Center tap fuse on JTM45

[keithv]

The JTM45 is a clone of the 5F6a for the most part, right?  But the 5F6a didn't have a fuse on the center tap of the PT secondaries.  What is the purpose of this fuse?

[sluckey]

That fuse provides protection for the PT HT winding in the event of an over current situation in the B+ circuit.

[keithv]

Why don't all amps use this?  Doesn't the mains fuse serve the same purpose as well?

[shooter]

Doesn't the mains fuse serve the same purpose

The mains might not be quick enough to respond to a DC *issue*?
I believe the HV fuse would be internal, hence a "qualified" service person checking things out verses a guitar player just jamin in another fuse

[tubeswell]

Why don't all amps use this? 

I put HT fuses in all my amps, but its extra expense

Doesn't the mains fuse serve the same purpose as well?

No


A mains fuse is there to stop the PT catching fire. An HT fuse is there to protect the OT from a short between the PT secondary/power rail to ground

[sluckey]

I don't like the idea of putting a fuse (or a STBY switch) in the HT center tap in an amp that has a bias circuit that is fed from another tap on the HT winding. When that fuse blows (or you open that STBY switch) the B+ filter caps will seek a path through the bias supply to the bias tap which now operates as an un-centered center tap. This will put a big positive voltage on the bias filter caps, often ruining them. That fuse would be much safer in the B+ line or even use two fuses in the PT HT leads prior to the rectifier.