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Version 1.20 1. Introduction This collection of Marshall Frequently Asked Questions (FAQ) is a work in progress. It is maintained by The Blame Productions dba Marshall Arts. The links herein are subject to change at the whim of the author, however the most current version is always available to the reader for online viewing at: http://faq.marshallarts.org.
A. The power up sequence is correct. On powering down, turning the standby switch to the silenced position leaves charge in the power filters. This is not a problem if you follow your normal sequence on power up, and may be easier to remember. However, with the caps charged, it would be extremely dangerous to then open it up and service it, and you wouldn't want to just turn both power and standby on at the same time a the next power on. If you leave the standby switch alone when you power down, this drains the charge out of the power filters as the tubes conduct until they're empty. Properly, you would turn the standby back to "silent" once the thing has gone completely silent after turning off the power switch, as this readies it for the next power on. (R. G. Keen)
P.S. A source for quality replacement tubes for Marshall amps is Q. I noticed on my JCM900 model 2100 SL-X amp that it has two positions for impedance: 4(16) & 8. What does the 4(16) mean? Someone told me that in this position, it "knows" what kind of cabinet (either 4 or 16) ohms that is plugged into it. Is this incorrect? A. There is no way for the amp to automatically recognize the impedance of your spaker cabinet, hence the need for the switch in the first place. However, there is no one right answer to what the impedance selector should be set for, since Marshall uses two types of impedance selectors on JCM 900 tube amps, depending on whether your amp is a combo or a head model. According to Marshall Guitar and Bass Amplification catalog M0001/91/1, page 8, the impedance selector choices are 4 or 8 ohms on combo models and 8 or 16 ohms on amplifier heads. Just to double-check, the SL-X owner's handbook verifies what's said above: "Impedance Selector Switch - Two-way switch for matching the amplifier to speaker impedance. Amp heads feature 8 or 16 Ohm operation. Combo versions feature 4 or 8 Ohm options."The confusion stems from the parentheses. Marshall actually labels the rear panels of the Dual Reverb and SL-X amps with one setting for "(4) 16" and one for "8". If you pay no attention to the parentheses, slide the switch to 16 if you use a 16 ohm cab, such as a single JCM 900 1960A (set in mono mode, and using the 16 ohm jack). If you have an 8 ohm cab, such as the Park G412, then slide the impedance selector to 8 ohms. Don't forget: when using two cabinets of the same impedance in parallel (i.e., two cabinets are "daisy chained together"), the impedance is halved. Thus two 16 ohm cabs require you to set the impedance selector on your amp to the 8 ohm position. (Blamepro Admin) Q. I have a JCM 800 2203 100 watt head with a matching 800 series 4x12 cabinet. I plan on buying another cabinet, and I'm wondering if there is any way I can run the amp in stereo with the 2 cabinets? Do I need a stereo pan pedal? An A/B box? Can I use the DI on the back to do this? Help! A. You need a second amplifier to run in stereo. You also need to have a preamp or processor that can simulate a stereo signal from your mono guitar signal. (ToneToDaBone)
2.2.2. Valvestate
A. You'll have to take the chassis out of the cabinet. Simply remove the screws holding it in from the top, and then lower the metal chassis down, then out of the cabinet. There should be a single 12AX7/ECC83 preamp valve in there. Remove the retainer clip, and ease the valve out of the socket, and replace with another one. (Liam)
Q. What's the difference between the model 1936 and 1922 2x12 cabinets?
A. The model 1936 is wide enough to put a standard-sized Marshall head on top of it, while the 1922 is the narrower width of the JCM 900 2x12 combos. Both cabs come standard with Celestion Speakers (GT-12 70's, I believe) and can be used as an 8 ohm mono cabinet or as a stereo cabinet, 16 ohms per side.
The 1922 has plastic jacks on the back and rather small wire connecting the speakers to the jacks. I have a 1922 and it sounds good with various amps. It sounds better now that I have replaced the wire with 16 gauge cable. (David Swanger)
  P.S. Included above are diagrams from a late 1980's Marshall catalog that provide dimensions for those who want to visualize the difference between the 2x12 cabinets. (Blamepro Admin) Q. I am very interested in the Marshall amps which the bassists used in the mid-sixties. Did they use 4x12 cabinets? Can I use a modern 4x12 guitar cabinet for bass playing without hurting the amp or speakers? A. The first answer to your question is that Marshall made 4x12 and even 4x15 bass cabinets. The bass cabs were larger in size than guitar cabinets, since bass cabinets are designed to use the volume of the cabinet to help develop the lower bass notes. I'd say that a 4x12 guitar cabinet rated for 300 watts (such as a JCM800 or 900 1960A equipped with 75 watt Celestions) would sound great for bass if the amp produced 100 watts or less. The sealed back cabs will prevent extensive cone travel. Remember that a single 15" will move an amount of air roughly equivalent to that of a pair of 12" speakers. The sound will be somewhat brighter with 4x12's, and may have an unwanted emphasis on guitar frequencies, since the volume of a guitar cab is smaller. A caveat, however: guitar speakers may handle the wattage, but the cone construction of guitar transducers is such that they are designed to be crisp and responsive within the frequency range of a 6-string guitar. Bass speakers are designed with stiffer cones, since they are subjected to more punishing amounts of cone travel. So a word to the wise: if you own a guitar cabinet and want to play bass through it for extensive periods, (a) don't crank the amp; and (b) watch those cones! You may be able to find used JBL, Altec, EV, or Celestion bass speakers to replace the guitar drivers in the cabinet. You can then sell or trade the guitar speakers to help offset the cost, after you're sure that you like the sound. (Blamepro Admin)
A. The Marshall Arts site has a page of detailed information and features for this valve-powered rack unit located at http://www.blamepro.com/mar1730.htm. There are also links to recorded examples of sounds produced from the JMP-1 at the site.
