Impedance again

[shafty2.3]

A D rated valve amp that has an output rated in both 8 and 4 ohms. What are the advantages/disadvantages of pairing with 8 or 4 ohm cabs?

[Phil Starr]

Your question doesn't make total sense. Which amp are you talking about?

The output from a valve amp goes through a transformer which is designed to match the speakers. Depending upon the amp the output may be matched to 4,8 or 16 ohms or even all three. You may also have just a four ohm output with two sockets, plug in two 8ohm speakers into these two sockets and you have a 4ohm load. (so that's [b]either[/b] one 4ohm [b]or[/b] a pair of 8ohm speakers)

You'll only get full output if your output and speaker impedances match. A fifty percent mismatch isn't going to break anything but will reduce the power available.

You need to read the manual to check which situation you are dealing with.

[Bill Fitzmaurice]

There are no advantages or disadvantages. If you have am 8 ohm cab use the 8 ohm tap, if you have a 4 ohm cab use the 4 ohm tap. If you want to use two cabs then they'd have to both be 8 ohm, which when run together would go on the 4 ohm tap.