Using Different Ohm Cabs - 3 Channel DR

[blacksmithjones]

I have a 4 ohm marshall cab and the 8 ohm recto cab. Am I correct in assuming when using both cabs that both should be connected to the 4 ohm outs.

 

[mikey383]

I don't recommend using both those cabs at the same time. The outputs are wired in parallel and it will yield a 2.6666 ohm load, which is pretty low and can damage your output transformer.

 

[blacksmithjones]

Thanks. I came up with the same 2.7 loading but I wasn't sure. Is it possible to re-wire the 4ohm cab (2x12 at 8 ohm ea) in series/parallel (not sure which one) to make it an 8ohm mono. If this is do-able I would get a 4 ohm load and then things should be safe. Is this correct and/or possible.

 

[MrMarkIII]

You need four speakers for series/parallel. You could re-wire the 2 x 12 to 16 ohms (series). That would give you 5.33 ohms total when paralleled with the 8 ohm cab.
Here's a calculator for speaker impedance matching:

http://colomar.com/Shavano/impedance_proc.php

Values entered horizontally are for series connections.
Parallel connections are entered vertically.
Be sure to show all your work. This will be on the final.

 

[mikey383]

Here's a general rule: If you have 4 speakers of a certain impedance (say 4 ohms), your total load will be 4 ohms due to series/parallel wiring. Now, you could wire it up for 16 ohms (series/series), but you'll never get an 8 ohm load from four 4 ohm speakers. To do that, you'll need to wire the upper 2 speakers in series, plus the bottom 2 speakers in series, and then wire those sets in series.

http://www.colomar.com/Shavano/4x12wiring.html

Check out the first diagram.