Ohm Sanity Check...

[binky_bass]

Right, 

This is a topic I know has gone round the houses, but I just want an absolute sanity check on my math to make sure I don't do anything stupid and kill a rather expensive amp straight out of the box!

So, the head in question is a Mesa Boogie Strategy Eight:88.

The cabs... PJB 12B and PJB 6T.

The 12B is 6 ohm and the 6T is 12 ohm.

I believe the equation for mismatching ohm cabs is as follows: (cab 1 x cab 2) / (cab 1 + cab 2)

For these cabs this would be 6 x 12 = 72 divided by 6 + 12 = 18, which in my head gives a joint impedance of 4 ohms. 

So first sanity check is: Is my math right? Is a 6 ohm cab run in parallel (daisy chained) with a 12 ohm cab going to then run a joint impedance of 4 ohms? 

The second sanity check is with the amp... the picture below shows the speaker inputs on the Mesa, the duel 4 ohm inputs us what is throwing me slightly... can I just simply daisy chain the 2 cabs (if my 4 ohm math is correct) and run them into one of the 4 ohm outputs? Or should I run the 6 ohm cab separately into one 4 ohm output and the 12 ohm cab separately into the 8 ohm output? 

I'm thinking daisy chaining and running into one 4 ohm output is the right option. 

Can anyone give me some absolute clarification on this please! And sorry if this is old hat, but I want to really be 100% on this.

Cheers!

Russ.

 

Edited December 30, 2019 by binky_bass

[nikon F]

 6 and 12 ohm cab in series would give you 18 ohm ,,,,in parallel it would give 4 ohms,,,,,,daisy chaining cabs dosent generally make it running in series ( it depends on how the cabs are wired up ) but generally cabs are wired for parallel .so daisy chaining them would give 4 ohms

[HazBeen]

In series you stack resistance, but daisy chaining is the correct way as cabs are wired in parallel. You get 4ohms, and the 4ohm output is the correct one to use, however do keep in mind that 2/3 of power goes to the 6ohm and 1/3 to the 12ohm cab. This is not ideal.

Oh, and you plug both into the 4ohm outputs.

Edited December 30, 2019 by HazBeen

[binky_bass]

Amended to correct my series/parallel blunder...

@HazBeen, given the outputs on the Mesa and the inputs on the cabs (pic below) what would you suggest is the best way to run both cabs into the head safely? Link one Mesa 4 ohm output to the input of the 6 ohm cab, and link the 6 ohm cab to the 12 ohm cab? 

2/3 power going to the 6 ohm and 1/3 going to the 12 ohm sounds like that's be what I want as the 6 ohm cab is the 12B and the 12 ohm cab is the 6T.

Edited December 30, 2019 by binky_bass

[HazBeen]

8 minutes ago, binky_bass said:

Amended to correct my series/parallel blunder...

@HazBeen, given the outputs on the Mesa and the inputs on the cabs (pic below) what would you suggest is the best way to run both cabs into the head safely? Link one Mesa 4 ohm output to the input of the 6 ohm cab, and link the 6 ohm cab to the 12 ohm cab? 

2/3 power going to the 6 ohm and 1/3 going to the 12 ohm sounds like that's be what I want as the 6 ohm cab is the 12B and the 12 ohm cab is the 6T.

Yes, I’d agree. Snce the Strategy has a jack output, I would daisy chain the two cabs with speakon, and then run a speakon to jack into the strategy (so only one jack into the 4ohm jack output)

Edited December 30, 2019 by HazBeen

[binky_bass]

So Mesa 4 ohm output to 12B input with a jack to speakon, then 12B 'link' to 6T input with speakon to speakon. 

That is exactly how I've been running the cabs into my previous head, and how I would have run them into the Mesa without seeking clarification, so I'm glad that does indeed appear to be right!

Always good to seek a second opinion on these things, much appreciated!

 

Edited December 30, 2019 by binky_bass