i just wanted to know if anyone could clarify this for me:

recently, i hired for a gig a Marshall JCM 800 100 watt guitar head, running into two flat face Marshall 1960 4 x 12 cabs....i wanted to run the cabs in mono (giving me a supposed 300 watts per side as opposed to 150 watts in stereo) so i set the head to the 4 Ohm selector and ran each loudspeaker output from the head into the mono (4ohm) input of each speaker box. is this correct to run everything in mono 4ohms, or, as my friend said, should i have set the amp to 8 Ohms and ran into the same inputs on the cabs? sorry if this sounds confusing, just wanted some clarification....thanks in advance!

I can't say with absolute certainty that a multi-channel JCM800 has the speaker outs wired in parallel, but every Marshall I've worked on has all the output jacks in parallel, including the single-channel JCM800s. Assuming that's the case, if the cabs are 4 ohms, you'd need to set the output impedance selector to 2 ohms (which isn't an option)... because two 4 ohm cabs in parallel represent a 2 ohm load. So if that's what the speakers really are, out of the available choices, the amp should have been set for 4 ohms since it's the closest to 2.

Are you sure the cabs were 4 ohms each? I wouldn't expect that Marshall had it worked out so their cabs and heads are mismatched like that. But if the cabs have four 16-ohm speakers each, wired all in parallel, they would indeed be 4 ohm cabinets. If they were wired as a series-parallel hybrid, though, then they would be 16 ohm cabinets, which is what I would have assumed a Marshall cab to be.

If I wrote this shorter...

If you add two speakers (or two speaker cabinets) of the same impedance, in parallel, you get half as much, not twice as much. In series, it's twice as much. And the standard Marshall amp output jack wiring is parallel, at least what I've seen of their wiring from '69 to '79.

Paraphrasing Scott, if you run two cabs of equal impedance in parallel, you want the amp set at half of that impedance. In your specific case, it sounds like you're using the 1960 cabs that have a stereo/mono switch, which allows you to access two sets of two speakers each at 8 ohms per set, or all four speakers at either 4 or 16 ohms. In the latter case, one jack on the cab is for 4 ohm operation, and the other is for 16.

The short answer:

1) Set the head to 8 ohms

2) Set the cab switches to mono

3) Plug speaker cables from the head to each cab's "16 ohm" input.

-Tom

thanks for the help. i am indeed running from the head into the mono jacks on the cabs, so i should be running the head at 8 ohms not 4 ohms right? this weekend on tour with my band i ran two cabs (Soldano brand that only have a 16 ohm single jack option) into my hiwatt head and set the ohmage to 8 ohms....this is the correct setting to match the speakers, but i found that when i used to run the head at 16 ohms along with both cabs i found it to be both A) louder and B) a cleaner, more head room kinda tone......or in other words, it did not break up quite so early. is this normal?

anyone?

benjaminwayne wrote:thanks for the help. i am indeed running from the head into the mono jacks on the cabs, so i should be running the head at 8 ohms not 4 ohms right? this weekend on tour with my band i ran two cabs (Soldano brand that only have a 16 ohm single jack option) into my hiwatt head and set the ohmage to 8 ohms....this is the correct setting to match the speakers, but i found that when i used to run the head at 16 ohms along with both cabs i found it to be both A) louder and B) a cleaner, more head room kinda tone......or in other words, it did not break up quite so early. is this normal?

Hi benjaminwayne,

I know (and love) that Hiwatt of yours, so take good care of it or I may have to fix it some day.

The Hiwatt's speaker output jacks may not be wired in parallel, like the Marshall's. They could be wired in series. Some amplifier manufacturers did this so you could run multiple cabinets from your amp head without having to do any mental arithmetic.

There are ways and means of testing what the individual output impedances of each speaker output are. If you're in town, I can open her up for you and work it out. That way you can rest assured that you're not going to blow your output transformer (which is what will likely happen if you run a 16ohm amplifier into an 8ohm speaker load).