Fusing for Bias Supply? Fusing for the OT?

[makinrose]

Hi makinrose

Of course, TUT3 details the use of fuses in tube amps  - and everything else, really.

Every winding on a PT should have a fuse or multiple fuses depending on whether the winding has taps. The rule of thumb is that the number of fuses for a winding is the number of leads for that winding minus one.

All AC fuses should be slow-blow. This includes mains and all secondaries of the PT. Secondary fuses protect the individual windings of the PT. A single mains fuse with no other fuses allowed for provides little protection for the PT or amp, and primarily protects the mains source.

An OT fuse protects the OT and the tubes and is always a good idea. This fuse can be slow-blow but is aesthetically more pleasing as a fast-blow.

Fuse sizing is based on the load current and a fudge factor for heating of the fuse and a second factor to avoid nuisance fuse failures while still retaining some degree of protection. Each factor is 1.25, so you have Amps x 1.25 x 1.25.

Have fun

Thanks!  For OT fuse is there preference between fusing the cathode vs the plate supply?