Hooking up MarkV combo to 2x12 extension

[cmpolk08]

Hello all,
New to the site so sorry if this post is not allowed. I have a Mark V combo and last night purchased a 2x12 three quarter back cab. I used to be very good at this but been put of the game for awhile so just trying to figure out the best way to hook this up. With a normal combo I would disconnected the speaker to the combo to avoid mismatching impedence but this cab also includes a switch to turn the speaker off. So should I leave the speaker turned off but plugged in and hook the cab to the 4ohm outputs? Speaker has 2 8ohm connections. Or what would be optimum? Bonus points, I have a 4x12 three quarter back as well that I will be selling but it would be cool to be able to hook them both up just for fun until I get it sold. This can has 2x 4ohm connections and an 8ohm but as far as I can tell the 8ohm connection does not work.

 

[domct203]

Welcome to the forum!

Is your 2X12 8 ohms? Is your combo speaker 8 ohms? If yes, move the combo speaker to a 4 ohm jack, plug the 2x12 into the other 4 ohm jack. Two 8 ohm loads in parallel is 4 ohms.

For the 4X12, the 8 ohm jack is usually all 4 speakers, the 4 ohm jacks are 2 speakers each, left and right pairs.

You can’t plug the two 4 ohm jacks in, that would be 2 ohms in parallel. Mesa output transformers are strong but don’t risk that low of an impedance. You would need to run the pairs in series to get an 8 ohm load, that’s how the 8 ohm jack work.

If the 8 ohm jack was working on the 4X12 you would do the same as with the 2X12.

Have you tried cleaning the 8 ohm jack with a good contact cleaner like DeOxit? Just be careful testing the cab, no speaker load to a tube amp is bad.

Dom

 

[Sinner's Swing!]

That's actually a good question. I wonder if it would be ok to just use that speaker off toggle if you had another 8 ohm or 4 ohm load. The stock creamback speaker in the combo is 8 ohms so if the jack isn't working, then the speaker in your combo is not working, or you have it plugged into one of the 4 ohm jacks?