Q. When I try and re-initialize my JMP-1 via the process in the owner's manual, I just get an 'EE' message on the LED and the unit locks up. The next time I power it up, the unit displays a brief 'E' message. It's fine the time after, though. Any ideas?
A. The JMP-1 internal battery may need replacing. See page 10 of the JMP-1 manual.
(Rahker)
A. The models produced in the mid 1980's are housed in black metal cases. The effects boxes introduced in the late 1990's are in taller, more compact silver metal cases. The Marshall Arts site has detailed information and features of each pedal located at http://www.blamepro.com/mar/pedals.htm. The Guv'Nor and Bluesbreaker pedals from the first series were reintroduced as the Guv'Nor II and Bluesbreaker II models, respectively, in order to avoid confusion with the earlier editions.
A. The PB100 Powerbrake allows the amplifier power section to run at high power levels, and soaks up some of the power output via a series of resistors before the amp output is sent to the speakers. Marshall also uses what is known as a reactive load (the impedance of the load changes, depending on frequency) in the PB100.
Note that running an amplifier at a higher output setting increases the amount of wear on the power tubes. This is offset by the increased versatility of using the Powerbrake to control the amplifier tone at lower volume levels for recording, practicing, and live performance.
The Marshall Arts site has more information on the Powerbrake PB100 available at http://www.blamepro.com/mar/marpbrak.htm.
Q. How is this different from a master volume (MV) control? If my amp already has a MV, is there any advantage to also using a Powerbrake?
A. A Master Volume-equipped amp works in a different way than a Powerbrake. The MV control reduces the level of the power amplifier section so that the output tubes do not work as hard. While this accomplishes the goal of lowering the volume, the power amp tubesare not driven to a high enough level at low volumes to produce distortion. Using a Powerbrake on a MV amp will allow power tube saturation to occur, resulting in a more pleasing, musical distortion.
Q. How old is my Marshall amp?
A. Early models featured a serial number stamped on the chassis. Later models have a serial number barcode sticker affixed to the rear of the chassis. (Blamepro Admin)
A. See Michael Doyle's book, The History of Marshall, for detailed information on determining dates from the manufacturing codes of Celestion speakers.
6. Settings for Marshall Equipment
The following JCM900 SL-X tone settings are from Marshall product support tech John Teto, and previously appeared in the Fall 1994 issue of dealer magazine Marshall Feedback:
Note: All settings are approximate, depending on guitar used.
6.1.2. Slash Reissue
The following information was submitted by M. Chu and appeared in the June, 1996 issue of Guitar Shop magazine. These settings should work equally well with a 50/100 Silver Jubilee head or the recent JCM Slash Signature model 2555 amps:
6.2. Other 6.2.1. Using the Effect Loop Appendices
A. History
B. Contact Information
C. Players That Use Marshall Equipment
D. Sources for Spare Parts
E. Modifications
"I'm not some sort of technical tube guru, just a musician who's handy with a soldering iron. Here are a couple really easy things I did to take care of the brittleness in my 2500SL-X. Please note that these are only possible if you have the 5881 output tube version. If you have EL34s, remove them, install 5881s, and skip step 1. Disclaimer: this amp can kill you even when its not plugged in and if this concept troubles you, find a technician to do these mods for you.
"1. The main power supply filter capacitors, big blue things, are dual 50+50 uF. On one cap both taps are used. On the other, only one of the taps is connected. Get more bass response by wiring the yellow tap to the red tap.
"2. Raise the grid bias on the 5881's from -50v (stock) to about -45v. Turn the trim, VR2, clockwise as far as it will go.
"There is a certain amount of risk for joe average user to do these modifications. I hope I made that clear." -- Ken
Thanks, Ken, for your submission to the FAQ! I appreciate your concern for user safety, and I can't stress that one enough. Tube amps can generate over 600 volts AC, which is lethal if you come in contact with it. If you're at all unsure or unskilled with this type of work, take Ken's advice and have the mod done by a professional.
F. Buying New
G. Marshall Resources in Print
H. FAQ Update History
NOTICE: The information contained herein has not been reviewed or endorsed by Jim Marshall Products, Korg USA, or any other entity. The user assumes all risks of damage or failure to equipment, persons, or property arising from the use of the information contained herein. This page is presented for the sole purpose of presenting the author's opinions on the topics shown. It is not intended to be an all-inclusive or definitive source on the subject. More detailed answers to user inquiries may be obtained from certified Marshall amplification repair centers or dealers. In no event shall the author, MarshallArts.org, or The Blame Productions, or their sponsors, affiliates, or assigns, be held liable for acts arising from the use or misuse of content contained at this web site.
Trademarks and copyrights used herein are the property of their respective holders. Marshall Arts is not affiliated with Jim Marshall Products, plc.
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