Checking against other amps from the era as well as component markings speakers, potentiometers, transformers can help to confirm a year of production. You must be logged in to post a comment. Marshall amplifier serial numbers made simple Posted on April 25, April 27, by Phil by Phil. JTM block logos changed to gold-plated plastic script, these are fragile and many have been replaced.
Cabinet feet are small round and gray.
Handles changed to plastic with larger end cap. Power tubes are KT66, 6L6 and Single W Drake transformer, rated at ohms, part number , for W models. Single 50W Drake output transformers rated at 3, ohms, part number , for 50W models. GZ34 rectifier no longer used. Cabinets have corner-lock joints with a quality control signature inside. Gold Plexigas back panel, changed from cream-colored. Dagnall transformers used in the JTM, changed from Drake. Chassis construction transitions from aluminum to steel in late First Watt Majors appeared, with treble, bass and volume controls, active tone circuitry and KT88 tubes.
The watt Major was modified to match the normal panel layout and the active electronics where dropped. The output transformer turned 90 degrees in comparison to earlier models to reduce hum. Larger black cabinet feet starting in July , changed from small and gray. Welcome Dumble Amplifiers to the Amp Archives. Marshall wanted to up the power of its amplifiers because players of the time were demanding it.
What differentiates these between the later JMP plexi Marshalls is the fact that they employed very large transformers and the filter caps for the power supply were all placed underneath the chassis. All Marshalls during this era were hand-wired point to point with no printed circuit boards. The larger transformers and high plate voltages applied to the earliest watt plexis meant that they had a lot more headroom, were very loud, and also wore down tubes fairly hard.
Again, they were available in lead and bass versions that were somewhat similar, many of the smaller differences disappearing after the volume was cranked up. In , Marshalls plexi amps were labeled with a JMP logo on the front panel, and the sizes of both the output and power transformers and corresponding plate voltages were reduced somewhat, leading to a bit more reliable performance.
The transition from filter cap placement from the bottom board chassis, to being top-mounted occurred gradually during this era as well. JMP plexi Marshalls are the most commonly available and are certainly the most affordable of the Marshall vintage plexi amp lineup. Sonically, JMP versions of the plexi had a touch more gain, some of this in theory can be attributed to the tone of the output transformer and resulting saturation occurring when it was downsized a bit.
The bottom line is if we had to place these amps in a comparison between the earliest JTM 45 amps and the next series of metal-panel Marshalls, while the JTM 45s would be considered the bluesiest and the early metal panels, the most aggressive with loads of distortion, we could safely put JMP plexis somewhere in-between the two. This makes them very popular rock and blues amplifiers that happened to have also been very well built. Now like the JTM 45, there were various versions of these plexi amplifiers.
There were Bass 50 watt , Super Bass watt , Lead 50 watt , and Super Lead watt heads in addition to a range of p.
There also consisted of Tremelo and Super Tremelo heads that of course added — you guessed it — a Tremelo effect that could be footswitched. The watt Major is rather rare and a different circuit and tube set. Why are they cheaper you may ask? On the contrary, p. The earliest models also had slightly less brightness and crunch of the later plexi amps.
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In this regard, they are favored for their loud bluesy tones and warmth. In , the JMP-marked plexi amps arrived and had reduced plate voltages that were easier on the power tubes and a bit more brightness in the circuit to add some crunch. Super Bass amps pack quite a wallop of power — or at least they feel that way because of how the circuit is tuned. In any case, the basic Super Lead and Super Bass circuit are both extremely similar and conversion to either type is very easy to do.
How can my plexi Marshall sound best — what types of guitars and cabinets should I use? Reference the tone tips and tricks article on Marshall Super Leads as it applies in particular to these amps. Beyond that, from my personal experience, if playing a guitar equipped with a humbucking pickup, a PAF-style to medium gain pickup works best, rather than a high output or super-high output ceramic-based model. High output pickups have extended lows and highs that are not very clear often so the resulting mix through these era Marshalls is a muddied type of tone.
