Fusing for Bias Supply? Fusing for the OT?

[makinrose]

I have a couple of questions on fusing. 

A.  I recently started using some transformers with a separate bias winding that I plan to use with a full wave bridge rectifier.  Is it good idea to protect it with a fuse?  I'm assuming if I did fuse it I'd use a slow blow 2 to 3 times the current rating of the winding?

B. While I fuse the windings on the power transformer is advisable to use fuses for the OT too? Or would that be redundant?   If I should so what's the most effective way to do that?  

Thanks everyone for the help!

[K O'Connor]

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

[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?

[K O'Connor]

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

[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!