Proper place for HT fuse?

[Dingleberry]

Hi.

What is the proper place for HT fuse?
I've seen them wired different ways. (schematics attached)

Picture 1, 2 or 3?

-T

[markr14850]

Proper? Dunno. But a possibly important point is that in 1 and 3, the ripple current will be flowing through the fuse, whereas it won't in #2. This might affect the fuse rating that you'd want, as the RMS current through the fuse will be different in those two configurations. It might also affect where you'd want to physically mount the fuse, as an increased area of wiring carrying ripple current pulses may couple more rectifying noise to the rest of the amp.

[pdf64]

My preference is before the rectifier (to protect the B+ winding from a rectifier short, in addition to the other failure modes).
Pete

[Dingleberry]

My preference is before the rectifier (to protect the B+ winding from a rectifier short, in addition to the other failure modes).
Pete

Like this?
(picture 4)

-T

[martin manning]

Yes, one for FWB, two for FW. Look up R.G Keen's "Immortal Amp" series for more discussion.
https://tubeamparchive.com/viewtopic.ph ... 255#219255

[Bob S]

FWIW in my latest build I used wired fuses in both legs feeding the bridge rectifier & another between the rectifier & 1st resevoir cap.
Rating & type (slow blow etc.) of fuse are important.
Good luck - it's worth protecting something that should outlive you.

[gingertube]

Before the rectifier, in both legs of the AC feed is the best/maximum protection scheme.

For the centre tapped HV winding, 2 diode Full Wave arrangement do not use the single fused Centre Tap to ground arrangement, that gives no protection against shorted diodes and I have seen one destroyed power tranny in an amp with this arrangement (both diodes were shorted putting a short across the entire HV winding). It was a classic HiWatt SA212. I had to get the power tranny rewound to repair it.

Cheers,
Ian

[Dingleberry]

FWIW in my latest build I used wired fuses in both legs feeding the bridge rectifier & another between the rectifier & 1st resevoir cap.
Rating & type (slow blow etc.) of fuse are important.
Good luck - it's worth protecting something that should outlive you.

I've seen that arrangement in some boutique amps. Probably the safest route anyway. Got to rearrange things inside a bit, but I think I can fit them with no major modification. I'm building the amp from scratch and "designing" the layout on the fly (PTP with terminal strips).
What about the fuse ratings if used on both legs, comparing to one after rectifier (and probably first filter cap) with dc on it?
Thank's guys already!
-T

[Roe]

Before the rectifier, in both legs of the AC feed is the best/maximum protection scheme.

For the centre tapped HV winding, 2 diode Full Wave arrangement do not use the single fused Centre Tap to ground arrangement, that gives no protection against shorted diodes and I have seen one destroyed power tranny in an amp with this arrangement (both diodes were shorted putting a short across the entire HV winding). It was a classic HiWatt SA212. I had to get the power tranny rewound to repair it.

Cheers,
Ian

So the PT blew before the mains fuse blew? not good

[pdf64]

Maybe type 2 and type 4 then.
With type 2, a quick blow fuse could be used, as the next B+ cap downstream would be fed via the choke or resistor (probably >1k), thereby limiting surge current, so avoiding the necessity for an anti-surge fuse.
This fuse would give good protection from tube or OT faults, with the type 4 fuse protecting for rectifier/reservoir cap faults.

With regard to protecting the B+ winding of a full wave circuit, I had already fused the CT ground return in all my FW amps prior to becoming aware of the weakness that Ian has raised.
Plus, with an internal surface mounted fuse, it has the benefit of running at 0V.
So to retain the CT fuse but also mitigate for rectifier faults, I've added series diodes to each leg, as per RG Keen's immortal amp mods.

Bob, why fuse both legs of a B+ winding feeding a FWB?
Pete