13ohm speaker load: 16 or 8ohm tap?

[EETStudent]

Just received my new Gibson GA42RVT, really sweet amp. They had the speaker cable plugged into the 4 ohm jack, but the two speakers were in series measuring 13 ohms. Should I use the 8 or 16 ohm tap?

[Rob Mercure]

Don't confuse the DC resistance (DCR) of the speakers for the AC impedance - the DCR is always lower as it doesn't includes the reactive component so what your've got is two 8 ohm speakers in series or 16 ohms. Perhaps the person who hooked it up before was confused about speakers in series and parallel and thought the combinations was 4 ohms. Anyhoo you should now know which impedance to use (and if they are, for some reason, dissimilar speakers wire them in parallel and use the 4 ohm OT tap - identical doesn't matter).

Rob

[Robert M. Martinelli]

What Rob said.

Always expect a lower reading ( e.g. about 3 Ohms for 4 Ohm speakers, some 6 Ohms for 8 Ohm speakers, some 12-13 Ohm for 16 Ohm speakers ) because you' re only reading DC resistance, not impedance.

Your current situation is not risky, though, we covered this in another thread about impedance mismatches, anyway, it' s safer to have an impedance mismatch on the high side ( speakers' impedance > output impedance ) than on the low side ( speakers' impedance < output impedance ) because a lower-than-expected load impedance forces more current to flow; on the other hand a higher-than-expected load impedance imposes less current to flow and this can reduce the voltage drop on the OT primary raising +B voltage a bit ( true with tube rectifiers which have a higher differential resistance and tend to "sag" with high current demands ).

Hope this helps

Regards

Bob

[EETStudent]

Another thing I notice is that even though the speakers are connected in series(about 16ohms), the 4 ohm tap provides a much bigger sound. Is this because of the mismatch? Or does a 4 ohm load generally differ tonally from a 16?

[Rob Mercure]

There's a couple of levels to your question. As long as the impedance between the amp and speaker matches the different impedances won't sound different enough to really notice. But there is a difference between series and parallel since the series circuit current is common to all the speakers. If you've got speaker with differing impedance vs. frequency profiles you can have, for all practical purposes, one speaker dissipating all of the power at a particular frequency. So, as a sweeping generalization, parallel combinations are preferable - and almost mandatory with dissimilar speakers.

Oh well, meant to go into this more but breakfast is burning - you can research from here.

Rob