12 Technology Drive, Ste.13 East Setauket, NY 11733
Note: This page is always being revised with additional information and details. Check back often!
There are many print and online resources that have detailed and sometimes confusing information about the different models, production runs, cabinet and speaker variations, etc. this page addresses the service, maintainace and electronic restoration of these amplifiers from a technical, non-collector point of view.
MARSHALL AMP REPAIR/OVERHAUL RESTORATION/MODIFICATION
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Classic Marshall amps: JTM45'S, Plexi's, 1959 Lead , 18W & 20W, PA's
JTM45's/Plexi's:
Perhaps the most revered and well loved Marshall amps of all, these amps are usually treated as conservation/restoration items, but definitely "players" rather than museum pieces. If not changed in the past, all power filter caps in the B+ and bias sections are changed. The original tone caps in these amps tend to hold up very well over the years and if not leaking are never changed. If they have been replaced over the years I usually restore with Mallory 150's or if requested SoZo "Mustard" caps. Personally, I find no sonic difference, but as the SoZo's have a following on the net, I do not let my own view get in the way. The most important points in repairing these amps are reliability and faithfulness to the original concept of the item. I advise addressing any safety and reliability factors first. Power filter caps are replaced with the same values as the originals, I replace all bias related resistors with new high quality parts as failure of these components can cause a lot of damage, up to burning out the transformers. I install one ohm saftey resistors on the output tubes as this does not affect sound and protects the output transformer in the advent of tube failure. Next are electromechanical issues. Power and standby switches can fail and become noisy, selector switches fail and output jacks break down. There are retrofit and repro parts that function quite well and do not detract from appearance, and we stock these and advice replacing old worn out parts before they cause a problem. Most of these amps used the KT66 output tubes, but can easily be rebased to El34's. A bit of a different tone but not a non-reversible issue. There are some good KT66 clones , I use either the Russian Gold Lion reissue or the Valve Art KT66, which is probably the closet sounding of all to the original GEC and really the only Chinese tube I like. I really try to keep these amps as close to original as possible as They are true landmarks in the history of modern music. I will decline to modify these amps in any way as they really should be maintained as close to original for future generations to enjoy.
18W/20W Marshalls:
The 18W Marshall is treated much in the same manner as the models listed above. The so called 20 watt version of this amp is not a very good sounding amp and does not have the sweet saturated sound of the older 18. Since it is a pretty looking amp, I have no problem altering it to the original configuration. I am not that much of a purist that a poor sounding amp should stay "original" PS: real 18W amps have three 12AX7 tubes, 20W's have two.
PA amps:
Marshall made a lot of PA heads, mostly 100w units. These were really lousy PA amps being only slightly reworked guitar circuits. With a little bit of work these can be great sounding guitar amps. My favorite Marshall "back in the day" was a unmodified 100W PA head that was a absolute tone monster with any humbucker fitted guitar.
The most important and common of these are the many variations of the JCM 800. There are a bewildering variety of these , 2210's, 2205's being the most common. Quite a lot of information is available on the net about these amps and suggested modifications abound, but from a service point of view there are really just a few areas to address. The main filter caps are usually pretty well shot by now and should be changed. The overall response of the amp can be tunes by selecting the right filter cap value. A lower rating will encourage earlier breakup, while a larger value will give a tighter, crunch tone. In my opinion, the red molded coupling caps used in these amps do not age well and can sound harsh. As the circuit board has to be lifted to replace one of the filter caps, I often swap these out with Mallory 150's, yielding a sweeter, more vintage tone. These are loud amps, both the 100w and 50W versions can be more versatile if a post inverter master volume control is added.
In the early 80's many of these amps that were to be sold in the US were the so called "Unicord" amps, using 6550 output tubes for greater reliability as the old production of EL34's were coming to an end. The 6550 is not a good lead amp tube as it does not break up until well into overload and tends towards a harsh sound when it does. If not done already, I always advise replacing these with EL34's. This only requires the change of one resistor in the bias circuit and yields a vast improvement in sound quality. Some JCM 800's were fitted with really lousy preamp tube sockets, causing noise and intermittent signal dropout. It is relatively simple to change these out and we use high quality Micalex sockets for this work.
90's / 2000's Marshalls and the DSL/TSL's:
Most problems with these amplifiers are related to soldering defects and simple component failures although there are some specific pronlems worth mentioning:
2000 DSL/TSL 100W Heads:
Output tube bias runaway: At times these amps will suddenly draw too much current and blow the mains or HT fuse. Sometimes they will just run incredibly hot, actually melting and distorting the glass of the output tubes. This is caused by bad connections in the small bias board at the rear of the chassis. Rather than bother trying to correct an inferior design, i retrofit a more classic bias circuit. It is a simple mod, does not cost anything extra in a repair and actually improves the performance of the amp.
Signal fade: As the amp warms up, the volume drops out. After turning off the amp for a while, full volume is restored, but will fade out after 20min to an hour of play. This is caused by a undervalued rectifier that supplies heater voltage to the first input tubes. We replace it with a better quality part and the problem is cured forever. This is referred to on the 'net as the " Haunted Marshall" problem.
Power mod: These amps are LOUD. For use in most clubs they tend to not sound their best unless cranked to the point of overkill. A switch that disables two of the output tubes reduces the output to about 40W and results in a more manageable amp.
Tone quality: The TSL is really a high gain/metal/shred amp and is best left as is. I have moddee quite a few DSL's into a more classic Marshall tone quite successfully. This involves changing tone caps, adding a power choke and a few changes in tone stack resistors, along with a post inverter volume control. Additionally retrofitting with Mercury Magnetics transformers will turn these amps into real tone monsters but this will run into some serious $$$.
DSL 401 combo amps: Numerous pcb problems and component failures plague these amps, but after carefuyl reworking they can perform very well. I always use JJ output tubes in these amps as most other tubes either fail or sound excessively brittle.
Reissue / Special edition Marshall's
We are revising this list check back soon
The Vintage Modern & Our Drive Volume Preset Mod:
The Vintage Modern is really an excellent amp, perhaps the best Marshall made in twenty years. A customer of mine has four of these and they have been a ongoing project for a while, one of them is in my studio now as this is being written. They are surprisingly nice amps right out of the box, but a few a few minor changes really bring out the best in these amps.
Tubes: These are set up with KT66 tubes standard, but changing to the venerable EL34 improves the tone, being a bit sweeter and softer in bass response. I guess that KT66's were used as a way of getting some JTM45 mojo into the mix, but the 34's just seem to work better.
Caps: Replacing the hard molded epoxy tone caps with Mallory 150's really " vintage up" the sound. I also do a "secret sauce" mod in the power supply that gives a more touch sensitive feel to the amp.
Volume preset: One problem in this amp is that when the drive feature is enabled, the amp get really loud. Not a problem if it is really nailed out anyway, but as a clean/lead function it is not as useable as it can be. I add a adjustable volume control on the rear of the amp as a preset feature. When the drive is enables the volume can be preset to any level you want, but it is completely out of the circuit when clean. Have done this on about 20 VM's so far and everyone seems to like it.
Extra gain stage EF86 mod: This was an experiment to get more touch sensitivity and saturation into the amp. I fit a Vox style EF86 input stage with a gain control as a internal booster, an alternative to a OD pedal. I also fit a switch to use this as the main input stage, giving a really different feel and response to the amp, more like a DR.Z sound and response, but with the ability to go back to the stock setup or use as a internal OD at the flick of a switch. I really like this one. So far I have done it on all four of the amps mentioned above, including the semi permanent test amp I have in my studio.
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02/28/2011