Fusing for Bias Supply? Fusing for the OT?

[makinrose]

Hi Guys

OT fusing protects the high-current plate path and it is about the only fusing ugmentation above mains fusing that you see in OEM products.

If the tube shorts, then the screen can still conduct a lot of current and that path is limited only by the screen dropping resistor or choke - so not much current limiting. As you can imagine, the mains fuse won't do anything about this... at least for a while....

If the output stage is wired ultralinear, the plate fuse to the OT CT protects the OT.

Cathode fusing will protect against any common failure mode in the output stage.Va, Vs and -Vb are all still present on the tube and depending on the failure mode of the tube there could be current from -Vb to Vs or Va. These should be limited by the grid circuit resistances.

Overall, cathode fusing is probably better than plate fusing. It also has the advantage of being safely ground-referenced so you do not have to worry about violating the fuse holder rating if it is a chassis-mount type.

have fun

Thanks for the enlightening answer!