Service Notes
Tubes
Tubes are consumables, as they have a given usable lifespan. They are part of the
heart of the tone, so keeping correctly operating tubes is essential. Tubes can fail
catastrophically or gradually, and it’s good to know what to look for if they start
to go bad. Periodically inspect them and look to see if anything inside the tube is
glowing cherry red other than the normal orange glow of the filament. This would
indicate a situation where the tube is conducting more current than it is capable
of handling and most likely about to fail. Two other conditions to observe are:
1) filaments not glowing or 2) a miniature fireworks display inside the tube. Any
of the above conditions indicate serious problems with the tube and should be
taken care of immediately. Tubes quite often are the cause of spurious noise in the
amp. Microphonic tubes will squeal or rattle with the vibrations of the cabinet.
If suspected, tap each tube lightly with a pencil with the amp powered up—the
suspect tube will let you know. Note that there is a normal metallic clinking when
doing this, but a microphonic tube will be quite loud.
Replacing preamp tubes will not require any adjustment, but the power tubes will
need rebiasing to assure proper operation. After power tube replacement, initially
inspect the tubes often to assure there are no “cherry red” components within
the tube. Tubes today can have a wide variety of tolerances and a re-bias is highly
recommended with new ones.
Power tube bias should be adjusted to 30 mV, +/- 5 mV. If power tubes are
mismatched by less than 5 mV, average the two around 30. Mis-matched tubes
beyond 5 mV will induce noise and may cause a degradation of tone.
NOTE! Capacitors may retain an electric charge and can be dangerous even when
the unit is off, unplugged, and has not been played for an extended period of
time. USE CAUTION!!!!
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LIT-AMP-MAN-JMOD REV B
John Mayer J-MOD 100
User’s Manual
