I want to fuse my PT secondaries. Help.

[leadfootdriver]

The Express PT is putting out around 300ma's. Does that mean each leg of the secondary puts out around 150ms'a?

[passfan]

Customarily , fusing of the B+ occurs after rectification making your question moot.

[Phil_S]

Fast blow 500mA on the CT is what you'll find in Marshall designs. Remember, if you start pulling current through the CT, it means you've got a serious problem and you want it to blow right away. As long as everything is running normally, that 500mA fuse should last forever. It's a reasonable safety measure, but don't over think it.

[leadfootdriver]

The safer method as per the Wizard is to fuse the PT secondaries. If a rectifier fails, you'll toast the PT.

[Cygnus X1]

The safer method as per the Wizard is to fuse the PT secondaries. If a rectifier fails, you'll toast the PT.

Not always.
I've seen many instances where diode rectifiers go open, and the main fuse goes...without harming the power transformer.

Fuse location depends on what you are trying to protect.
When I use a secondary fuse it goes on the positive leg after the rectifier to keep downstream caps from blowing up.
That is...if the filters are pulling too much current, then the HT fuse opens.

But I'm really interested in seeing all sides of the argument here, my mind could be changed easily!

[tubeswell]

The safer method as per the Wizard is to fuse the PT secondaries. If a rectifier fails, you'll toast the PT.

You can always put a couple of 1000V 1A diodes in series between the HT winding and the rectifier tube anodes (1 x SS diode on each side of the winding). This rectifies the signal on the HT winding (before it gets to the tube rectifier) so that if the tube rectifier fails, it won't dump high VAC onto the filter caps (which would otherwise cause them to short). The voltage drop as per the tube rectifier will still have effect (as will sag etc) because each SS diode's forward voltage drop is only 0.6V (compared to 20V or more from the tube rectifier). Then you can get away with just having the Marshallesque HT fuse (and this will take up less real estate than multiple fuses)

[diagrammatiks]

ya fuse them both before the rectifier if you want.

I think 500ma should be ok?

[VacuumVoodoo]

Mains fuse on primary side should protect against overload exceedeing nominal VA rating by ca 10%
Fuses on secondary side (before any rectifiers) should protect the transformer against short circuit load on any winding. Otherwise you may get a short circuit on heater winding which will not blow the mains fuse but will certainly cook the transformer.
A fuse on HV winding after the rectifier and 1st ripple filter cap should blow before any power tube meltdown.
A lot of fuses in total but a cheap prevention of PT melt down and/or fire.

[leadfootdriver]

Thanks for the replies!

[katopan]

The other issue with fusing secondaries that makes it difficult is you don't know the transformer thermal curves. If this was an industrial application you'd overlay the device curves with the fuse time-current curves and pick the most appropriate. But not knowing squat about the transformer time-current withstand means all of this is guess work with a bit of rule of thumb. You end up putting in the smallest fuse you can which doesn't blow, and hope that if you have a fault it actually protects your PT. Not something you want to test just in case it doesn't work.

Having said all that I always use a secondary HT fuse and have had it save my PT once when I accidentally brushed a probe across the wrong spot during bench testing.