HT fuse in a 5E3 - what value to use?

Keefy

About 3 years ago I built a 5E3 Deluxe clone from a @Modulus_Amps kit. I never use the Standby switch so rather than just bypass it I was thinking of replacing it with a panel-mounted fuse to protect the HT. Despite extensive Googling, I can't find a recommended value or type for this.

I have calculated an average current draw on the PT secondary of about 50mA, so would a 500mA fuse give sufficient protection against shorting, without risk of blowing all the time due to heavier start-up current? Quick blow or slow?

Thanks in advance.

Modulus_Amps

Good call, a HT fuse is the best mod to do to these amps, I normally fit one on the back panel though, but the standby switch location will work too.

The amp has a slow start up with the valve rectifier so heavy start up current is not as much an issue, A 250mA fuse would be fine if you are running it with 6V6's, and you may even get away with a T100mA.

This will save your power transformer if either of the power valves fails catastrophically, it wont save the PT if the Valve rectifier dies a horrible death

ICBM

A big problem with a 5E3 circuit is that a screen short - which seems to be a common failure mode for modern 6V6s - will burn out the choke resistor, at a fairly low current draw since it has a very high value. To have a chance of stopping this you really need a fast-blow fuse as well as the lowest possible value, so I would go for the lowest F value above the normal maximum current draw that doesn’t nuisance blow. I can’t remember off the top of my head but I would also guess around 100mA - easy to meter, just check both the start-up draw and at full power.

Keefy

Thanks chaps, that’s really useful

Merry Christmas!

Keefy

I measured the HT current at 65mA. Took the standby switch out, only to find that (1) the two old Bulgin fuse holders I had in my bits box take teeny tiny fuses and (2) I haven’t got any 100mA fuses anyway. Parts ordered...

Keefy

The parts came within a few days so I installed the fuse holder and fitted a 100mA fuse. The current draw doesn't vary that much but I will carry a 200mA fuse with me when I take the amp out, just in case!

Cirrus

This thread came in handy tonight - I'm wiring up the PT on my 5e3 build and was wondering what value fuse made sense. Though TBH the main reason I'm including it is for future testing/ troubleshooting - I don't want a standby switch, but sometimes it's nice to be able to turn off the HT voltage...

Keefy

I’ve been gigging that amp steadily for the last 2 years and that 100mA fuse has never blown.