Fuse Sizing for Amp With Crazy Heater Current

[The New Steve H]

I forgot something. The rectifier current comes from the PT at 275 V, so it's going to be less on the way into the PT at 125.

(125/275) x .125 mA = 57 mA, right?

I was trying to avoid adding up the potential currents from all the tubes, but I guess I didn't save much effort.

Anyhow, now it's looking like around 1.2 A @ 125 V, total, unless I messed something else up.

[Roe]

scandinavian amps often fused the heathers. two ac50s I saw recently both used 10A for two el34s and 4 ecc83s

[husky]

scandinavian amps often fused the heathers. two ac50s I saw recently both used 10A for two el34s and 4 ecc83s

The problem with that is the fuses and surrounding can get VERY hot! Usually you need to use a very low resistance fuse and check for heat around the connections. No Slo Blo, Canadian Marshalls fused the heaters and it was stupid as the fuse holders would unsolder themselves from the PC board if a slo blo was used.

[wsaraceni]

so what's the math involved to pick the proper mains and HT fuse sizes?

figure for an amp with a pair of el84s, 3 12ax7s and an ez81 rectifier?

[Cliff Schecht]

For a pair of EL84's I like about 120mA (at full power and including the preamp tubes). Let's choose a B+ of 300V because that's where I like 84's at.

For the heaters you have 2 x 0.76A (EL84's), 3 x 0.3A (AX7's) and 1A (EZ81) giving us 3.42A total, all at 6.3V.

So 300V*120mA=36W
and 6.3V*3.42A=21.546W
This gives us 57.546W total.

Divide this number by the primary winding voltage to find your primary winding current.

In this case we have 57.54W/120V=~0.48A in which case I would use a 0.5A slow blow fuse or a 1A fast blow fuse.

I don't ever fuse the HT in my amps but typically you just put a 500mA fast blow fuse here. You could go smaller but remember that peak current and RMS current are two different things and we typically work with RMS values (this simply means that peak currents can be higher than RMS current so don't be fooled).