead, Super Bass, Super Tremolo, and Super P.A. models. There are also some 50 watt amps wired up this way too.

Thanks for figuring this out. Perhaps Amp Archives would be interested in adding this catagory to the Marshall section.

[neikeel]

Of course the extra dropper will make the preamp a bit browner