If you check the plate voltage for example and it reads low, then the plate load resistor R6 is probable going bad. If the tube is good, then the two most likely sources of problems will be R6 (plate load resistor) or R7 (screen grid resistor). The voltages shown are usually the first thing to check.
Basically the signal is routed to the input grid (pin 1) of the 6AU6A tube for amplification.
If you had a problem with the non-vibrato sound, it would likely be in this area. If the vibrato switches are set to vibrato off, the signal from the matching transformer goes into Point “A”, a solder terminal located on the preamp.
