As preamp tubes age, the sound begins to distort very quickly at lower volume settings. This can be undesirable, especially if the musician seeks a clean sound. Is is likely that the tube closest to the larger power tubes is the one which needs to be replaced.
In vintage amplifiers, effects controls and equalizers are generally controlled by the preamp tubes. Unresponsive controls, or controls that produce strange, uncharacteristic effects can also be an indicator that preamp tubes need to be replaced.
If the signal coming out of the amplifier has become noticeably weaker, it may indicate that the preamp tube located closest to the instrument input has become defective, and needs to be replaced.
Vintage tube amplifiers typically produce reverb effects from a preamp tube. If the reverb control is no longer responsive, it is possible that one of the preamp tubes has become faulty, and needs to be replaced.