PRS Guitars Custom 50 User Manual

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. 

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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2 Channel Custom Guitar Amplifier 

User’s Manual