Tube Diagnosis

[kingster911]

How do I diagnosis a broken tube? I own a RKII Combo and I think there's a blown preamp tube (12AX7?) because I started getting this low growl of a crackle that I know wasn't tube vibration on Ch. 2 even only when it was on Fat with under half gain. Anyone know what it is? Also how do you remove the AX7's? I tried to unscrew but it felt like it wouldn't budge. Maybe I was being too delicate to my baby...

 

[Lurch]

First check the power tubes by turning on the amp and gently tapping on each tube with a pencil eraser. If a tube squeals or makes the offensive crackle sounds, then replace the power tubes in matched pairs.

To remove the preamp tubes, (make sure they are cooled down!!!!) first remove the silver metal cover by pushing down on it and twisting a quarter turn counter-clockwise. Then grasp the tube itself and pull straight up (don't twist it, it's attatched to the tube socket with straight pins)

A cracked preamp tube usually has a cloudy white top that indicates air has leaked into it. If you gently shake it and there are loose parts inside (like a burned out light bulb) then it has been phisically damaged. You can also replace each preamp tube one-at-a-time with a known good tube to find a bad one.

For a very detailed description of the process, read page 21 of 38 in this Mesa Dual Recto manual: http://www.mesaboogie.com/manuals/2chRecto.pdf

 

[JamesD]

+1