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From 20th Century Guitar Magazine Anniversary Article By Mitch Colby
Terminology
Plexi
A "plexi" Marshall is one from around 1965 through the middle of 1969. What distinguishes "plexi" Marshalls from others is the material that the front (and sometimes back) panel is made from. The "plexi" panel is a plastic called "perspex" that is clear with lettering and gold on the back surface. It is mounted to the front of the amp chassis and becomes the front panel that the jacks and pots are mounted to. The amps before the plexi models had either white panels or on some, lettering was screened to the aluminum chassis. There are some JTM45s and JTM45/100 (the first production 100 watt heads) amps that have plexi front panels and a cream colored back panels. Plexi does not mean see-through.
Bluesbreaker
The term "Marshall Bluesbreaker" (when it comes to vintage amps) was coined for the amp used by Eric Clapton on the album "John Mayall and the Bluesbreakers". The amp is pictured on the back of the album cover and is the 1965/6 version of the model 1962. The 1962 was a JTM45 (model 2245) with tremolo in a 2x12 combo. The speakers were 20 watt Celestion Greenbacks. Since these amps have become highly collectable, other Marshall combos have been dubbed Bluesbreakers as well. In my mind, the only true Bluesbreaker combos are those that have the same amp chassis (JTM45 with tremolo) including the first version which had a slightly different cosmetic configuration. In the earlier version , the wood sides were thicker, they had two Vox style brass vents and 15 watt AlNiCo speakers. Included in the Bluesbreaker category would be the 2x12 and 4x10 combos from 1964 through about 1966. The front of these amps had the split look with vinyl on the top and pinstripe grill cloth on the bottom. Later combos used the 50 watt chassis that had EL34’s rather than the KT66’s used in the JTM45. Most of these later amps had basketweave grill cloth on the entire front of the amp. An even rarer version of the later combos had vinyl on the top and salt and pepper grill cloth on the bottom. While both of these 50 watt combos are fine amps, and actually preferred by many because they sound more like "true" Marshalls, they are not Bluesbreakers. The biggest "stretch" of the Bluesbreaker name is its use for the 10, 18 and 20 watt combos of the 60's and early 70's. These amps came in all the cosmetic configurations described above and came in 1x12, 2x10 and 2x12 versions. Reverb was an option although very rare. The 18 watt amps sound far superior than the 10 or 20 watters.
The other "Bluesbreaker" products are Overdrive pedals. They are the "Bluesbreaker" and the "Bluesbreaker II".
Model Numbers
Many of Marshall's model numbers start with "19" such as 1959, 1987, 1962, etc. These are model numbers only and do not represent a date. So the amp with the model number 1959 is the 100 watt Super Lead. Manufacturing on this amp did not start in 1959 since Marshall started making amps in 1962!
JTM
Stands for Jim and Terry Marshall, Terry being Jim's son.
JCM800
JCM800 is a name for a range of amplifiers, not a single amp. The JCM800 range included both Master Volume (2203 and 2204) and Split Channel Reverb (2210 and 2205) heads as well as a variety of combos. The Split Channel Reverb amps far outsold the Master Volume ones but the Master Volume ones are usually the amps people refer to when they say JCM800 because they were preferred by touring bands.
JCM900
Similar to the above situation in that it is a name for a range of amps, not a single model.
JCM2000
Ditto
Hand Wired
There are various techniques of wiring that are considered hand wired. Each has a varying degree of hand work. A totally hand wired amp has it’s components mounted either on rails (with or without a board and from part to part), on a board with turrets (early Marshall), on a board with eyelets. Some amps that have a printed circuit board are partially hand wired with hand wired pots, switches, tube sockets and jacks that are mounted to the chassis.
Point to Point wiring
True point to point (sometimes abbreviated "P2P") wiring uses rails without any circuit board. The components are basically mounted in air. A board that is used with either turrets or eyelets is also wired by hand but is different than rails in that they will have some effect on the sound of the amp. I do not make a judgment on whether the effect is positive or negative because that depends on whether the designer took this effect into account when designing the product.
Myths
The JTM45 is a 45 watt amp
Actually, the JTM45 puts out about 30 to 35 watts of clean power. JTM45's will however, put out 45 watts into full distortion.
British watts are different than American watts
This is an interesting situation. About 25 years ago, Rose Morris (the worldwide Marshall distributor) and Unicord (the US Marshall distributor), put out ads that said something like "the reason Marshall's are louder is that we use British watts which are louder than American watts". The way that this came about is that in England, the people in charge of standards chose RMS watts as the way to measure power. At that time, in the US, there were different ways that companies could use to measure power. They were RMS, peak power and I??. Peak power and I?? allowed the companies to rate their amps with higher numbers while the amps put out less power than amps with a similar power rating using RMS as the standard. What confirmed this situation to me is that I recently saw old Goodmans speakers that had two ratings, one in British watts and the other in American watts where the American rating was twice the British rating.