Rumble 210+210 impedence setting

[G-Dog]

... ... I own a Fender Rumble stage 800. Just bought a Rumble 210 extension cab. Do I set the ohms to 400watt 8ohms, or 500watt 4ohms? Thanks!

... ... ...
>>> Set switch to match total impedance rating of external cabinet(s); <<<
... ... ...

8 ohm cabinet set to 8 ohms.
4 ohm cabinet set to 4 ohms.

Very simple

... ... ... There is a Rumble Club on TB, maybe you can get some experienced responses there? ... ... ...

Yes! There is a very helpful Rumble Club here on TalkBass. And it concurs with these answers. Come introduce us to your Rumble and get your free membership!
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[InhumanResource]

Hopefully this helps clarify.

Specs:
View attachment 4815448
Note the area in red. The internal speaker is 8 ohms. The minimum impedance is 2.67 ohms. You can run either an 8 ohm or a 4 ohm extension cab. FYI, Using an 8 ohm extension results in a 4 ohm load, and using a 4 ohm extension results in a 2.67 ohm load.


View attachment 4815449

View attachment 4815450

Notice the area in red. The switch is specific to the impedance of the extension cab, rather than the total impedance. Per the specs, the amp makes 800W at either 4 ohms or 2.67 ohms. If you use an 8 ohm extension, set switch to 8 ohms. Total power is 800W and each cab will get 1/2 of the power (400W).

If you use a 4 ohm extension, set the switch to 4 ohms. Total power is still 800W but it will not be shared equally. [266.67 + 533.33= 800] The internal 8 ohm cab will get 266.67W and the 4 ohm extension cab will get 533.33W--a little more than the back panels says.

I don’t know if I’m impressed or annoyed that they just assumed we don’t know how impedance works and said “just set it to the number on the cabinet.”

 

[G-Dog]

I don’t know if I’m impressed or annoyed that they just assumed we don’t know how impedance works and said “just set it to the number on the cabinet.”

IME, unfortunately, it's a valid assumption for a great many musicians! The "Set Switch to Ext. Spkr Impedance" is very straightforward and helpful.

But, the first units didn't have that straightforward and helpful message on them! So, the wording on the amp near the switch was very confusing for those who do understand how impedances work! Plus, many had never heard of a 2.67 ohms stable amp, which is what the Rumble Stage 800 & the Rumble 800 Combo amps really are. Most amps were either 8 ohms, 4 ohms or 2 ohms.

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[agedhorse]

Im pretty clueless on the subject, but Surely the technology must exist for the amp to simply sense the load of any speaker combination applied and supply power accordingly? Why is that NOT standard on many (most?) Amps? Not being snarky, just curious. That would seem to aptly fit the term "plug and play".

It appears to be quite simple in theory, but in fact is quite difficult in practice.

It’s similar to tube power amp auto-bias circuits… simple in theory yet in practice every one so far introduced (by some very experienced designers) has proven to be “reliability challenged”.

I second that last advice, it's the easiest way to get close to solving.

Basically there is also Watt's law, like Ohm's but relates to power, and it's when both work together that it gets slightly odd. W=V2/R, where R is the speaker impedance, V is voltage, and W is the power, watts. Without getting to details like frequency-depenance of impedance, it's important to see that SMALL changes of voltage can make big changes in power, it goes up as a 'square law'. The numbers don't matter, but that square law thing DOES because it trumps all other scales of intensity in the calculation.

In practise what that means is it's only the strong transients in the output that will drive anywhere near the full power, if you're careful to avoid blasting all controls to 11 and trying to break your guitar. The usual signs of hard transients is a farty sound on the attack of the note. So long as you back off when that happens (assuming you're not making a preamp do it deliberately), it will be safe unless something else is wrong somewhere.

Just turning up volume goes with a non-linear curve so anything but full throttle on a 400W amp into a 400W cab will actually drive a lot less than 400W. In other words, get the impedance matched, that's important, but matching power is far less so if you're careful to listen for signs of stress.

You are ignoring mechanical limitations of the speakers, and how most speakers are rated using thermal limits yet most speakers are more vulnerable mechanically. A speaker rated at say 400 watts thermally may only be rated at 250 watts mechanically.

IME, the OP’s Rumble 210 will be fine with his amp.

 

[Lostgallifreyan]

You are ignoring mechanical limitations of the speakers, and how most speakers are rated using thermal limits yet most speakers are more vulnerable mechanically. A speaker rated at say 400 watts thermally may only be rated at 250 watts mechanically.

IME, the OP’s Rumble 210 will be fine with his amp.

It seemed that way to me too, but about the ratings, I hope when a cab is rated for a stated output that this possible ambiguity is solved at source, i.e. the lowest rating given so that people are less likely to get nasty surprises they don't have easy reason to understand.

When I was building diode lasers there could be several limits of safe effective power, but one method of testing is to the point of stress under intended system conditions, then after some time, to the point of destruction (and the margins can be small with those). It was cheaper than fending off unhappy buyers, and I didn't get complaints. I'd just stay within the lower margin for error reduction and hope the diodes were make consistently enough. They weren't, very, but the method gave good outcomes even with the cheap diodes.

What I'm getting at is, if the cab were rated as a mechanical system, regardless of what's written on the drivers in it, then that might get safe outcomes based on the calculations for the whole design. I'm not sure what the practice is with makers of cabs, or if it varies between them.

 

[agedhorse]

It varies, but because of marketing competitive pressures, most manufacturers choose to publish power ratings of thermal limits (or even higher!).

Even with the higher numerical ratings, most speakers will hold together short term, but long term reliability will suffer based on the power versus time curves under high level conditions.

We publish both thermal AND mechanical power handling numbers because I feel that it’s important to the customer that understands (or wants to understand) this stuff. Fortunately, in the companies I have worked for, the engineering perspective is accepted as the appropriate way to represent a product. This is especially true for products that carry a 5 year warranty (for obvious reasons).

 

[Old Garage-Bander]

I don’t know if I’m impressed or annoyed that they just assumed we don’t know how impedance works and said “just set it to the number on the cabinet.”

The lawyers make them pander to the lowest common denominator. There are people who genuinely have never gotten into this end of it all. I assume the OP is in that category, and make that assumption without any judgement. Then there are the idiots. It just takes one to force us all into confusion or annoyance.
That being said, I give Fender a C- for the combination of labeling on the switch and how the instructions were written. Count me not impressed.