run 4 ohms into 8 ohm cab?

The power rating applies when hooked to a 4 Ohm cab.
Usually you can use any cab that has an impedance of 4 Ohm or higher (some amps handle loads of 2 Ohm, but 4 is the standard).

It's no problem at all to hook a 8 Ohm cab to your amp, but it'll deliver about 30% less power than @ 4 Ohm.

 

Your amp will put approximately 150 watts into the 8 ohm cab. No damage, no worries.

Caveat: this assume a solid state amp. If a tube amp, you must have an 8-ohm tap to run an 8-ohm load.

 

What do you mean with tap??

 

You do not 'run Ohms into a cab'. Ohms is the measuring unit for the impedance of the cab.

An amp puts out power (Watts). The higher the impedance of the cab hooked to the amp, the lower the power output. (The cab 'impedes' the flow of current, so to say)

And if the impedance of the cab(s) is too low, too much current will flow and the amp will smoke off sooner or later.

With all-tube amps things are a little different: The cab has to match the head exactly. To be able to run the amp at different loads, there is either a selector switch or different speaker outputs for 8/4/2 Ohms.

Matthias

 

Tube power amps had different transformer taps (connections) that allowed the tube output to run with various different speaker impedances. The intent is to closely match the power amp impedance to that of the cabinet.

A solid state power amp has no taps. It is designed to operate with a specificed minimum impedance, and works fine with impedances higher than the specified minimum. For example, my QSC minimum impedance is 2 ohms. I run an 8 ohm driver and it is fine. If I had a tube power amp, I would use the 8-ohm tap to connect my 8-ohm cabinet.

 

THANX!!!