Cab ohms question with VH4

[geese_com]

Can I run an 8ohm cab and a 16ohm cab at the same time with a VH4 (not an S version). I have a Diezel 2x12 and am looking to pick up a 16ohm cab to go with my Marshall DSL100. If I ever wanted to run both cabs at the same time, I want to make sure I am not going to do any damage to my VH4.

Thanks.

 

[kahawe]

Better re-wire one of the cabs before you walk the dark path of ohm-mismatching...

 

[steve_k]

Yes, you can safely run them in the 4 ohm outputs. 1-16 and 1-8 = 5.3 ohm equivalent.

 

[geese_com]

Thanks for the replies guys. I was thinking that it was more complex than pluggin the 8ohm cab to the 4ohm jack on the VH4 and then the 16ohm cab to the 8ohm jack. I was just basing that on the "Speaker Impedance Martching & Hook-Up Guide - WIRING SCHEMES...Amplifier to Speaker Cabinets" section from my old Mesa Mark V manual.

http://www.mesaboogie.com/manuals/MkV_101005.pdf - see page 60 of 70 on the pdf.

(As you can see I am completely stupid when it comes to this stuff)

 

[SinDes1]

Here's a better way to do it and still keep the impedance matched -

16 Ohm Cab into one of the 8 Ohm jacks

8 Ohm Cab into one of the 4 Ohm jacks

Doing it this way allows you to run both cabs while each cab dissipates the same power and the valves get the proper impedance reflected at the primary. This works because of the different voltages present at the different taps of the output transformer.

 

[geese_com]

Here's a better way to do it and still keep the impedance matched -

16 Ohm Cab into one of the 8 Ohm jacks

8 Ohm Cab into one of the 4 Ohm jacks

Doing it this way allows you to run both cabs while each cab dissipates the same power and the valves get the proper impedance reflected at the primary. This works because of the different voltages present at the different taps of the output transformer.

Thanks for the advice! I actually did this last night at rehearsal. Everything seemed fine.

 

[SinDes1]

And it will.

Assuming you have a steady 100W output, the following voltages will be present at each of the speaker jacks -

16 Ohm Jack - 40VAC
8 Ohm Jacks - 28.28VAC
4 Ohm Jacks - 20VAC

When we connect the 16 ohm cab to the 8 ohm jack -

(28.28V^2) / 16 Ohm Cab = 50W

When we connect the 8 ohm cab to the 4 ohm jack -

(20V^2) / 8 Ohm Cab = 50W

As the math shows, each cab will see 1/2 the total power even though one cab is a 16 ohm cab while the other is an 8 ohm cab. This is what I was referring to when I stated "This works because of the different voltages present at the different taps of the output transformer".

 

[Peter Diezel]

This is main transformer math. The result depends
also on the feedback loop. In this case we need
some more math to calculate the AC response of
different speakers.

 

[geese_com]

This is main transformer math. The result depends
also on the feedback loop. In this case we need
some more math to calculate the AC response of
different speakers.

So are you saying that I probably shouldn't run the cabs like what I stated?

 

[Peter Diezel]

It will work like Sinister said. Please try also
to use both 4 ohm jacks. It just a matter of taste.
You cannot destroy the OT.

Best,
Peter

 

[geese_com]

Thanks for the advice Peter.

I guess I am going to have to test out both ways and see which one my ears like best.

What do you think I can expect from the two scenarios?