HT fuse placement

[lowell]

In my custom amp I have the HT fuse on the CT of the PT. This transformer has a bias tap on the secondary winding. My issue is that if the HT fuse blows, the bias voltage skyrockets upwards of 400v and my bias caps are at risk of blowing. As of now I have the caps rated at 450vdc but would prefer not use such large caps. I'm thinking of placing the HT fuse in series w/ the B+ supply line but am curious if this is bad practice or if the fuse is at risk of blowing intermittently when the standby switch is thrown. I have seen this done in commercial designs but want some advice.

[txstrat]

I've never placed the HT/B+ fuse elsewhere than in series with the B+ (usually right before the stand by switch). Never had an issue. I use slo blo.

[Bruce / Mission Amps]

In my custom amp I have the HT fuse on the CT of the PT. This transformer has a bias tap on the secondary winding. My issue is that if the HT fuse blows, the bias voltage skyrockets upwards of 400v and my bias caps are at risk of blowing. As of now I have the caps rated at 450vdc but would prefer not use such large caps. I'm thinking of placing the HT fuse in series w/ the B+ supply line but am curious if this is bad practice or if the fuse is at risk of blowing intermittently when the standby switch is thrown. I have seen this done in commercial designs but want some advice.

Maybe you have your plan confused with having a fused link in the center tap of the OT, not the PT.
Simply move it to the OT center tap lead.

[lowell]

Bruce wouldn't that only protect against power amp issues and not protect against the rest of the power supply?

thanks txstrat... I'll just move it there.

[Bruce / Mission Amps]

Bruce wouldn't that only protect against power amp issues and not protect against the rest of the power supply?

thanks txstrat... I'll just move it there.

Well yes, but the power tubes usually draw +90% of the B+ rail's available current so it seems like that's where the most protection is needed.
It's pretty rare that a filter cap shorts out causing excessive current draw through the B+ rail.
If it worries you, I don't see why you couldn't use a steering diode (1N4007) over to another low current fuse and feed the rest of the amp from there.

[Steve Conner]

If you put the fuse in between the rectifier cathode and the first filter cap's positive terminal, that's electrically the same as putting it in the PT's centre tap. It passes the same current, so it'll have the same protective effect.

The only difference is that the bias tap is no longer affected when it blows, and your chances of getting a jolt off the fuseholder go up a little.

Bruce's advice also applies, though: the OT centre tap is the only thing that really needs fusing. All of the other power supply nodes (screens, preamp tubes, etc) have resistors in series with them, and these will limit the short-circuit current. The screen resistors are the lowest valued ones, but even they usually burn out before the fuse blows, if a screen shorts.

[Diablo]

Maybe I'm confused? If you fuse the center tap of the PT, and that fuse blows, that cuts off AC voltage to the rectifier and also cuts off AC voltage to the rectifier diode for the bias supply. I don't see why there is a problem. Where is the 400+ volts coming from on the bias cap when the fuse blows?

[lowell]

I believe it's because the PT secondary's CT is no longer DC ground referenced so the secondary basically turns into a non-centertapped coil that sits at a much higher voltage throughout it's winding. That's the best I can make of explaining it. Someone else can probably do better.

[Diablo]

I believe it's because the PT secondary's CT is no longer DC ground referenced so the secondary basically turns into a non-centertapped coil that sits at a much higher voltage throughout it's winding. That's the best I can make of explaining it. Someone else can probably do better.

You're right, and I see it now. The return path for the current comes from the bias resistor ground. It looks like the PT centertap fuse won't do much good, because if the fuse blows, you just wind up with a PT with an offset centertap - the bias tap now is the ground and you'll send the HT to your rectifier - maybe 50 volts higher than your normal B+ on one side and 50 volts lower on the other side.