Adding HT Fuse

[UltraHookedOnPhonix]

I’d like to add a HT fuse to my amp for added protection.

Current power supply specs: PT is 345-0-345 @ 350mA, 6.3VAC @ 7A using the non-precision power supply running two EL34s. Main filter caps are placed before the standby.

From what I’ve understood there’s two places where I can put a fuse. One is from the B+ center tap to ground and the other is after the rectifier/before the standby switch like a Sovtek MIG-100H: http://www.schematicheaven.com/newamps/ ... ig100h.pdf

I’ll use a fast-acting fuse but the question is: what amperage?

Also, why do so many builders not use these? Seems like a good idea if a power tube were to fail, threatening your OT etc.

[fp2000]

Use one along with the stand-by switch. I don't know if the other way you mentioned would work? Hmm, center tap to ground? Remember the current flows (theoretically) from + to -. So you want the current to flow through the fuse before it burns everything else.
Of course, adding fuses is all for protection in a theoretical world. It should work.
I always put very good transformers to make sure I protect those high quality fuses in case of a short somewhere.

F

[bnwitt]

The center tap position will work and has been implemented by many manufacturers. If you think about it , it's a pretty good solution but the pre-standby location is fine too.
With 350ma being your H.T. operating current max, a 0.5 amp 250 volt 3AG fuse will be fine. Don't worry about the 250 volt rating as these are used in many major brand amplifiers. If you get nuisance blows with the 0.5 amp fuse due to spikes (like at start up), you can go to a 0.75 but I'd try the smaller fuse first.

Also, why do so many builders not use these? Seems like a good idea if a power tube were to fail, threatening your OT etc.

Most manufacturers rely on the main fuse to do the job. I would assume that many manufacturers added H.T. fuses after some tube related damage caused by tube failures in the warranty period cost them some dough. If you think about it, a different size and blow action (fast or slow) fuse could be used in several stages of an amp to give ultimate protection from tube failure that causes collateral damage.

I don't see what that 2 amp fuse in the schematic you posted would do. It is too big to protect the PT's H.T. winding. I would think you'd have a lot of damage before it blew in that amp. Marshall 100 watt amps use a 750ma H.T. fuse for example. So does the Komet 60.

[heisthl]

One minor drawback on the H.T. (high tension) fuse is the pilot light stays on when the fuse is blown, so you have to educate the end user. I like to use them in amps that don't have a center tap for the heaters!!! That might sound stupid but depending on why the tube is bad, a bad tube more times than not will burn those ground reference resistors before it takes out the regular fuse or damages the O.T.

[UltraHookedOnPhonix]

Guys, thank you very much for the replies. Very informative! I'll start of with a .5A 250V on the CT.

I found a quote by Ken Fisher on the HT fuse:

Certain amplifiers may have an HT (HIGH VOLTAGE) fuse. Even when unplugged, these fuse holders may contain high voltages.
Use caution when changing the HT fuse. ***NEVER*** touch the metal end of the HT fuse with your bare fingers!

I guess it's a good idea to take a look at the Littlefuse shock-safe holders: www.littelfuse.com/data/en/Data_Sheets/ ... ckSafe.pdf

[llemtt]

A better protection makes use of two fuses from the high voltage windings before the diode strings. A single fuse from the ct can't protect from a short between the two windings.

Put a plastic safety cover on your fuse holders too.

my .02
teo

[UltraHookedOnPhonix]

So, there's essentially three ways to do it:

1. Secondary's CT to ground.
2. Placed in series with full-wave rectifier and Standby switch.
3. On PT secondaries before rectification.

Many ways to protect!

[heisthl]

A better protection makes use of two fuses from the high voltage windings before the diode strings. A single fuse from the ct can't protect from a short between the two windings.

Put a plastic safety cover on your fuse holders too.

my .02
teo

Help me out by giving me a senario where this could actually happen that having 2 fuses would matter.

[llemtt]

Not considering human error the only event I can think of is diodes shorting (reverse breakdown), unlikely but...

[heisthl]

Not considering human error the only event I can think of is diodes shorting (reverse breakdown), unlikely but...

Shorted diodes usually act like a fuse in that they puff smoke, split open and no longer conduct. There must be a better reason to even consider adding 2 H.T. fuses and I can't think of any. Has anyone actually seen an amp with dual H.T. fuses?