Je me permet de mettre ici un mémo que j'ai trouvé sur plusieurs forums US :
SOUND “DROP-OUTS” / INTERMITTENT SIGNAL
One Possible Cause
Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower” tube stages in their preamps. Tube selection is CRITICAL in these stages.
Specifically, in a 12AX7 tube used as a “cathode follower”, the voltage difference between that present at the cathode, as compared with the heater voltage, can be withstood or tolerated by certain types of tubes, whereas other tubes will fail. The failure of a “cathode follower“ tube will cause sound dropouts or signal loss.
For the past few years, Mesa has been using two types of 12AX7 tubes: ones originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek) tube is NOT reliable as a cathode follower. Of the tubes we are using today (March 200, ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.
In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp, suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked as “CHINESE” (silk-screened on the tube itself).
Below is a partial list of Mesa amps and cathode follower tube locations:
GUITAR AMPS BASS AMPS
Lone Star & LS Special: V3
M-Pulse: V2
Stiletto: V3 & V4
Venture: V2
Road King I: V3 & V4
Big Block 750: V4
Road King II: V3 & V5
Titan: V4
Roadster: V3 & V5
M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4
Bass 400+: V2
Tremoverb: V3 & V4
ANOTHER POSSIBLE SCENARIO may occur in the “SPONGY” (or on some models, “TWEED”) power setting: the reduced ilament voltage may cause very low output from a RUSSIAN preamp tube. Again, the recommended fix is to replace the "sagging” tube with the CHINESE type of Mesa 12AX7 - which are more immune to this type of failure.