Celestion Greenback speakers have a nice full sound with good bottom end and smooth mids and exhibit nice breakup in their sonic characteristics.
G12H30s are also a good choice and have larger magnets and power handling capability. Ironically, the larger magnet does NOT seem to make it have a stronger or fatter bass response as one would tend to think. These are a cleaner speaker that still retains warmth. Now as to reissue versus the originals?
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G12H30 anniversary reissues are said to be more accurate in construction and design to the originals and indeed sound good. Plenty of people also like the G12M 25 watt greenback reissue though it does exhibit quite a bit more brightness in its tone. Some of this may be due to aging since speakers will roll off some of their highs after years of use. Sounds great and better to my ears than any of my other cabinets, even when playing newer Marshalls!
This can be tricky and is easier for 50 watt models in particular because they had codes on the transformers that could reveal and indicate the year of manufacture.
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Here are some other general spotting clues. Look at the chassis of the amplifier as it may have its inspection tag still attached with a legible test date. Otherwise, some general things to look for:. JTM 45 identified on front panel for watt amps indicates first of the series.
Legendary Tones
Many models had no option to run U. Filter capacitors all located on underside of chassis filter capacitors are the circular blue or silver cans that look like tubes that are on the top side of most Marshall chassis standing vertical with the other tubes around them. Gold brittle logo along with small gray plastic feet on all head boxes. Head boxes have flat front lip. Thinner than normal top screen vent for watt models. No vent on top of head boxes for models under watts. Voltage and impedance selector on back will be a plug style, the shape and size roughly of a larger vitamin capsule style pill.
MK II labeling on all amps. Enlarged normal top vent screen on watt models. Some filter capacitor cans not mounted on the top, but NOT the complete set of 6 as was traditional on all later watt heads from later in and onward. Serial number scheme inconsistent with window-etched types or ink stamped being the norm. Gold brittle logo along with small gray plastic feet on all head boxes continues. Head boxes continue flat lip style. JMP now labeled on front panel.
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Fifty watt JMPs now have solid-state rectifier. Serial numbers now consistently stamped in engraved black on rear plexi panels. Mid marked the beginning of the 6 filter capacitors typically 50 x 50 uf each, though some have been equipped with 32 x 32 uf as well being mounted consistently on the top of the chassis. This configuration continued with watt models until midway through the JCM series. Gold logo along with small gray plastic feet on all head boxes continues.
Head boxes revert to rounded-front lip style in early-to-mid before the end of the plexi era. Both will be powerful and of course loud. In truth, they are about the same but have a different feel and response. Also, whether I play a 50 watt or a watt plexi amp, I still need to use an attenuator such as the THD Hot Plate to bring down the levels a bit.
Marshall amplifier serial numbers made simple
The actual volume difference between a maxed 50 or watt is only about a dB difference! I prefer the watt sound personally, but many prefer the response of the 50 watt models. In the plexi era, beginning in mid-late , Marshall made slightly altered versions of its plexi amps for the U.
This worked together with an attached two-prong AC cord and was used to eliminate any hum caused by a ground loop from the line. You simply plugged in your amp and chose the polarity setting that caused the least amount of hum. European amps were grounded, had detachable circular three-prong Bulgin connectors and no polarity switch on the front. What kind of changes should I consider to make my plexi sound better?
Should I modify it? If you enjoy the various rock and blues sounds from the known players that have used Marshalls, you should know that many of these Marshalls got their tone in stock, albeit cranked form. Also consider what are you looking for? For gain, I chose to incorporate a clean-boost effect to increase my gain when I wanted without affecting the tone you can read more about this in the Tips and Tricks for the Marshall Super Lead article. For reducing the brightness as well as overcoming what was to me a muddy sound that lacked definition, I did two final things.
First you should be familiar with what a Marshall plexi is and what it should look like. Basic amplifier with four inputs, controls for presence, bass, middle, treble and volume 1 and volume 2.
Legendary Tones - The Marshall Shopper's Guide Part 1
Look at the front and back of the amp. An extra switch sometimes accompanies this mod. Who cares about originality? Well, what about the output transformer?