Love and Care of a Marshall JCM 800 2103 (circa 1981)

The AMP Doctor: Scott Russell 1-248-914-1177 (he does all of Junkyards amps). Note: he lives in Clarkston across from Pine Knob (DTE).




JCM800 MODELS

Tubes :Lead models 1959, 1986, 1986 bass, 1987, 1992 lead, 2103, 2104, 2203, 2204, 4010, 4103, 4104 V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. JJ ECC83s and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN ECC83/12AX7s are recommended for this position. We understand that some technicians recommend Ei or JJ ECC83s for this duty.V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or JJ ECC83s for this duty.Power tubes are 2 x EL34 or 6550 in 50-watt models, 4 x EL34 or 6550 in 100-watt models. Bias adjustment is MANDATORY.

Source:
http://www.thetubestore.com/inonmaram.html#Lead%20models%201959,%201986,%201986%20bass,%201987,%201992%20lead,%202103,%202104,%202203,%202204,%204010,%204103,%204104


Amp Specifications :

All specs can be found at:

http://www.drtube.com/marshall.htm#JCM800



Modifications recommended to Achieve Maximum Tone :

All specs can be found at:

http://www.tone-lizard.com/Ultimate_JCM800.htm

http://www.tone-lizard.com/Marshall_Myths.htm#Marshall


Trouble Shooting:

Q
Hi, I have this marshall jcm800 that I use not to often, recently I was playing and I noticed that the sound volume and the quality drops for few seconds, after that It is ok for a while and drops again ....I am a real doomie with electronics, please help me out with this ...thanks......GIO -- Gio

A
Gio- I think that everybody playing a tube amp should have plenty of spare tubes so you might first try replacing all of the tubes to see if that solves the problem. (Mark the old tubes with their position before you take them out.) If that does solve the problem you might try putting the old tubes back in to figure out which one was the culprit. If that DOESN'T solve the problem you'll probably want an amp tech to check it out... it could be a bad solder joint or a bad contact on a jack.
Good luck! -- Steve

A couple Great Trouble shooting sites:

http://www.rickonslow.com/mods/ampfaq.html

http://acruhl.freeshell.org/mga/main/troubleshoot.html