HEAD TO CAB OHMMS ?

[JCMRobbo]

Hey chaps! I’ve never actually known this. So I thought you lot would know.

head to cab:

if I set my head to (4) 16 what do I need to set my cabinet too?

What if I set my head to 8 what do I need to set the cabinet too?

AND WHY DOES THAT MATTER?

Cheers! All for learning!

Chazz

 

[SkyMonkey]

It would help if you state the head and cab models.
Not sure what the (4) in brackets is supposed to mean?

But I will jump in and guess that it is a stereo 412 cab like a 1960, and the head is mono.



If the head is set to 16 ohm output, connect to the right hand cab jack with the switch to the left (MONO). Parallel pairs connected in series.
If the head is set to 4 ohm output, connect to the left hand cab jack with the switch to the left (MONO). All speakers connected in parallel.
If the head is set to 8 ohm output, connect to either of cab jacks with the switch to the right (STEREO). Isolated pairs wired in parallel.
In STEREO mode (8 ohm) each of the cab jacks will only connect to one pair of speakers in the cab, i.e. stereo split L/R.

 

[JCMRobbo]

It would help if you state the head and cab models.
Not sure what the (4) in brackets is supposed to mean?

But I will jump in and guess that it is a stereo 412 cab like a 1960, and the head is mono.



If the head is set to 16 ohm output, connect to the right hand cab jack with the switch to the left (MONO). Parallel pairs connected in series.
If the head is set to 4 ohm output, connect to the left hand cab jack with the switch to the left (MONO). All speakers connected in parallel.
If the head is set to 8 ohm output, connect to either of cab jacks with the switch to the right (STEREO). Isolated pairs wired in parallel.
In STEREO mode (8 ohm) each of the cab jacks will only connect to one pair of speakers in the cab, i.e. stereo split L/R.

thanks very much

 

[Lance Chambers]

General rule: The speaker cab can operate at equal/double the amp setting.

Ex: 4 Ohm amp output into an 8 Ohm cab is safe.

NEVER run a higher amp Ohm rating into a lower Ohm rated cab...................BOOM!

 

[LPMarshall hack]

The (4) on the Marshall impedance selector is for combos. They used the same chassis for heads and combos. This so what I was told by the forum many years ago.

 

[Ryan Morris]

General rule: The speaker cab can operate at equal/double the amp setting.

Ex: 4 Ohm amp output into an 8 Ohm cab is safe.

NEVER run a higher amp Ohm rating into a lower Ohm rated cab...................BOOM!

This is true of solid state amps, but NOT tube amps. Most tube amps can tolerate a 2:1 mismatch in either direction (ex. 8ohm head->4ohm cab or the other way around), however it will cause the tubes to run out faster. The one thing to NEVER do is to run a tube amp with no cab connected, because that presents the head with infinite impedance, which can definitely damage your amp. On the other hand, solid state amps are perfectly OK with not being connected to a speaker, but if you match a SS amp to a lower impedance speaker, the amp can overheat and get damaged.

As for the (4)16 switch, LPMarshall hack was right in saying that the (4) is for combos. Basically, for combos that switch will change the amp from 4 ohms to 8 ohms, while on heads the switch goes from 16 to 8 ohms. In fact, heads will say this: (4) 16, while combos will say this: 4 (16). The reason for the second number in parenthesis is because you can rewire the output transformer to work for the opposite setting. For example, you could rewire your head to have a 4ohm option instead of the default 16ohms.

In short, both heads and combos have the capability of being run at all three impedances (4, 8, 16) but they only put a switch with 2 options which is honestly pretty dumb.

The reason they picked 4 & 8 for combos is because the internal speakers are 8 ohms, so connecting to an extension 8ohm cab will give you 4 ohms total. For heads, the old greenback-loaded 1960 cabs were rated at 16ohms, so you unless you had a full stack, you needed that 16ohm option. Modern 1960 cabs can run at 4 or 16 ohms mono so its not really a problem anymore.

If you are interested in rewiring your head to run on 4 ohms, check out this article:
https://paul1179.wordpress.com/2016/04/22/jcm900-output-transformer-hookup-instructions/
I dunno if you have a JCM 900 or something else, but I'm guessing t=other amps are similar.