This morning I received a very good question about using the 4-ohm Thunderchild 112 as an extension cab. I assume we're talking about paralleling it with an 8-ohm speaker in a combo amp, and that the combo amp has a 4-ohm-capable head.
Instinctively you'd think the answer would be no, but the question arose because the TC112 is a "high 4 ohms", and the owner measured its DC resistance with a multimeter and came up with about 4.4 ohms, which is not too far off from the DC resistance of some "8 ohm" woofers.
Technically the TC112 is closer to being a 5-ohm load, and I could probably get away with claiming 6 ohms, but I claim 4 ohms because people are more used to cabs being either 4 ohms or 8 ohms. The DC resistance of 4.4 ohms shown by the multimeter arises from the series resistance of the woofer + crossover, the latter adding maybe half an ohm or so ballpark.
The actual minimum impedance is a little bit under 4 ohms, but it's up around 1.8 kHz. That's high enough up the scale to not be of any concern for a bass guitar application.
I think the Thunderchild 112 could be safely paralleled with some 8-ohm cabs and still give an acceptable "4-ohm" load, but it depends on the specifics. I figure the lowest impedance dip down in the upper bass /lower midrange region that is still safe for a 4-ohm-rated amp is about 2.5 ohms. This based on the theory that two Eminence Basslite 2010 woofers in parallel = ballpark 2.6 ohms minimum (that's DC resistance + the effects of voice coil inductance). and I have never heard of any amps having a problem driving a 2x10 cab that has two Basslite 2010's in parallel.
So I would say that the Thunderchild 112 can be safely paralleled with a speaker or cab whose minimum impedance is 5.7 ohms, but I have to withdraw my official stamp of approval if the other cab's minimum impedance is less than that. You can still try it of course, and with a little luck you can probably get away with it, but I suggest getting the opinion of your amplifier manufacturer first, and/or trying it out in practice before betting on it at a gig.
Here's an online calculator you can use to figure out the ballpark minimum impedance for two paralleled bass cabs:
http://www.1728.org/resistrs.htm
Consider the minimum impedance of the Thunderchild to be 4.5 ohms, as that takes the DC resistance of the driver + crossover + contribution of inductance in the 100-300 Hz ballpark into account; and as I said we can overlook the actual minimum up around 1.8 kHz because it's not in a high-power region for bass guitar. Note that the actual minimum impedance of an "8 ohm" cab my be as little as 5 ohms, so you should try to find out what the actual minimum is before you make the calculation. In most cases the minimum will be in the upper bass/lower midrange region; the actual minimum higher up on the TC112 because of the crossover, but that's very unlikely to be an issue in a bass guitar application.
(For the record, the impedance curve of the TC112 is quite benign as far as phase angle goes, but I think that's true of most bass cabs in the region of their minimum impedance.)
If any amplifier manufacturers come across this and would like to add insights and/or corrections, please do so.