Amp rating for ohm select switch in the cab?

[CitizenCain]

I found a nifty wiring diagram for a 2 speaker cab that uses a DPDT switch to change between series and parallel wiring.

No recommendation for the size of the switch on the drawing. I have a switch here that's rated 3a @ 125v. Would that be big enough, do you think?

My amps are in the 20-50w range.

[Blue Strat]

That switch should be more than enough.

[CitizenCain]

Awesome!

Thanks, Mike :D

[John Phillips]

Here's how to work it out: power = current-squared x impedance

So the current is the square root of the power divided by the impedance. The worst case is always with the lower impedance, and with the amp fully cranked you can expect up to double the rated power, so for a 50W amp (100W max) into 4 ohms, you get - sqr(100/4) = 5A.

This is an absolute worst-case maximum, and since switches are fairly conservatively-rated anyway, a 3A switch is probably enough.

Be careful though - if the switch fails, or even develops significant contact resistance, the amp will see an open circuit and you'll blow something. I don't like the complex switching on the modern Marshall cabinets for exactly this reason - the signal has to pass through not only a DPDT slide switch (four contact points) but also through the switches in the two jacks too (another four contacts), a failure in any one of which will leave the amp with no load. I suspect this may be a significant cause of the blown transformers modern Marshalls seem prone to, especially as the jack contacts tend to get bent if the cable into the cabinet is tugged or the plug pushed sideways. At least a heavy-duty DPDT switch (even only a 3A one) is a lot more reliable than that.

[CitizenCain]

I think this switch is pretty heavy-duty. It's sure not designed for looks :) It's definitely got an industrial look and feel to it.

Thanks!