# Thread: Fuse Calc

1. ## Fuse Calc

I looked in a few books I have, but they deal more with fuse placement/purpose, and do not seem to get into calculating value.
For the Mains Fuse, do I use:
1. Heater current for each tube.
2. Plate current for each tube. With power tubes, is it the Zero Signal or Max Signal value that is used.? I assumed it is Max, but I am not really sure.
Thank You

the amp has:
3 x 6SL7
2 x 6K6
1 x 6X5

2. Here's how I suggest doing it:

Calculate the maximum wattage that your amp will consume. For example:

Heaters: 6.3V @ 5A = 31.5W (I picked 5A at random, your actual tube count might well use less)

B+: Voltage on first filter cap * (power tubes max signal current + preamp current) Say for a 6K6 amp: 350V * 0.1A = 35W

Total wattage: 66.5W

Now divide by your nominal line voltage: 66.5/120 = 0.554A estimated line current.

Now double it for luck - so in this case you would use a 1 amp fuse. The mains fuse is normally slow-blow too, to help it cope with the switch-on surge of cold heaters, empty filter caps and possible saturation of the transformer itself.

3. Engineers are people who calculate things to three (or more!) decimal places, then double or triple the answer for a safety factor.

I like that about us. 8-)

For anyone doing something like this, a \$25 Kill-A-Watt is a huge simplification. It can read line current or actual power, and although it can't easily capture power-on transients, it will tell you the actual current out of the power line quickly and easily by plugging between the wall socket and the line cord. It makes figuring the line current at max power easy. And, as Steve says, double max normal power, and use a slow-blow fuse to get past the transients.

4. Thank You.
This is for an amp that has not (God willing) functioned yet.
I am sore Steve's B+ assumption will get me off the ground.
I will 2x check that after I have a working amp.
Thanks Again

5. Rectifier power supplys typically have a poor power factor. That means they draw more AC current than the DC Watts delivered would suggest. You need to measure the RMS current under actual playing conditions. Then double that. A 5F1 uses a 2 amp fuse.

6. Originally Posted by loudthud
Rectifier power supplys typically have a poor power factor. That means they draw more AC current than the DC Watts delivered would suggest. You need to measure the RMS current under actual playing conditions. Then double that. A 5F1 uses a 2 amp fuse.

That certainly makes sense to me.
But, what I am trying to understand is, before an amp is working (a new model Meas Boogie for example) what math is used to determine the mains fuse value.?
If you build a prototype (for lack of a better hobby word) how do you choose a value for the Mains Fuse.?
Thank You

7. what I am trying to understand is, before an amp is working (a new model Meas Boogie for example) what math is used to determine the mains fuse value.?
check Post #2

8. Originally Posted by J M Fahey
check Post #2

Check post #4.

9. Originally Posted by loudthud
A 5F1 uses a 2 amp fuse.
In a 110VAC-125VAC Mains caintry.

But in a 220VAC-250VAC mains caintry, it only needs a 1A fuse (in case peeps from these caintries are reading this thinking they need a 2A fuse in a 5f1)

10. Heater current, and max signal current for the power tubes, can both be got from the tube datasheets. At least, the good old ones from RCA and Philips that listed example operating conditions. So you can apply my method before even building the amp.

The poor power factor of the rectifier and magnetising current of the transformer are valid issues. That's why I recommend doubling the result, but I think RG suggests allowing even more.

Mesa Boogers probably just use the same fuse they used in their last amp model, and for the first amp they ever made, copied the fuse value from the Fender Bassman.

11. Dear tubeswell, great advice.
Please post it also in English, for the benefit of those living in caintries other than Oz/NZ

12. Originally Posted by Steve Conner
Heater current, and max signal current for the power tubes, can both be got from the tube datasheets. At least, the good old ones from RCA and Philips that listed example operating conditions. So you can apply my method before even building the amp.

The poor power factor of the rectifier and magnetising current of the transformer are valid issues. That's why I recommend doubling the result, but I think RG suggests allowing even more.

Mesa Boogers probably just use the same fuse they used in their last amp model, and for the first amp they ever made, copied the fuse value from the Fender Bassman.
Of all amps to choose from.....
...no idea why I dragged Randal Smith into this.

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