Close enough to the 2ohm not to matter, IMO and IME, riz. As for the sound, this comes down to all sorts of things including efficiency. Different makes of cab (even if of equal overall impedance) will have that problem. I’d only spend money on a cab after I’d tried it with the gear I’m intending to use it with.

ok ,
but I had read somewhere that having two cabs with non-matching impedance would cause having more power on one cab than on the other, therefore it was not a good thing because the sound would have been affected too, or something like that…
but having a 8ohm cab and a 4ohm cab in parallel would be something like 2,4 ohm, and that would not be that far from the 2 ohm position in an amp that has a selector, right?

The problem, riz, is that two different impedance cabs hooked up to a valve (or SS) amp gives you an odd overall impedance. Suppose you plug an 8ohm cab into one output and a 16ohm cab into the other output, your load overall on the amp is 5.33ohms. As I mentioned, Weber sell 50W and 100W OPTs with taps to match all of these oddball impedances. The two problems are (1) the expense and fiddle, (2) changing the OPT of an amp WILL change its tone from how it used to sound.

However, if you have a 4/8/16 selector, either the 4 or 8ohm will be close enough. There’s a loss in efficiency and the parts have to work a bit more than they should, but I don’t think it’s such a killer, even for the supposedly fragile Marshalls. If a valve amp is the type with different output sockets, I wouldn’t connect the 8 to the 8 and the 16 to the 16. Better to connect the first cab to the 8 output and daisychain the two cabs. I haven’t looked at the wiring or schems, but that’s my preference.

uhm…
so having 2 cabs with 2 different loads, hooked to a valve amp, is NOT bad, right?
maurizio

🙂 , Mack. I tend to look in and post a fair bit on the main guitar cat. Before MightyModMike created the Bass cat, this was the only place. Apart from bass guitar, my other interest is old, valve amps. If you have a search for threads started by Riz (another member here), you’ll see me answering this sort of Q.

Think of a cab as being just one speaker, and vice versa; to the amp, it doesn’t make any difference. A speaker driver, and the cab, have a +ve and a -ve terminal. From the jack lead, the tip is the +ve and the sleeve is the -ve. There are two ways of connecting a multiple of speakers; in series or in parallel. Connected in series, it goes like this:

SERIES
Jack tip —- speakerA +ve, speakerA -ve —– speakerB +ve, speaker B -ve —– [……….] speakerN +ve, speakerN -ve —– jack sleeve. I’ve pinched an image that shows this for a hifi’s black and red posts, but it’s the same thing for a jack input:

http://www.bcae1.com/images/gifs/impspksr.gif

Now, the maths for this is easy. The overall impedance is just the sum of the impedances for each driver:
Total ohms = Aohms + Bohms +…… Nohms

or 8ohms + 8ohms = 16ohms. 8ohms + 4ohms = 12ohms etc etc.

Alternatively, the speakers can be connected together in parallel, like this:

PARALLEL
Jack tip —- speakerA +ve, speakerB +ve —– [……….] speakerN +ve —- speakerA -ve, speaker B -ve [……..] speakerN -ve —– jack sleeve. Here’s a similarly pinched image for parallel, although a speaker cab for guitar would have the wires running from the driver’s +ve and -ve terminals and back to the jack socket:

http://www.bcae1.com/images/gifs/impspkpr.gif

Now here, the maths is a bit more complicated. This formula gets it roughly right; good enough for us to use:

1/Total ohms = 1/Aohms +1/Bohms +…… 1/Nohms

or 1/8ohms + 1/8ohms = 1/4ohms therefore overall impedance = 4ohms
and 1/8ohms + 1/4ohms = 3/8ohms therefore overall impedance = 2.67ohms

So, forgetting the maths, wiring up speakers in series INCREASES the impedance by simple sum. For a SS amp, this is a bad thing because a higher impedance means reduced output. No problem for a valve amp, because you just adjust the tap. Series wiring is a bad thing for both types of amp for one reason, if you remember doing the lightbulbs and battery experiment at school. If wired up in series, one dud speaker will take the whole chain out.

Wiring up in parallel REDUCES the overall impedance; a good thing for SS amps unless you wire up so many together that the overall impedance drops below the safe minimum (usually 4ohms or 2ohms, just check the spec). With like for like, maths is easy. 2x four-ohms = 2ohms, 2x eight-ohms = 4ohms, 2x sixteen-ohms = 8ohms.

