Determine Mains Fuse Required

[martin manning]

And the only protection from that is a mains fuse and maybe a quick reaction from the user switching off the amp?

Fast-blow fuses in the HT legs or CT will protect the power transformer from a rectifier or power tube short. You can also add one in the B+ after the rectifier. See: http://www.premierguitar.com/articles/t ... ash-pt-2-1

[Littlewyan]

I tend to use a slow blow after the rectifier. I wouldn't use one on the CT as if it blows then you'll have a floating secondary.

[martin manning]

I wouldn't use one on the CT as if it blows then you'll have a floating secondary.

True, but if it's needed you'll still have a secondary.

[Littlewyan]

I think I'll just stick with the mains fuse and HOPEFULLY the HT Fuse. I've always thought about having fuses for the heater supply but never had the room to fit them :/. As for the HT Fuse in the amp I'm building at the moment I haven't got room for a holder to be mounted on the back, so I'm looking for a holder than can be mounted inside on the chassis. I do have one but its exposed, trying to find one with a glass cover at least for a bit of safety.

[martin manning]

If you're space limited, and have room for only one fuse, the best thing is to put the fuse in the CT so that you are protected against both rectifier and power tube shorts.

[Littlewyan]

If you saw my original message ignore it, just read up on how the CT Fuse would work. I understand now that it'd stop all current from flowing. However as I'm using a Bridge Rect i cant use it anyway but I could put it before the rectifier as i'd only need it one side. Will see what I can do.

[martin manning]

Yes on a FWB one side will do it. For a FW I stand corrected. Since the cathodes are connected together, a short in one rectifier would let half-wave current flow through the remaining rectifier from one end of the winding to the other. So a fused CT in a FW will protect the transformer against a power tube short but not a rectifier short- the mains fuse would have to cover that.

[Littlewyan]

Its quite amazing how many fuses you really need in your amp to provide proper protection. I remember my Marshall 6100lm which had something like 8 fuses in it, something ridiculous like that. But now that I know a bit more about electronics I realise that I should thank Marshall for that.