VOX AC30C2 Standby Switch Blown Fuse

EL84 Power Vacuum Tubes (Matched Quad)

The most frequent cause of a blown B+ (standby) fuse in this circuit is a cathode-to-plate or control grid short within the power tubes. These shorts occur due to heat-induced structural degradation of the internal tube elements over time. Replacing the full set ensures balanced current draw across the output transformer.

T500mAL 250V 5x20mm Glass Fuse

This is the sacrificial component (F1) located on the internal PCB that protects the High Tension (HT) rail. It is designed to fail when a power tube shorts to prevent catastrophic damage to the power transformer or screen grid resistors. It must be replaced with a slow-blow (T) variant to handle initial inrush currents.

100 Ohm 5W Screen Grid Resistor

When a power tube fails violently, it often draws excessive current through the screen grid resistor, leading to thermal runaway and component carbonization. These should be inspected for physical scorching or cracked ceramic casing if the fuse continues to blow after tube replacement.