Cabinet Impedances

[Betos]

I have a cabinet which is 16 Ohms. I'd like to run an amp through it that has both 8ohm and 4 ohm outputs, but no 16.

Is it safe for both the amp and cabinet to hook up and run through the lower outputs? What are the possible consequences and effects of doing this?


I don't want to rewire the cabinet.


Thanks,
Betos

 

[VaughnC]

Due to flyback voltage effects, with a tube amp, it is much more risky running it into a higher impedence than the amp wants to see, than lower....especially at gigging volume. The major concern is voltage arcing inside the output transformer, and they aren't particularly inexpensive to replace. Your safest route would be to rewire the speaker cab for 8 ohms.

 

[John Phillips]

You can't rewire it for 8 ohms. If it's a 16-ohm cab it will have four 16-ohm speakers, so you can get 16 or 4 ohms only.

You could run it from the 8-ohm tap, but you will lose some power and there is a (very small) risk to the OT. If you want to do this from a four-tube amp, you could actually remove two of the four power tubes (either the inner or outer pair) - this will give the correct match with the 16-ohm cab plugged into the 8-ohm socket, and only give slightly less power than with all four tubes mismatched.

I would rewire the cab for 4 ohms though, it isn't very difficult.

 

[VaughnC]

He didn't say so, but I guess I assumed he was talking about a 2X12 cab with two 8 ohm speakers wired in series.

 

[Betos]

Hey thanks guys.


Thing is, this is an early 70's Marshall 4x12 with 16ohm 25w Greenbacks, and I'm likely going to sell it, so I don't want to change the wiring just to test it with a Mesa Boogie. Nor do I want to risk damage to the amp.

Betos

 

[gitarzilla]

Actually, I think there is supposed to be a way to wire it for 8 ohms using some series/parallel hybrid wiring voodoo. I'm not exactly sure how it goes, but a good amp tech would know.

 

[John Phillips]

No. A 16-ohm cabinet, either 4-speaker or 2, can be wired for only 16 or 4 ohms, not 8.

If it's a 2-speaker cab, it contains 2 8-ohm speakers in series, and the only other possible scheme is them in parallel, which is 4 ohms.

If it's a 4-speaker cab, it contains 4 16-ohm speakers in series-parallel (2 parallel pairs in series, or two series pairs in parallel, it doesn't matter which), and the only other sensible wiring scheme is all 4 in parallel, giving 4 ohms.

Betos, if you just want to see what the cab sounds like with the amp, pull two power tubes (assuming it's a four-tube amp) and run it from the 8-ohm socket. This is safer all round as not only is the impedance match now correct, you aren't running the cab anywhere near its power limit (which you would be with all four tubes and it rewired to 4 ohms).

If it's a two-tube amp, just run it from the 8-ohm socket. Mesa amps will tolerate that kind of mismatch quite safely.

 

[Betos]

Unquiet,

Thanks for the advice, I appreciate it you being a "racket scientist" and all

The Mesa is a MK series, so it has four tubes and a switch which takes it down to 2 tubes. Would that be ok?

I'd only be running the amp at low volume anyway, unless the addition of the cabinet just floors me, at which point I might consider changing it to the 4ohm configuration.


Betos

 

[John Phillips]

Wait a minute... you said 'addition' - I take it you mean you want to run both the 4x12" and the internal speaker at the same time?

If so, it's much easier - there are two ways.

Either plug both the internal and the cab into both 4-ohm sockets (it has two 4 and one 8, right?). This will give a total load of 5-1/3 ohms, which is totally safe from the 4-ohm tap, and will send 2/3 of the power to the internal speaker and 1/3 to the cab; or connect the cab to the 8-ohm socket and the internal speaker to the 4-ohm one - this will produce the correct match for each speaker, and distribute the power equally.

If you DO want to run the cab on its own, simply switch to two tubes and run the cab from the 8-ohm socket.

This impedance business is quite confusing if you're not used to it...