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I felt like I had a good grasp on this, but I'm doubting myself at the moment and hoping someone can be answer my stupid question so I can have clarity.

The Marshall JCM900 heads and the JCM2000 TSLs only have outputs for 8ohm or 16ohm.

From my understanding with most amps, if they have 2 x 8ohm outputs and you connect to 1 8ohm cab to each output, the impedance in parallel should be 4 ohms this should be fine.
(I have had a few amps that had the proper 4/8/16 ohm outputs that said to set the output to 8 ohms if running it this way, and set to 4 ohms if only running off of one of the outputs and using the in/out on the cabinets).

The Marshall manuals are less clear and have me doubting my understanding, so my question is: can I run the 2 x 8ohm cabs from either of those Marshall heads, using both speaker outputs and set to 8 ohms? Or will that risk blowing something up? (my current living situation has me using 4 x 1x12 cabs rather than my normal 4x12s).

Side note: I think it is dumb that the JCM900 transformers are capable of 4/8/16 but they chose to do a 2-way switch instead of a 3-way to cut costs.

Thanks.

Your amp has 2 or 3 speaker jacks?
My DSL50 has 3: a single 16 ohm jack and a pair that services either a total load of 8 or 4 ohms, depending on the setting of the switch. *as you are already aware, two 8 ohm cabs in parallel is 4 ohms, so set the switch to 4 ohms, plug into those two jacks and rock on!*
If it's a 2 jack output, the total load (for both jacks) is set by the switch. Heads go between 16 or 8 ohm loads (they assume you are using two 16 ohm cabs), the combos 8 or 4 ohms (based on an internal 8 ohm speaker and 8 ohm extension cab). IF it's a head and you have two 8 ohm cabs, no go. I don't mess with impedance mismatches on Marshalls. I don't feel the transformers are wound robust enough to adequately handle the heat from a mismatch and can melt and short out.
Older Fenders and Mesas, mismatch all day. Fender just overbuilt the amps and didn't care, Mesa recommends mismatches in the manuals for tone chasing.

bigtone23 wrote: ↑

Wed Apr 23, 2025 6:39 am

Your amp has 2 or 3 speaker jacks?
My DSL50 has 3: a single 16 ohm jack and a pair that services either a total load of 8 or 4 ohms, depending on the setting of the switch. *as you are already aware, two 8 ohm cabs in parallel is 4 ohms, so set the switch to 4 ohms, plug into those two jacks and rock on!*
If it's a 2 jack output, the total load (for both jacks) is set by the switch. Heads go between 16 or 8 ohm loads (they assume you are using two 16 ohm cabs), the combos 8 or 4 ohms (based on an internal 8 ohm speaker and 8 ohm extension cab). IF it's a head and you have two 8 ohm cabs, no go. I don't mess with impedance mismatches on Marshalls. I don't feel the transformers are wound robust enough to adequately handle the heat from a mismatch and can melt and short out.
Older Fenders and Mesas, mismatch all day. Fender just overbuilt the amps and didn't care, Mesa recommends mismatches in the manuals for tone chasing.

Thanks. The TSL and 900 only have 2 speaker jacks. The DSL was different in that regard.

My Hartke 5000 model bass amp is an example of one that has bi-amp 8ohm outs but was designed to go into a pair of XL 8ohm cabs (and the manual says to do this), so I got a bit uncertain about how my Marshalls would do, but I will just stick to running them with one cab. I should have my old rig back later this year or sometime early next year.

This might be something worth looking into.
I have found many amps have all 3 taps on the transformer, but only 2 connected to speaker jacks.
This could be a simple wire swap to get the 4 ohm load you desire..?