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December 17, 2010

Plexi amps explained

Marshall Plexi - the meaning behind this term seems to make guitar players everywhere drool and bring out their life's savings. So what exactly is this meaning? Well, that is hard to explain in words of course, but I want to mention a few things that might help clarify what a Plexi amp is.

First of all, the name "Plexi", refers to the face plate and back plate, which were made by plexi glass, whereas the chassis were aluminum. The "Plexi era was from about 1965 to July 1969.

Plexi History

Marshall started with aluminium fronts in 1962, and for a period they also had plastic fronts. The plexi face plates came in 1965 and was replaced with brushed aluminium mid 1969.

The aluminum chassis were changed to steel chassis around 1967. Around that same time, tubes changed from KT66 to EL34.

The models in this period were:

JTM-45 (65-66), JTM-45/100 (late 65-66),
JTM "Black flag" or "reverse logo" (late 66-mid 67),
JMP (mid 67-mid 69)

JTM50 was the 50w 1963 Super PA (JTM MKIII)
JTM100 was the 100w 1968 Super PA (JTMV)
(The JTM MKIV was the tremoloversion of the JTM-45)

The super loud 200 watt amp - "The Pig" - was made only in '67, and were replaced with the Major in '68.

For the first year, the amps had plexi front panels and cream white back panels. Soon, the back panels also changed to plexi.

The Lead (50w, model 1987) and Super Lead (100w, model 1959), changed a lot after the Plexi area, and the metal face amps from the early 70's sounds very different.

So if an amp is made '65 - mid '69 and has a plexifront - it's a Plexi.

From Wikipedia:

Other early customers included Pete Townshend and John Entwistle of The Who, whose search for extra volume led Marshall to design the classic 100-watt valve amplifier. Ken Bran and Dudley Craven, Marshall's developers, doubled the number of output valves, added a larger power transformer and an extra output transformer. Four of these amplifiers were built and delivered to Pete Townshend, and the Marshall Super Lead Model 1959, the original Plexi, was born. At the request of Pete Townshend, Marshall produced an 8x12" cabinet on top of which the 1959 was placed, giving rise to the Marshall stack, an iconic image for rock and roll.

Plexi amps are LOUD

They work this way:

  1. A direct signal through the whole amp. No loop, no channel switch. Just the things that makes the guitar signal sound good.
  2. Distortion from preamp tubes, power tubes, and speakers. It is that distortion chain that makes that Plexi sound. The good sound will come when the volume is at 6 or more. Guess what? It's prett loud at that point. That's why you may need an attenuator, or else get used to being asked to TURN DOWN.

Where to get a real Plexi amp

Now, to get an amp like those old loud Plexi Marshalls, there are some builders who can help you out. Look no further than George Metropoulos from Metro Amps. These guys build faithful reproductions. View the video on their site, and you'll understand how devoted they are to building these amps.

Another place where you can learn more about vintage amps (Marshall included) is Vintageamps.com.

Video Demos

By Robert Renman - www.dolphinstreet.com

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Posted by Robert Renman on December 17, 2010

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