Amplifier:
Fender Bassman ’70
Fault description:
No sound at all. Does not react to volume controls. Amplifier had died in the middle of rehearsal. Filaments light up normally.
Procedures:
Checked the internals, HT fuse had blown. The amplifier was previously serviced in 2016, fuse looked quite fresh. It may have been swapped around the time. The blown fuse was rated 500mA. Fuse rating was correct, but the fuse in place was fast type (F).
Fuse was replaced with a 500mA time lag type (T). After the fuse swap, the amplifier ran without issues.
Conclusion:
More attention should be paid to fuse types. Fast fuses may work fine for long periods, but surges are normal in tube amps and fast fuses are prone to premature failure. Using time lag type fuses makes the amp more reliable, while still keeping things protected nicely, as long as the fuse rating is correct.