What fuse in 230v amp?

[talbany]

Your fuse should be rated for about 1.5 to 2X the "normal" primary operating current. You can work this out by adding up the (max average) power used by each of the secondary windings, then divide by the mains voltage. The primary fuse should never be larger than the mains voltage divided by the transformer's primary winding resistance.

Tony

[norburybrook]

Your fuse should be rated for about 1.5 to 2X the "normal" primary operating current. You can work this out by adding up the (max average) power used by each of the secondary windings, then divide by the mains voltage. The primary fuse should never be larger than the mains voltage divided by the transformer's primary winding resistance.

Tony

Tony,

Anybody, could you give an example of how you calculate this with say a twin PT with 320-60-0(400mA)-320 .....3.15-0 (7A) - 3.15 power transformer.

average power? watts? how is that calculated? I know P= V x I but how does that relate here?

TIA





M

[martin manning]

Tony is quoting from here, see under primary fusing: http://www.valvewizard.co.uk/fuses.html

Basically it’s energy rate (power) in equals energy rate out, or Vp*Ip = sum(Vs*Is), so Ip = sum(Vs*Is)/Vp.

[erwin_ve]

When going from 110v to 230-240v do you to have to consider the 60hZ to 50hZ main frequency ac? Long Time ago I had to calculate this, but I forgot how and the outcome.

[norburybrook]

Tony is quoting from here, see under primary fusing: http://www.valvewizard.co.uk/fuses.html

Basically it’s energy rate (power) in equals energy rate out, or Vp*Ip = sum(Vs*Is), so Ip = sum(Vs*Is)/Vp.

so could you do the figures then Martin form the twin PT Values on a 240v AC amp ?


M

[martin manning]

Yes so you are dividing by a primary voltage that is approximately twice as big, so the fuse size is halved.

[norburybrook]

Yes so you are dividing by a primary voltage that is approximately twice as big, so the fuse size is halved.

I actually meant could you put the numbers form the transformer into the equation so i could see how it worked on paper

so the current draw from the output valves has nothing to do with it then? and valve wizzard says 1A is fine up to 60w amps so one could deduce a 100w amp is fine with a 2A fuse then like I've been using it would seem, hence I've had no issues


M

[donvan]

So getting back to the OP’s original question using the info from this thread:

If Dumble designed an amp using U.S. power that called for a 5 amp slow blow fuse, the 230v European equivalent fuse would be roughly 2.5 amps.

Please check my math:
Ip = sum(Vs*Is)/Vp

Using U.S. power, solve for sum(Vs*Is)

5A = x/120V
X = 5 * 120 = 600

Now for European power:

Ip = 600/230 = 2.6 amps. I’m guessing the closest value would be 2.5 amps.


For a schematic that calls a 3 amp fuse using U.S. power the European equivalent would be 1.565 amps so a 1.5 amp fuse.

Using U.S. power, solve for sum(Vs*Is)
3A = x/120V
x = 3 * 120 = 360

Now for European power:

Ip = 360/230 = 1.565 amps


So in the end, it goes back to what Martin said: just halve the fuse value.

[donvan]

I actually meant could you put the numbers form the transformer into the equation so i could see how it worked on paper

so the current draw from the output valves has nothing to do with it then? and valve wizzard says 1A is fine up to 60w amps so one could deduce a 100w amp is fine with a 2A fuse then like I've been using it would seem, hence I've had no issues


M

Marcus,
The current draw DOES have something to do with it.
Not sure if this helps but Mr Valve Wizard has another article that helped me figure out my current draw:

http://www.valvewizard.co.uk/smoothing.html

[donvan]

That article I linked to tells how to calculate tube current draw but you also have to consider heater current draw.

[norburybrook]

That article I linked to tells how to calculate tube current draw but you also have to consider heater current draw.

it's all getting complex now...I'm happy with Half the fuse value



thanks for the info though guys, I learn something new everyday


M

[martin manning]

Fender ‘65 Twin Reverb (which has two channels, reverb, and tremolo) uses a 4A slo fuse for 120V, 2A slo for 230 and 240V, so there you go.

[talbany]

Tony is quoting from here, see under primary fusing: http://www.valvewizard.co.uk/fuses.html

By far the 2 best sources of info pertaining to tube amp design (guitar) IMO are both Valve wizard and anything by Randall Aiken and another example of invaluable info the Amp Garage has turned me on to.In the past 20 years of working on tube amps I am constantly re-reading them and passing on there info here. Bookmark these sites!

Aiken
https://www.aikenamps.com/index.php/tech-info

Valve Wizard
http://www.valvewizard.co.uk/

Tony

[hywelg]

When my Ceriatone OTS had a new transformer (hammond, cant remember the model no) it would regularly blow the fuse on startup. That was with 2a (240v) I then upped it to 2.5A and its been fine since.

[talbany]

Hammonds are known to have higher amounts of in-rush current than most other transformers at the same rating. I've sometimes had to over fuse them as well. They have also been known to run hotter.
.Ceriatone has also been known to have crappy filter caps!.
Tony