speaker cabinet impedance

[tt0511]

I'm considering getting an extension cabinet for my combo amp. The manual says "minimum total impedance is 16 ohms". Most cabs I'm looking at online are rated at 8 ohms. The native amp speaker is 16 ohms. So, does that mean I can add a cabinet that is 8 ohms or does the cab have to be 16 or higher? (in other words, is the impedance additive so that an 8 ohm add on cab would produce 16+8 ohms = 24 ohms, and everything would be cool?)

[T.B.A.]

16ohms + 16ohms = 8ohms (which is a higher load)

If you want an extension cab with 2 speakers, they're usually 8 or 4ohms but a lot of them can easily be rewired to 16ohm in a couple of minutes.

[tt0511]

thanks

[Vim Fuego]

Ok, this is probably more difficult that it seems at first.
From your description I guess your combo is a solid state rather than valve amp, (I'll tell you why in a minute).
You've only had half the story so far. The impedance of speakers depends on how they are wired up.
Two 8 ohm speakers in parallel will give a total load of 4 ohms.
Two 8 ohm speakers in series will give a total load of 16 ohms.
Now does your combo have an extension speaker socket? If so is it this that is labelled minimum 16 ohms? If so then just plug a 16 ohm speaker into this and everything will be ok.
If not then will you need to add a socket yourself?
Let me know the answers to these questions and I can advise you further!
The reason I think your amp is solid state is that if you put too low an impedance on the output of a transistor amp it will try to supply too much current and could be damaged, any impedance above the minimum will work ok, hence the minimum 16 ohm statement. With a valve amp there is a transformer in the output stage that requires exactly the correct impedance to be connected to it, too much or too little is just as bad.
Jim

[tt0511]

Thanks Vim. My amp is the Peavey Classic 30 tube amp. I got a reply from Peavey this morning. Apparently the amp supports 16 and 8 ohm loads. The speaker in the amp is a 16 ohm speaker. They said adding a 16 ohm extension cab will bring it down to 8 ohms, ...so, that means the extension cab jack is setup in parallel with the "in amp" speaker. That helps me understand what to do.

But, ...I am having a very hard time finding 16 ohm extension 1x12 and 2x12 cabinets online. Almost all of them are rated at 8 ohms. However, a friend who has the Marshall 2x12 extension cab says his has inputs for both 8 and 16 ohms, but the online documention does not say that. I guess I'll have to spend some time in a store analyzing the input jacks of cabs to know. I could always buy a 1x12 and switch the speaker out for a 16 ohm speaker, ...but that would add up to too much money. I could also buy a 2x12 with two 8 ohm speakers and wire them up in series to get to 16 ohms I guess.

[Vim Fuego]

I've just looked at the Peavey manual on line and I can understand your confusion, bad manual!!
I think from the reply you had that when you plug an extension speaker in the back (in parallel) the jack socket switches the amp from 16 to 8 ohm operation, hence you'll need a 16 ohm extension cab.
That last option you mention is what I effectively do.
I have a Laney head and I wanted a couple of 2x12 cabs, but most were too big to go into the back of my car easily. I chose the Marshall Valvestate cabs which are small enough but come wired for stereo. I changed the wiring so that the two 8 ohm speakers were in series giving me 16 ohm cabs.
The head has different outputs for different impedances, so at practices I use one cab in the 16 ohm socket, and at gig use both cabs as a combined 8 ohm load!
I guess you *could* always get a Peavey extension cab?
But if I were you I'd probably by a nice Celestion speaker (which are available in 8 or 16 ohm versions) and make a cab for it to go in. The woodworking is pretty easy as unlike bass speakers you don't really have to worry about the construction other than it being a rigid box.
Jim

[tt0511]

Yeah, a friend told me that peavey cabs are the worst so I don't really know if I want their 2x12 16ohm cab. I've also read reviews of people who love the Classic 30 combo amp, but they tried it or the Classic 30 Amp Head through the Peavey 2x12 cab and hated it.

I did find this vendor though, http://www.portcityamps.com/wave1OS.html and I am impressed with their product (based on online information, ..I haven't actually plugged one in yet). I'm thinking of getting a 1x12 oversized cab with a Celestion Vintage 30 (available in 16 ohm) in it. Dave Weiner thinks very highly of their cabs apparently. His feedback on their site is very good. I watched the two videos on youtube last night and the tone sounded very nice.

The OS Port City cab is just barely larger than the Classic 30 combo amp so my amp will sit nicely on top of it. And, I don't know if I want to add a 2x12 since the internal speaker stays on when an external cab is plugged in, ...I don't know how that 30W amp will push 3 - 12" speakers. I think it might sound better with just an additional 1x12. I don't want to have to crank the amp all the time just to get good tone. My fear may be unfounded. I don't have any experience in this stuff.

I just know a friend who runs a Marshall J2000 (JCM 2000? dunno exactly) through the Marshall 2x12 and it sounds great. He gave me a recording of a song he did with that cab plugged into his Peavey Classic 30 and the distortion is very sweet with those 3 speakers running, ...but I think he had to crank the amp up pretty high to drive that 3 speaker configuration. Since I mainly play at home, I don't want to "have" to push everything to a high volume for it to sound good.

[Vim Fuego]

For someone who "doesn't have experience" you make a lot of sense!
I'd probably go with a 1x12 myself (sometimes 2x12s are a bit big to fit in small pubs!)
For reference, if you plugged a 16 ohm 2x12 in (two 8 ohm speakers in series in one box), then the power from the amp would be shared equally between the combo speaker and the extension cab each receiving say 15 Watts.
The two speakers in the extension cab would then share what the cab gets, so, each of the extension cab speakers would only receive about 7 Watts.
I'll clarify, the peavey speaker will get half the power and the extension speakers will each get one quarter.
We all know that guitar speakers are funny things and sometimes only sound decent when they have enough power going to them, so the extension speakers might sound a bit odd and quiet!
Important note:- more speakers doesn't make a valve amp any louder, it just spreads the sound out a bit better! (some transistor amps can deliver more power into more speakers though!)
Jim

[tt0511]

Sweet! That confirms what I was suspecting. Thanks for the "make a lot of sense" compliment. I am an engineer by profession, ...although not electrical or I wouldn't be asking these questions, lol. I tend to think very analytically because of my trade.

Sounds like a 1x12 is exactly what I need.

I notice that the Celestion Vintage 30 is rated at 60 watts. Would the 15W split to the extension cab drive it hard enough or should I look for a 20-30 watt speaker (they seem a little hard to find from my internet research so far).

[Vim Fuego]

Have a read of this...
http://professional.celestion.com/gu...replace_GB.pdf
In the article, the guy replaces the speakers in a Carvin 2x12 combo, he uses a Vintage 30 (which is probably similar to what you have already) and a G-12M Greenback. The Greenback is rated at 25 Watts so should be driven better by an amp at low volumes as you suggest, that's what he hoped anyway!
Jim

[tt0511]

That was a helpful article, ...only trouble is that I can't seem to find anything on the Blue Marvel speaker in the Classic 30 that says what wattage it is to direct me on how to choose a good speaker mating. Haven't found anything online yet either. I think the Vintage 30 will probably be great based on that article.

I know the Blue Marvel is warm and fat, and I think it breaks up good. I'll try to do more research on that speaker before choosing, but looks like that Port City Cab with the Vintage 30 will be awesome paired up with the Classic 30.