# Mod suggestions for Marshall 100W MKII Plexi Reissue?



## taskforcestudios (Dec 21, 2007)

hey guys,

i've got a late 90's reissue of a Marshall 100W MKII Plexi head and I was looking to do some mods to the amp. I actually really like the tone of this amp, but I know there could be some major improvements.

Is this circuit fairly accurate to the older Marshalls, or are there changes that need to be made to it?

I just feel like sometimes the tone is really muddy and the EQ and prescense aren't as responsive as they should be.

Right now I have the amp modded to have a master volume, which I like, but I'm more looking to improve the tone and EQing and maybe even get a little more gain out of it. Just need to really open up the sound of the amp.

Also, has anybody tried running KT88's in this amp before? I've herad its possible, but I'm not sure how crazy the mod is. I've switched my Bassman over to EL34s, and it wasn't too difficult, I was hoping this would be a similar kind of mod.

If you guys have any advice, comments or suggestions on how to do these things, please let me know it would be much appreciated.

Take care,
Brian


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## Wild Bill (May 3, 2006)

taskforcestudios said:


> hey guys,
> 
> i've got a late 90's reissue of a Marshall 100W MKII Plexi head and I was looking to do some mods to the amp. I actually really like the tone of this amp, but I know there could be some major improvements.
> 
> ...


The circuit is accurate. What you are experiencing is a common complaint of the original 100 watt Plexis! In the 'old days' it was commonly believed that the 50 watt Plexi had better tone, even though techs knew the circuit was the same, except for that necessary for the extra pair of output tubes.

What's REALLY happening is that the amp needs to be played LOUD! This is usually a problem and it more quickly becomes a problem with the more powerful amp.

Once again I'll repeat that you have to understand a guitar amp gets its distortion (TONE!) from two places, the preamp section and the output tubes. The two types of distortion are quite different. Preamp distortion is "crunchy" and a bit "fizzy". It is the more modern sound. Power tube distortion is the classic rock and roll tone from the Golden Years, i.e. the late 60's and early 70's. It is thick and warm, the way Randy Rhodes originally sounded.

Those vintage amps never had a master volume control. That means all the distortion came from the power output tubes. The amp was cranked up. Early PA's weren't as sophisticated as those of today so the guitar amp was responsible for most of the guitar's volume. Clubs were bigger and higher volume was the norm.

Master volumes came about in the late 70's. You can drive the preamp hard and then the master volume will cut the signal going into the power tubes. So the overall volume can be quite low. However, you pay a price. ALL your distortion will then come from the preamp! You only get power amp distortion when the output tubes are working hard. Turn the volume down and the power tubes go all clean and "hifi". 

It is impossible to get that classic rock tone just from the preamp. You can get a good tone (modern) but NOT the classic tone! You have to crank the volume up to get it, period and end of story! THERE ARE NO MODS THAT WILL DO IT FOR YOU!

You get power tube tone when the main volume is cranked up above 6 or 7, the more the better. This leads to a simple solution for playing small clubs. Get a smaller amp! Many times I've seen someone take a 50 or 100 watt amp into a small club, figuring they will just play at a much lower volume. Every time, they start cranking up the volume, bit by bit. They're not trying to be too loud! They're just trying to get some good tone! So they get themselves into trouble.

This is the main rationale for the genesis of all those 18 watt Marshall designs. A 20 watter on 8 will sound FAR better than a 50 watter on 3, if you're wanting power amp distortion! A carpenter has more than one size of hammer and a gigging guitarist needs more than one size of amp!

With your amp, if you leave the Master Volume on 10 and just crank the amp loud with the input volume your amp will give you that classic tone. Having the MV on 10 is like not having one at all, as far as the circuit is concerned. However, a 100 watt Marshall is LOUD! If you don't want to get a smaller amp for those smaller venues you might try a smaller speaker cab and some kind of "Hot PLate" attenuator to soak up the speaker power.

That's my story and I'm sticking to it!

WB


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## fraser (Feb 24, 2007)

say, if youre going to wind her up all the way, let us know. im only 8 kms away, ill be able to hear you if i go sit on the porch.


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## parkhead (Aug 14, 2009)

Marshall Hand Wired 1959-HW EricJohnsonTourAmpCOA Clean - eBay (item 140444188710 end time Sep-22-10 09:22:05 PDT)


have the amp recapped, replacing the 50/50 cans with 32/32 types 
this will increase the saturated quailty of the wide open distortion and allow it to happen at relativly lower volumes 

this can be done in two stages the EJ amp has all caps replaced 
You can also leave the main filters as 50/50 for maximum volume and bottom end 
replacing the pre amp, screens and inverter caps with 32's will make the amp softer in the pre amp but leave the big power 100 watter sound 

all 32 can caps is closer to the 66 -67 clapton Hendrix Van Halen* sound than the later amps with 50/50's 


In the ebay auction you see Eric Johnson's touring reissue and clearly he has had the 50's replaced with 32's 

there are other mods but I would try that First If you already almost Like the sound of the amp 

next I would remove the 5k bright cap on the bright volume control (the one that makes it full volume on 2) 

then 1/2 the feedback resistor value from 47k down to 27k 

finally I would convert V1 back to shared cathode 

as you move through these changes you are converting the amp back to an earlier style plexi 

stop when you get to the "year" you like 

each change except the first make it more bassy vs pure mid cut 

*
If you've seen the HOM Doyle book they describe EVH's magic plexi 
as the Earlier type with the caps inside the chassis vs on the chassis like the re-issue 

those in the know realized instantly that this means the EVH amp has the earlier lower filtering values 
making it more SQUISHY ... 

IMHO the EVH amp is in the transitional crossover point between early low filtered power supply and 
fully the mid range oriented 68-69 lead pre amp
they made very few amps like this 

They would have maximum distortion due to the agressive pre amp design and the limited power supply filtering 
making the amp grunt and groan with the players touch 

I believe most people will find what they are looking for in classic Marshall sounds by starting with the filters 
and backdating stepwise from there ........

You local tech will be happy to take you back to 66 

http://www.youtube.com/watch?v=Mr9IksFJutk

and this should be that reissue Plexi in action 

http://www.youtube.com/watch?v=0IUoSTaD7aw


P


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## Emohawk (Feb 3, 2006)

Just a side note...Didn't RR have his plexi's modded to cascade the two input channels for more gain in the front end?


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## parkhead (Aug 14, 2009)

yes the RR mod is called the 
One wire mod 

do a search 

its crude but effective 
p


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