Converting 8ohm Cab Into 4ohm?
 

How hard is it to convert an 8 ohm cab with a tweeter into a 4 ohm cab? Is it as simple as switching out the speaker into a 4 ohm? Will doing that blow the tweeter?

If you are replacing the 8 ohm driver with a 4 ohm driver, you should be all right.
Chances are the tweeter will be a piezo and the driver change won't affect the tweeter that much.
Some others might think different and offer their opinions too, but basically it will work and the tweeter will be fine.

There are a few minor factors that are not really too importaint, depending on the cabinet passive crossover type, the 4 ohm driver may change the crossover frequency a little, but i don't think you would notice.

Speakers must be replaced, yes. Even most bass cabs with "regular" tweeters only have a HPF in them and changing woofer impedance won't hurt a thing, though there are exceptions.

The bigger question is why? If it's to "get all the watts" from your amp, forget it. Adding a 2nd cab will be the better option. But, you can't do it anyway, unless you replace all the spkrs. Just get a 2nd cab if you want louder.

Absolutely correct. Doubling your audio (amp) power gives you less than 1 dB increase in acoustic power (SPL), whereas adding a second cabinet will increase your amp power and also double your cone area which will roughly double your acoustic SPL (+3dB).

ALSO - if you need to do this for other reasons, if the cab has a REAL crossover with large inductors and caps, it will also need to be changed, as the values depend on the speaker impedance to crossover at the right point.

4 ohm 15" speaker
 

If you are looking for a 4 ohm, 15 inch speaker (Eminence). I just converted my 4 ohm cab to 8 ohms. A simple speaker swap. My tube head likes 8.
LMK

The bigger question is does the Eminence like the cabinet you shoved it into??

Fill in the gap:
(Troubleshooting)

[] I want to have all the amp wattage
[] I don't have enough loudness
[] My amp is farting out

yes to the first box only.

??????? Acoustic power is not SPL, speakers convert amp power into sound and we measure it on a logarithmic scale of dB SPL.

Doubling the power available potentially offers +3dB of SPL but it's unlikely the 4ohm version will actually respond well to double the power through it.

Adding a second cab gives you +6dB of SPL if your amp can deliver double the power into 4 ohms.

Bassamatic has this weird math troll going on every few months where he will next say I don't understand what he was saying.

i think in theory it's supposed to give you +3dB, but at real bass amp volumes most speakers run out of room and "eat" the extra power before that happens.

maybe if you went from like 10 watts to 20 watts into a 400 watt cab you'd get the full +3dB.

I thought so. So, here's the deal- "getting all the watts" from your amp is not all you think it is. BUT, adding a 2nd cab, prefferably one identical to the one you have now, WILL get you all your watts, PLUS, it will actually be a worthwhile improvement on several issues, more overall volume, and more low end. You have to understand first, the wattage ratings on spkr cabs mean next to nothing- your current cab would not really make use of the "extra" wattage, (there are exceptions), so putting full power to the one cab is not much of a step up- but a 2nd cab IS a big step up. There is lots of info in the amps forum to back this up, and I hope you take the time to understand how this all works.

+1, no need to squeeze out every watt into one cab.

Simply replace the speakers ...

Or just run a 2nd 8 ohm cab ... then you will have a total of 4 ohms.

Higher impedance drivers are more efficient for a few reasons that are actually kinda cool. Z(ohms)=E(volts)/I(amperes), so higher impedance means lower amperage at a given power level. Fewer amperes means a given wattage can pass a smaller diameter of wire without melting it, which coincidentally increases the impedance of the wire. For example AC transmission lines for power grids run at crazy high voltage so more power can be distributed with less expensive networks. In the case of bass speakers the power limits are generally a matter of cone excursion limits, so higher impedance drivers result in better efficiency in a given cabinet from having lower voice coil weight and higher magnetic field density.

So while your amp output is best at 4 ohms, you want to use several higher impedance drivers wired in parallel to achieve it. Long story short, adding a 2nd 8 ohm cab is much better than replacing an 8 ohm driver with a 4, like folks have said.

4-ohm drivers are typically 1dB less efficient than their 8-ohm counterparts, part of what I like about my 4-ohm 4x10, it uses 16 ohm drivers, even more efficient than 8's.

Eminence speaker swap
 

No, the cabinet and speaker hate each other. However, the Vas, Qts, and Fs are compatible with the cabinet volume and port tuning. Sounds great, and my tube head is in love with the impedance.

I just converted my 4 ohm cab to 8 ohms. A simple speaker swap. My tube head likes 8.