Years ago I found an old Reußenzehn cabinet with two 12" speakers in an old rehearsal room that we cleaned up. Since neither the speakers nor the cabinet beared any markings regarding impedance and wattage, I was reluctant to use this box, and so it kind of hybernated for quite some time now.
In the last weeks I worked on two tube amps, changing power tubes, setting the bias, and one transistor amp, where I repaired a blown power section. So it turned out that I have a need for a relatively small and compact cabinet to test the amps. This was when I remembered the old Reußenzehn.
So finally I measured the ohm resistance of the speakers (6.4ohms each, so I guess they are rated 8ohms impedance) and by inspecting the wiring, consisting of two switchable input jacks (one of which had fallen inside the box, not mounted at the terminal, but still connected to the wiring), I figured out that this cabinet is most likely of 4 ohms impedance.
But what's up with the second jack? I measured some more, with jacks plugged in and without, and so it seemed that with the current state of the wiring this cabinet can also run with 8ohms impedance, but then only one of the two speakers is connected to the signal.
Well, 8 and 4ohms is not bad, but one of the tube amps had two power tubes pulled to get it down to 50 watts - so the impedance switch has to be set to the half of the actual speaker impedance. Which means in order to use the Reußenzehn, I need to set the amps impedance to 4 ohms, and use the 8ohms circuit with one speaker only. For testing purposes this might be ok, but… the two 8ohms speakers could be wired in series for 16ohms - so the amp in question could make use of both speakers, with a setting of 8ohms at the amp.
I thought about the wiring… with two switchable input jacks, maybe it is possible to make one work in series, and one in parallel? Switchable input jacks act like a switch: if not plugged in, the two pairs of connectors are closed, if a jack is inserted, this connection is broken and only the tip and the ring of the jack is connected to one side of the input jack, but not the other.
I scribbled a bit and finally came up with this wiring:
If the upper jack is used, the two speakers will run in series,
the lower jack will switch the wiring to parallel
Nice! Now I have 16 and 4ohms at my disposal.
Wouldn't it be great to have the 8ohms setting as well? After pondering about inserting a switch that may select a mono/stereo mode, it dawned on me that I can use my wiring as is - the only thing needed is a 'blind' jack that's inserted in the upper input.
Now the lower input uses only one speaker, and tadaa, 8ohms.
But it is important that the 'blind' jack is not connected to anything or shortened internally - there's this connection from the upper input's tip to the speaker that's used by the lower jack. If this would also carry a signal, I fear this wouldn't end too well.
Another thing to remember: in the 8ohms single speaker mode the cabinet can only work with half of the total power. Since the Reußenzehn doesn't feature any markings, I still cannot tell how many watts the cabinet can stand. Since I wouldn't crank a 100w amp to eleven here at home, I think I can take the risk, though.
For the moment I'm quite pleased with this setup -- switchable input jacks are a great invention. :-)