HV fuse after filter caps?

[Gunner]

Hi,

Finally building me a 100w

Som thoughts on the HV fuse. Are there any reasons for the suggested postion after the first filter caps instead of before? For example between the rectifier bridge and the first filter caps, as i many amps. If you get any problems with the first filter caps there must be a increased risk of frying the PT?

Usually, when I build other amps I even put the HV fuses before the rectifier bridge.

/Gunnar

[flemingmras]

Hi,

Finally building me a 100w

Som thoughts on the HV fuse. Are there any reasons for the suggested postion after the first filter caps instead of before? For example between the rectifier bridge and the first filter caps, as i many amps. If you get any problems with the first filter caps there must be a increased risk of frying the PT?

Usually, when I build other amps I even put the HV fuses before the rectifier bridge.

/Gunnar

Yes. On a 100 watter the HT Fuse MUST be after the first filter caps. This is because the bridge and the 1st filter caps actually form full wave doubler circuit since it uses the center tap of the PT to balance the voltage across the first filter caps. If you fuse it BEFORE the first filter caps in a doubler arrangement, when the fuse blows it will only knock out one side of the doubler (the bottom filter cap will still be live).

If you want to fuse it before the caps, you'll have to disconnect the center tap of the HT winding. But then you'll need to install bleeder resistors across the filter caps for voltage balancing (220K should suffice for this). This will allow the HT Fuse to knock the entire HT supply out when/if it blows when it's place between the rectifier and the first filter caps. You'll also need to use a slo-blo type fuse as the inrush current that the fuse sustains on power up will increase since you'll now have more filter caps drawing charge up current through the fuse.

[Gunner]

Thanks! I get the point.

But there must still be an increased risk for frying the PT with the fuse after first filter caps, right?

Hm, strange. In the 1959 HW scheme the fuse is actually placed between the rectifier bridge and first filter caps. That amp has the same arrangement with CT to the point between the first filter caps, no bleeding res and so on, similar to the old stock 1959 scheme. Got any explanation to that too?

/G

[flemingmras]

Thanks! I get the point.

But there must still be an increased risk for frying the PT with the fuse after first filter caps, right?

Maybe. Typically if you short a filter cap or a rectifier diode the PT will draw more than enough current to take out the mains fuse.

Hm, strange. In the 1959 HW scheme the fuse is actually placed between the rectifier bridge and first filter caps. That amp has the same arrangement with CT to the point between the first filter caps, no bleeding res and so on, similar to the old stock 1959 scheme. Got any explanation to that too?

/G

"Designer's oversight" is pretty much what that boils down to. The positive rectifiers and the top filter cap may be protected, but the negative diodes and bottom filter cap have to rely on the Mains Fuse to get taken out.

Just because Marshall did it doesn't mean it's right. God knows they made plenty of mistakes on their earlier amps (such as running 4 x EL34s on a 1.7K plate-plate load on a 500V B+, when the proper load should've been 2.5K for that voltage).

Typically if you want to fuse it prior to the first filter caps and do it CORRECTLY you must remove the CT connection, then add bleeder resistors to balance the cap voltage. You can then place the fuse prior to the first filter caps.