TA-30 OHM configuration for Marshall cabinet?

Hi, I just bought a TA-30 head which I love!

I already had a Marshall 1960a 4x12 cab and have decided to use this with the TA-30 for now.

I have no complaints about the tones I can get out the the Mesa - they are insane - and they sound amazing out of the Marshall cabinet too. What I'm curious about the the head --> cabinet OHM configuration. I'm going 4 OHM out of the amp into the cab at 4 OHM mono. From reading the TA-30 manual it sounds like I could possibly use the 8 OHM out into the 16 OHM mono input on the cab. Before trying this I'd like to know if (a) this is a bad idea, and (b) what the difference might be. According to the Marshall manual I should always match the impedance (eg. 4 to 4, not 8 to 16).

Can anyone enlighten me on on this? Thanks.

I am very sure that there are some great technical experts on this site, who probably can give you a very detailed answer. My basic understanding is that there is a risk in using a lower resistance output from the amp into a higher resistance speaker. If the amp is trying to power into more resistance (16ohm) than it is putting out (8ohm), the power amp section has to work harder to overcome the resistance and there is some risk that the amp will overwork itself.

The way it was explained to me years ago was to think of electrical resistance like water pipes. Lower resistance means a bigger pipe, which means more water can go through it. As you use a smaller pipe, less water can flow through per minute. So, if you are running water full bore through a 12" diameter pipe and suddenly try to move that same quantity of water through a 3" pipe, it is going to back up, restrict and possibly burst a pipe or spring a leak. Same idea with electrical signals. Higher resistance equals a smaller pipe.

Mesa literature traditionally says you can mix/match output jacks with different speakers' resistance. Usually, the suggest trying using the 8ohm output jack into a 4ohm speaker. However, that is the opposite (running a small pipe output into a larger pipe).

Anyway, I hope this helps. And, I hope I am not so far off base that I get slammed.
Brent

my understanding is that the amp should not have higher impedance than the speaker, that would be an unsafe mismatch.
You can check the TA-30 Manual on the Mesa webside, your situation is described on the page 29: 8 ohm amp into 16 ohm cabinet=SAFE MISMATCH,
the amp will sound and feel a bit different but according to mesa you are safe

Yes it is a safe mismatch to go into a higher speaker ohm value but not a lower one. The sound may be a bit warmer and it will not get as loud as a perfect match but you may prefer it. When I was gigging with a Mesa MKIV I preferred the sound of the 8 Ohm cab plugged into the 4 Ohm out and as it was an insanely loud amp volume was no problem!

OK, so it sounds like a safe mismatch, but would it be working the power amp harder than a 4 Ohm -> 4 Ohm match (as babow2's theory suggests)?

Also, how come the Marshall cab manual does not recommend this, yet the Mesa TA-30 manual says it's OK?

Driving a higher Ohm speaker won't make the amp work harder the output tubes will actually deliver less current and power to a higher impedence speaker.