Connecting an external 8-ohm cabinet to a Mark IV

[rottenchops]

Hi, everyone. First, my apologies if this has been answered already and I've failed to find it...

I've got a Mark IV combo to which I would like to connect an external 8-ohm cabinet, while keeping the internal speaker connected (to the 8-ohm output).
The external cabinet in question has 2 jacks which are wired in parallel.

My question boils down to: what would be the best way to make use of this cabinet (remember, I'd like to keep the internal speaker connected if at all possible)?

Here are 2 scenarios that come to mind:

1. Connect the cabinet to one of the 4-ohm outputs and deal with the high-side impedance mismatch.

2. Not sure if this is possible, or just stupid - connect both 4-ohm outputs on the amp to both jacks on the cabinet. Would this in effect present a 4-ohm load to the amp?

From what I gather from the Mark IV manual, the two 4-ohm outputs on the amp are internally wired in parallel but I'm not clear on how the 8-ohm output (for the internal speaker) is wired in relation, if at all. For example, if I connect an 8-ohm cab to each of the 4-ohm outputs, as recommended by the manual, can I leave the internal speaker connected to the 8-ohm output and still be perfectly matched?

I hope I'm making sense... Thanks in advance!

 

[trem]

Welcome to the board. In opening, I suggest you read the manual more thoroughly.

In your scenario, plug the internal speaker to one of the 4 ohm output, and the external cab to the other 4 ohm output. Since the outputs are wired in parallel, and since both the cab and the internal speaker are 8 ohms, the total load is 4 ohm, matching the output transformer tap.

 

[rottenchops]

Thanks, I hadn't thought of that.

For the record, I've read the manual quite thoroughly many times since I bought the amp 18 years ago... having just acquired a new cab today, I have not given this particular situation any thought - I haven't needed to.

You could have replied without the snide attitude, but hey, whatever makes you feel superior.

 

[rottenchops]

Still looking for an answer to the second part of my post. Trem, don't bother if you're going to be a dick...

From what I gather from the Mark IV manual, the two 4-ohm outputs on the amp are internally wired in parallel but I'm not clear on how the 8-ohm output (for the internal speaker) is wired in relation, if at all. For example, if I connect an 8-ohm cab to each of the 4-ohm outputs, as recommended by the manual, can I leave the internal speaker connected to the 8-ohm output and still be perfectly matched?

 

[trem]

Sorry, didn't mean to offend you in any way. That was written without any attitude in mind. Internet is horrible in conveying emotion, or the lack of it.

 

[rottenchops]

No worries... it's late here and I'm tired and irritable...

Thanks again

 

[Mark Fore]

Good on ya Trem for helping a newbie out. That is a pretty important Mesa lesson with the 2 x 4 ohm wisdom nugget. It's like everyone remembers where they were when JFK was assassinated. For us I remember where I was when I learned that 2 x 8 ohm cabs go into the 2 x 4 ohm jacks.

 

[MrMarkIII]

Plugging an 8 ohm cab into each of the 4 ohm jacks and leaving the 8 ohm internal speaker connected results in a total in a total of 2.67 ohms. Not fatal, but don't go lower. The sound will get "thinner", too. You can experiment with how the sound changes by just switching the internal speaker between the 8 and 4 ohm jacks.
From the Mark III Manual:
"One 8-ohm and two 4-ohm jacks are provided.
The Boogie is not very sensitive to speaker mismatches
and will not be damaged by them except that very low
ohmage loads will cause the power tubes to wear out faster.
A single twelve-inch 8-ohm, speaker should
generally be connected to the 8-ohm output on the amp.
When using two 8-ohm speakers, connect them both
to the 4-ohm outputs provided
(because the total load is 4 ohms in that case)."
QED