07-03-14: Jmcgrogan2
I have been told that the lower ohm taps will drive more current, whereas the higher ohm taps will drive more voltage. I don't know if there is any truth to that though. Maybe our local electrical hero Al (Almarg) will drop in and enlighten us on this phenomenon.
Thanks, John :-)
That's true, although there are a number of other factors which also come into play when comparing the two taps with a given speaker.
Usually a tube amp having 4 ohm and 8 ohm taps is designed such that its maximum power capability when driving an 8 ohm load via the 8 ohm tap is the same or similar to its maximum power capability when driving a 4 ohm load via the 4 ohm tap. For a resistive load:
Power = Voltage x Current = (Current squared) x Resistance
= (Voltage squared) / Resistance
From that it follows that delivering the same amount of power into an 8 ohm resistive load connected to the 8 ohm tap as into a 4 ohm resistive load connected to the 4 ohm tap requires 1.414 (the square root of 2) times as much voltage on the 8 ohm tap as on the 4 ohm tap. And it requires 1.414 times as much current flowing into the 4 ohm load connected to the 4 ohm tap as into the 8 ohm load connected to the 8 ohm tap.
Also, since the output transformer transforms impedance in proportion to the square of its turns ratio, while transforming voltage in direct proportion to the turns ratio, it follows that the output impedance of the 8 ohm tap will be approximately twice what it is on the 4 ohm tap. Which means that for a **given** speaker impedance the damping factor (which is inversely proportional to output impedance) will be twice as high for the 4 ohm tap as for the 8 ohm tap.
To address Grimace's questions:
1)No, you will not hurt the amp by using the 4 ohm tap with your nominally 8 ohm speakers. That is often done, and as John indicated it generally pays to experiment. And the comments by Kevin Hayes which he referred to are of course correct.
2)I couldn't find an impedance curve for your speakers, but of course the impedance of most "8 ohm" speakers varies significantly as a function of frequency. So the 4 ohm tap may be a better match for the amp at some frequencies than the 8 ohm tap.
Also, as I indicated above, damping factor will be greater when using the 4 ohm tap with a given speaker than when using the 8 ohm tap. That figures to be a major reason for the better defined bass you noted.
Also, tonal balance will be affected by the interaction of amplifier output impedance with the speaker's impedance variations as a function of frequency. That can be thought of as a
voltage divider effect occurring between the amplifier's output impedance and the impedance of the speaker at a given frequency. If the speaker has been voiced with the expectation that it is likely to be used with a solid state amp (nearly all solid state amps having negligibly small output impedances, amounting to a tiny fraction of an ohm), the effects on frequency response of that interaction using the 4 ohm tap would come closer to the interaction that was intended by the designer than would be the case with the higher output impedance of the 8 ohm tap.
Finally, distortion introduced by the amplifier will be affected by how well matched or mismatched the tap and the speaker impedance are at various frequencies. How that nets out subjectively, in cases where speaker impedance varies widely as a function of frequency, can be expected to not have a great deal of predictability.
The bottom line: It pays to experiment with the different taps, and no harm will result regardless of which tap is selected. Assuming, of course, that volume levels are not turned up to the point that the amp is asked to deliver significantly more power than it is capable of, in which case it would most likely clip and distort very noticeably.
Best regards,
-- Al
