Fuse Calc

[C Moore]

This is for a Mains Fuse.
I have not had to do this for a long time.....
Can you guys check the Math/Formula for me.?
Thank You

2x6K6
heat 800 mA
plates 64 mA

6X5
heat 600 mA

3x6SL7
heat 900 mA
plates 14 mA

heat 2.3A x 6.3V = 14.5W
plates 80mA x 300V = 24W

14.5 + 24 = 38.5W
38.5/120 = 320 mA

So in theory I would need a 320 milliamp fuse.
Would a Slo-Blo of 500mA be too risky.?

[Phil_S]

You didn't put the screens into the calc (maybe 2W). Still, I think you should be OK with 500mA slo.

[C Moore]

Good Eye.!
I did forget the screens.
Kind of hard to figure....looks like in Class A at 250V they are saying 5.5mA.
So that would add another 11mA to the mix.
I have a 315mA fuse, but it is of the "small"size. What is that called.? is that style 3mA.....or 5x20mm.?
Anyway.....I do not have a fuse holder for the smaller fuses. And the lowest value fuse I have in the bigger style is 500mA.
Not a big deal at the moment, as I am just trying to test a few things.
If you guys think 500mA is OK, then I have no worries.
Thanks

[Colossal]

3AB fuses for slow blow application
3AG fuses for fast blow application

[JazzGuitarGimp]

3AG type is 1 1/4" x 1/4" and it was the standard in the U.S. for decades. The 5 x 20mm size hails from Europe (I think) and has been popular in the U.S. for 15 - 20 years or so.

[Phil_S]

120V*500mA=60VA. If that's too much for your PT primary, then you are taking a chance. The fuse protects the PT on the primary side from overload.

[Bob S]

Just my limited experiences with this.
The current inrush to the PT when you power up is a major factor.
Not all slow blow fuses are created equal.
Nuisance popped fuses are a pia when the transformer is ok.
Good luck with 500mA.
Seems awful small.

[C Moore]

Just my limited experiences with this.
The current inrush to the PT when you power up is a major factor.
Not all slow blow fuses are created equal.
Nuisance popped fuses are a pia when the transformer is ok.
Good luck with 500mA.
Seems awful small.

When you say "small", do you mean for a circuit calculated at 320mA.?
I guess if it snaps all the time, I could just use whatever value comes next.?
Not sure if I have seen a fuse between 500 and 1 amp. is there one.?
thanks

[katopan]

It's fairly normal to size the fuse 2-3x the normal primary load current. That and using slow blow will stop the fuse popping inadvertantly. Contrary to popular belief the mains fuse is there to protect your house from burning down - the basic assumption is that your PT is toast anyway. In practice we would love the fuse to also protect the PT from a short on the secondary. Problem is you need a heap of info from the PT manufacturer to size the fuse properly for this, and it's info that you'll never get for an amp transformer. What you're after is the thermal current-time withstand curve of the transformer, then you'd set the fuse curve 90% under that. I've never seen those curves for appliance transformers, especially ones for valve amps.

[Bob S]

Maybe one of our gurus will chime in.
I use the total VA rating of the transformer for the calculation.
Then use that value or the next higher value slow blow fuse.
You can also protect the secondaries with the appropriate fusing.
HT & filament supplies.

[C Moore]

Per usual.....Thanks for all the info.
I understand most of what you guys are saying.
Sounds like I will start with the 1 Amp Slo-Blo and take things from there.
Thank You

[JazzGuitarGimp]

I think 1 amp is appropriate.

[C Moore]

Thanks

[pdf64]

Primary power must be higher than (total) secondary power, as the transformer can't be 100% efficient, there will be magnetic and electrical losses, most obviously manifesting as heat.
Pete

[katopan]

You normally allow for 80% efficiency even though the actual value is often closer to 90%.