As you've probably figured out already there are no instructions in the manual to cover this so knowing how attenuators work here is my opinion. The Air Brake is probably optimized for an 8 ohm load. As you apply more attenuation the amp will "see" proportionately more of the Air Brake's resistance and less of the speaker cabinet's impedance. So what that means is at the first click the total load to the amp will be around 3.5 ohms (estimated) and by the time you get to the last click and into the Bedroom settings the load will be close to the Air Brake's 8 ohm load. Fender amp output transformers will normally tolerate a 100% impedance mismatch. If I were using an Air Brake with that amp I would disconnect one of the three speakers in the Vibro-King's cabinet. That way the total load would be 4 ohms; an acceptable match with the Air Brake. Also the two remaining speakers would each be driven harder than would three and probably sound better.
Post by benttop (Steve) on Oct 9, 2016 6:33:10 GMT -7
If you click the Amp Specs link in my signature, then choose the Speakers tab (down at the bottom) there is a table of effective impedances provided by Mark Campo here on the forum. It doesn't address your specific question, but it gives an idea of what is going on with the Brake Lite when you switch the attenuator. If you follow the advice already given and use 4 ohms, you can see exactly what is happening with this table.
I'm pretty sure that you can. I would suggest getting with the folks at Dr Z to find out which speaker output to plug the Air Brake into. It would either be the 8 or 16 ohm speaker out.
Just out of curiosity I just measured my Brake Lite (sorry, I don't have an Air Brake) with a 16 ohm speaker plugged into it and at full atten. it reads 24.8 ohms. So I'll bet they are going to tell you to use the 16 ohm output jack.
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