Let's see your rig!

[Basvarken]

Oh wait they're 10,7 to be precise. Not that it makes a lot of difference.. ;-)

[Granny Gremlin]

Is that DC resistance (you measured) or actual Impedence (listed on the back of the cab or whatever)?

[Basvarken]

That's what its says on the back of the cabs.

Impedance per speaker is 32 ohms  There's three of them inside.
Total impedance per cab is 10,7 ohms. So that makes sense.

[Granny Gremlin]

It's not ideal obviously, but I wouldn't worry about it (even with a tube amp).

[Basvarken]

I always thought that it had to be an exact match for tube amps.
But on the back of the amp it says "8 ohms minimum load"
Which leads to think that its doesn't matter as long as it's not less than 8 ohms it's okay.

[Granny Gremlin]

Nothing (when it comes to impedence matching amps to speajkers) is ever exact - remember an 8 ohm speaker isn't actually 8 ohms; it is a nominal rating meaning that on average over the intended useful range of the device the thing is approximately 8 ohms.

E.g.  the Z curve for some random "8 Ohm" speaker:



That said a 16 Ohm rated speaker would not be a good idea (you might/probably be able to get away with it depending on the speaker and the amp)  - when I say average above, that is a weighted average (i.e. "over the intended useful range") and in reality many speakers Z curves bottom out at the rating (or an ohm or 2 below it) and even then only for the span of a few Hz in the midrange and are well above it for most of the useful range.

FYI - the peak at about 100Hz identifies the speaker's resonant frequency (lowest freq the speaker will usefully produce in typical situation like a sealed or ported cab)

[Basvarken]

Thanx Jake. But that's over my head already 

[Psycho Bass Guy]

The reason you don't want higher impedance for tube amps in general is that the output transformer starts feeding back voltage to the output tubes. It's called flyback, but the mismatch has to be pretty significant. Since the factors involved literally range from the amount of wall voltage and the ambient temperature of the room, the math involved gets very serious when it come to calculating precisely which combinations of speakers are more dangerous than others, and as your amp shows, it's not necessarily a hard rule every time, just like you can get away with 1 and 2 ohm loads on solid state amps that aren't rated that low.

However, as Jake's graph shows, speakers have absolutely HUGE low frequency impedance peaks at the resonant frequency and depending on how the cabinet is wired and what the speakers are, these can be astounding, literally over the hundreds in some cabinets. Good tubes are made with this in mind and the order of voltages they can handle from flyback feedback are numbers that start sounding very scary. The rule is to avoid higher general impedances than the amps are rated, but as your PA135 shows, there are caveats to that.

Other things factor in as well. Ported cabinets will have higher, more dangerous resonance than sealed and the type of signal being run through the amp makes a huge difference too: guitar is many orders of magnitude safer than bass or anything with lots of lows, again because of the resonant peaks at the bottom of the speaker's frequency response. You're fine, but I still tell all the kids who have heard the solid state output impedance rules being applied for tube amps that they should avoid higher ratings because there will always be some idiot who hooks up an SVT to an 8 or 16 ohm ported cabinet and cranks the lows only to watch the one or two cheapo old Chinese 6550's (new ones are pretty good stuff) that his guitar player buddy stuck in the amp arc internally and the SVT starts throwing fireballs as it blows up spectacularly: they used to be famous for doing it. Nowadays, the internet and not the grapevine keep players better informed, helping to keep more old tube beasties alive.

[Basvarken]

Thanx for all the advice guys.
I know I'll be safe with my Eden cabs, which are 8 ohms each.

[drummer5359]

My Ampeg Rig is finally complete. I got the second cabinet this past Saturday.

Ampeg SVT Classic-SVT212AV-SVT212AV

300 watts of all tube Ampeg goodness through four 12"s seems like the top of the bass amp mountain, at least for me it is.

The bad news is that I currently have an ear infection, so I've yet to be able to crank it up and really let it breath.

Here is a shot of it with some old bass in front of it.

[Pekka]

Just bought another LAB Series amp. A 200W model L4. Sounds as good as the L2 above. They are still dead cheap so if anyone's looking for a great sounding SS amp with plenty of decibels I heartily recommend these.

[Granny Gremlin]

Nice!

[slinkp]

Interesting about impedance peaks for ported vs. sealed cabs. How do other designs fare?  Transmission line? Folded horn?  Open back guitar amps?

[Psycho Bass Guy]

Interesting about impedance peaks for ported vs. sealed cabs. How do other designs fare?

In terms of electronic impedance and frequency response, there are only two types of cabinets: sealed and ported. The deciding factor is the density of the air column below the cabinets' tuning frequency inside the cabinet itself. In any kind of ported cabinet: bandpass, open back (a port is simply a mechanically tuned open-air column), etc, the voltage applied to the voice coil of the speaker below the port tuning induces little cone movement and is dissipated almost entirely as heat in the coil; too much of that and the speakers will blow without producing any audio other than the harmonic distortion artifacts generated by the amp itself. The speakers will lock their cones at their mechanical limit and stay there until the amount of power passing through the speaker exceeds the thermal limit of the voice coil and the coil burns open, unloading the amplifier completely at all frequencies and destroying the speaker. In a tube amp, as the frequencies below the tuning of the cabinet were always "seen" by the amplifier this way, the output transformer may have sustained internal arcs.

Transmission line? Folded horn?

Those are, in theory, simply sealed cabinets that "cheat" by manipulating the speaker's pressurized air column in various ways and behave electronically accordingly. The difference in these cabinets and "normal" sealed cabinets is that these types have a much lower mechanical resonance. Since these types depend on their "natural" tuning for extreme lows, boosting low frequencies below tuning will still have an order-of-magnitude larger impedance spike which will make for a very uneven low frequency response. That's also why "normal" sealed cabinets have remained popular while these types remain esoteric novelties: the relatively high low frequency cutoff of a normal sealed cabinet can be compensated for electronically at predictable intervals.

[slinkp]

Fascinating stuff,, thanks.

I suspect the much lower cost to design and build a sealed cab may be a factor ...