Fuse question--SF-VC export conversion

[Ben N]

I am switching the power transformer in my SF Vibro Champ to an export-style PT, switchable 120v/220v/240v primary. The original fuse is a 250v 1A slow blow. What should I put in for 220v/240v operation, and is that safe when I use the 120v tap? Should there be separate fuses for each tap, and if so, where can they go?

TIA,
Ben

[Tom Phillips]

When switched to 240V line operation you would use a 1/2 A Slo-Blo fuse for the Vibro Chanp.
It is not usual practice to include separate fuses when changing the line voltage. Just change the fuse value in the existing fuse holder.
There are additional line input circuit changes. The attached schematic shows the Fender USA version and "Export" version side by side.
Note that the document shows the old style USA version which used the 2 conductor line cord and ground switch. There are many discussions in the forum which discuss upgrade of that configuration to modern standards.
Also note that the layout page of the attachment does not show the "Export" wiring.
Vibrochamp AB764 W Export Line Ckt.pdf

[bob p]

> and is that safe when I use the 120v tap?

In case Tom's answer wasn't clear, that would be No, one fuse would not be good for both voltages.

1 amp for 120 VAC only.
1/2 amp for 240 VAC only.

P = IV

[Ben N]

Thanks all. That schematic makes it all much clearer--including the fact that I didn't have to buy a new transformer, since my VC has the 010020 with the Euro and NA voltage taps on the primary. :headsmack: Doh! (It's ok, I have another Princetonish amp that can use this one.)
Two more questions:
1) Can I use the existing DPDT slide switch (currently wired as SPST) as well?
2) How does Ohm's law help me with fuse values?

[Enzo]

Your amp consumes X amount of power running and playing. If we want to fuse it for 120 watts, then that means 1 amp at 120v or 1/2 amp at 240v. The circuit and operation are not different for either mains voltage, only the mains voltage and current change. Power is current times voltage as bob points out, so 120 x 1 or 240 x 1/2.

Clearly the amp is not expecting to draw 120 watts from the wall, or we'd be blowing the fuses. But converting 120v to 240v and back for fusing, it is a simple 2 to 1 ration. (Or 1 to 2 is we go the other way)

[pdf64]

Assuming that it’s still working ok on 120V, the power switch should also be fine at 240V.
The current and voltage ratings should be noted on the switch somewhere but to see them it may need removing.
However I doubt Fender used different switches for the domestic and export models.

It’s an old switch though, and sometimes those slide switches don’t have a very positive latching action eg they can be put in a ‘halfway’ position. That could cause problems so make sure to check its action in the light of the above.

[bob p]

> and is that safe when I use the 120v tap?

In case Tom's answer wasn't clear, that would be No, one fuse would not be good for both voltages.

1 amp for 120 VAC only.
1/2 amp for 240 VAC only.

P = IV

In case my answer wasn't clear:

The designers of your amp decided that a fuse needed to blow at a power level of 120 VA to prevent a house fire. That 120VA rating is going to be the same no matter where in the world the amp will be operated. For use in the USA they determined which fuse to use by solving for I in the following equation:

P = IV
120 VA = I * 120V
120 VA / 120 V = I
I = 1 amp

And for export to a 240 VAC country they decided:

120 VA = I * 240V
120 VA / 240 V = I
I = 1/2 amp

In practice, we don't know the VA rating of the transformer, it's not stamped on it. So we determine the VA rating by looking at the fuse value printed on the fuse socket or schematic and do the math:

In the 120V case you have a 1 amp fuse:

VI = P
120V x 1 amp fuse = 120VA

Now we know that the fuse was selected to blow if 120VA is pulled by the transformer. (To keep things simple, I'm going to assume power factor is 1, or at least it remains constant, so you don't see it in the equation. If you don't know what power factor means, just ignore it.)

Ohms law tells us that if you switch to 240 VAC the existing 1 amp fuse would blow at:

240 VAC x 1A = 240 VA

That 240 VA figure is twice the intended value of 120 VA. If you keep the same 1A fuse when you switch over to 240, the fuse will blow late. You don't want that. You want it to blow at 120 VA, so to determine the size of the fuse that you need:

P = IV
120 VA = I * 240V
I = 0.5A

[Ben N]

Thanks for the very clear explanations.