adding a PS fuse to a widowmaker

[Randall]

I am wiring in an iso transformer in a Harmony H400 amp from 1965 I believe. I also want to put in a line fuse per this drawing, but I am not sure 3 amps is the right value. If I am correct, what would be a better one?

http://www.geckoamps.com/design_ideas/h400-iso.pdf

[Jazz P Bass]

ISeeing that the schematic indicates that the amp 'pulls' 25 watts (0.25 Amps) in use, the 3 amp fuse is confusing

Maybe they are covering for inrush.

Suggest that you monitor the current requirements with a meter.

Pick a fuse accordingly.

[Randall]

To be clear, the original amp had no PT nor fuse. This drawing is someone else's mod. I wired mine up the same way, but it doesn't seem like a 3 amp fuse is right. I have a 1/2 amp in it now.

[J M Fahey]

1/2 amp looks fine.

[R.G.]

The line fuse has to withstand the inrush surge on the new isolation transformer, and then the power used by the amp. The schemo rating says it pulls 0.25A without the new isolator. The isolation transformer won't add more than another 5-10% to that. All you have to cover is the inrush.

I'd start with 1A slow blow.