It gets complicated with unlike cabs or drivers. I’ll do the maths for everyone. 4ohm + 8ohm = 2.67ohms overall, 4ohm + 16ohm = 3.2ohms overall, 8ohm + 16ohm = 5.33ohms overall. There is a big advantage with SS amps; they can handle these weird numbers no problem as long as you stay above the minimum rating. For valve amps, it’s not so good because most amps only offer 4/8/16. You either fit a Weber OPT which has all those weird taps, or you go for a nearby number on the old selector (not too risky for the amp anyway).

Thirdly, it’s possible (and very commonplace in 4x driver cabs like Marshalls) to mix the wiring as series and parallel:

http://www.bcae1.com/images/gifs/spk4sp4.gif

With a pair in parallel serially connected to the other parallel pair. Lets keep the maths simple. If all four drivers are the same impedance (eg, 8ohms), then the total impedance of the cab is the same value (ie, 8ohms). If you start mixing drivers of varying impedance, you get a headache working it all out.

Finally, and this is important, you can connect speaker drivers in serial or parallel but most ways of connecting multiple cabs are usually parallel. Your amp head has two speaker outs? They’re in parallel. Your combo has an “ext speaker” socket? It’s in parallel (unless it disables the internal speaker, as some do). You’re using cab #1s extra socket to daisy-chain up another cab? It’s in parallel. There’s a good reason for this; if one of the cabs duffs out, the other(s) will still carry on; if linked in serial, they’d all go down and you’d have no sound.

The thing to remember is that cabs connected up will follow the same maths as I wrote in the parallel blurb, above. There is one way to connect cabs in series and that is to make/have made for you a jack lead where the tip of one end is connected to the sleeve of the other end and vice versa. This will then daisy up the extra cab in series. If you ever make one, DON’T choose black cable – go for fluorescent pink or something so that you never use it except for that particular job in mind.

The very last point is that when hooking up a cab to a combo (or amp head), use unshielded amp-speaker cable; NOT the standard guitar lead. Special speaker leads are heavier duty than instrument leads, which could get hot under the strain.

Phew! sorry about the tech-speak; hope some of it is of use/makes sense 😕

Mack, there’s no reason why other brands of cabs can’t be tried with amp heads. I’ll tell you where your chum may have heard some concerns, particularly regarding Marshalls, but the bottom line is that the AVT50 won’t have a problem. Similarly, that cab can be used just fine with any amp that can handle an 8ohm load. This following tech-stuff is of interest (maybe) to you or anyone who’d like to know a bit more, otherwise just skip to the last para:

Any amp’s output needs a load, usually provided by the speaker. Many solid state amps (I’m a bit hazier with SS) are happy with anything from 16ohm down to 4 or even 2ohm, without any switch-flicking. The lower the impedance, the higher the output so that explains why SS stereo power amps might be rated as 1000W but, if you read the small print, that’s at 2ohms and a pair of 8ohm cabs will only punt out about 300W.

With a valve amp, the output transformer (OPT) will have various “taps” so that the impedance of the speaker cab(s) is matched up to the OPT. Typically, valve amps have a selector that can be altered to 4, 8 or 16ohms (some amps have different jack sockets that you choose according to the cab). Vintage Marshall heads had a 4/8/16 selector and the 4×12″ cab (the 1960 A and B types) were 16ohms overall each. A halfstack required setting the selector to 16 and the fullstack required setting to 8. Valve amps don’t really appreciate being “told” one impedance and being given a cab with a different impedance. The OPT and the valves (tubes) have to work harder. While this is true of all valve amps, the Drake transformers used in old Marshall heads were notorious for failing quickly if the wrong impedance was selected.

Your mate’s AVT is Advanced Valvestate Technology. This means that there’s a preamp valve in there (a 12AX7 or ECC83 as it’s known in Europe) but this has nothing to do with providing the watts. It’s just for the initial gain stage and it does a very good job of making the amp sound “valvey”. The actual power output is solid state and I don’t know all the details but what I’ve gleaned is that it’s rated at a minimum 4ohms. The AVT 4×12″ cab has an overall impedance of 8ohms.

In practice, the AVT50 can be connected to a single 4×12″ or any other cab you like with an overall impedance of 4ohms or greater. Any commercially sold cab will have the impedance rating somewhere in the spec. A single AVT cab or any other 8ohm cab will, I would have thought, punt out less watts than two AVT cabs (or a single 4ohm cab) would with the same head. Make sure, with the AVT50, that the overall impedance never drops below 4ohms as this could fry the amp.