Help, 1st switchable OHM Ext Cab settings


Senior Member
Oct 8, 2011
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I found a B-52 ST212s on craigslist for $175 on craigslist this weekend, so pretty cheap score. Its constructed of 13 layer birch ply and comes loaded with 2 celestion (Chinese) V30. Not the best, but certainly could do worse for the money.

Anyway it has a mono (8 ohm) and stereo (16 ohm) with one switch to go between impedance and 2 input jacks.

I can't for the life find any other example of this kind of switch online it seems to be 8 ohm stereo 16 ohm mono.

I run a Marshall 401 with a 16 ohm stock V30 speaker.

I think I'd like to try to run the cab in 16 and the on board speaker in 16, so I can hook them both up at 8ohm.

If I understand it correctly the switch needs to be plugged in on stereo mode (16 ohm), but does it matter with input jack I plug into?



Marshall & Moonshine

Senior Member
Jul 21, 2010
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There are two 16ohm speakers in the cab. Running stereo means you can plug two different amps (set to 16ohms) into the back of the cab, one amp for each speaker. The Mono jack places both 16ohm speakers in parallel for an 8ohm load.
So, your choices (if you want to match impedance, which is another conversation...) are to run the internal speaker in parallel with ONE speaker from the cab, and set your amp to 8ohm, or just run the cab in Mono and disconnect the internal speaker.
Now, most people say a rule of thumb for matching impedances is that you can go one step in either direction with very little risk. If you want to run all three, you could probably run all three speakers on the 4ohm setting (or 8ohm) as you'll have a 6ohm (I think---I'm a bit tired...) speaker load. That SHOULD be plenty close enough.
So the internal speaker (16ohm) plus the Mono cab (8ohm) in parallel makes a 6ohm load.

Big John

MLP: mid life pimple
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Mar 18, 2007
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M&M nailed it.

Dave, that cab wasn't the right choice IF your goal was to run all 3 speakers at once. Doing so will present a 5.33 ohm load to the Marshall. Since your particular model only has 16 or 8 ohm selections, your output transformer will be at risk.

Again, this is if you want to run all 3 speakers at once. Unplugging the combo's speaker and using just the 2x12 or just using one of the 2x12's speakers with the combo's speaker are your only safe options, IMO.

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