impedance question

[opus]

i am plugging an 8 ohm cab into an amp that wants 4 ohms (ab763 bandmaster), how is that affecting my tone/headroom/etc? thanks for the help.

[da-boogieman]

i am plugging an 8 ohm cab into an amp that wants 4 ohms (ab763 bandmaster), how is that affecting my tone/headroom/etc? thanks for the help.

Should have a greater voltage swing so less headroom. Weber says browner sound. O'Conner says potential for greater bandwidth. I don't know which is correct but I'm inclined to side with O'Conner unless and until I can work through it myself.

Output tube life improves to some degree and the iron may run a little cooler...good things.

You probably know some folks object to the practice of double impedance because of the higher flyback voltage. I'm not one of them.

Enjoy the experimenting...

[John Phillips]

Irrespective of the theory about bandwidth, the tone becomes more midrangy... you just have to listen to it. You get a flatter, tighter, 'drier' type of tone with less deep bass. You also get a little less power and headroom so you'll need to turn up further to get the same final volume, and the sound will be more distorted.

Low mismatches do almost the opposite - you still get less power, but now you get a squashier, flubbier, softer tone with less top-end.

The amp will probably be fine with either a high or low mismatch of up to a factor of two (ie half or double the correct impedance). Don't push it any further than that no matter what anyone says about theory - high mismatches can and do blow things like output transformers or tube sockets; severe low mismatches stress the tubes heavily.

(I suspect a lot of the theory applies to clean sounds - high mismatching is quite normal for hi-fi designs, and does seem to produce a more 'linear' sound. But these amps are never under any circumstances driven into distortion or even close to it.)

[tmac]

My Bassman head seems to like an 8 ohm cab rather than 4 ohms but I just may be hearing more speaker/cab difference than anything. I do agree that it seems midrangy/browner and a bit less bass. It does sound great though!:BEER