Contrary to what you may have read or heard, I would expect (with one irrelevant exception described below) that for any given speaker there would be LESS strain on the amp using the 4 ohm tap than using that same speaker on the 8 ohm tap. Although as I and others have said, it is extremely unlikely that you would have a problem either way.
The 4 ohm tap puts out lower voltage than the 8 ohm tap, which would result in less current flowing into a given load impedance. Also, the load impedance seen in the plate circuits of the output tubes would be higher (again meaning less current flow) if the speakers are connected to the 4 ohm taps rather than the 8 ohm taps (since the load impedance seen on the primary side of the output transformer is equal to the load on the secondary side multiplied by the square of the xfmr turns ratio, the stepdown ratio to the 4 ohm taps being greater than the stepdown ratio to the 8 ohm taps).
The irrelevant exception I alluded to would be if the speaker impedance at some frequencies dropped down to outlandishly low values, say 1 ohm or so. In that case the output impedance of the amplifier would become a significant determinant, and perhaps the major determinant, of how much current would flow from the amplifier. And since that output impedance is lower on the 4 ohm taps than on the 8 ohm taps, greater current might flow from the 4 ohm taps in that situation. However, the only speakers that I am aware of that have impedance curves dropping down that low are certain planar speakers like the 1980's Apogees. I am not aware of any box speakers that do that, and certainly none that are rated at 6 ohms nominally.
I suspect that those who may have asserted the contrary are misled by the fact that a lower impedance speaker, or a speaker that drops down to low impedances at some frequencies, which would typically be connected to the 4 ohm taps, represents a more difficult load than a higher impedance speaker, which would typically be connected to the 8 ohm taps. But here the speaker is not a variable -- we are talking about connecting the same speaker to one tap or the other.
Finally, take a look at the video Pacific Valve has about your amp: http://www.pacificvalve.us/MDMC34AB.html. Fwiw, it is described as an amp that "loves difficult loads."
Not to worry!
Regards,
-- Al
The 4 ohm tap puts out lower voltage than the 8 ohm tap, which would result in less current flowing into a given load impedance. Also, the load impedance seen in the plate circuits of the output tubes would be higher (again meaning less current flow) if the speakers are connected to the 4 ohm taps rather than the 8 ohm taps (since the load impedance seen on the primary side of the output transformer is equal to the load on the secondary side multiplied by the square of the xfmr turns ratio, the stepdown ratio to the 4 ohm taps being greater than the stepdown ratio to the 8 ohm taps).
The irrelevant exception I alluded to would be if the speaker impedance at some frequencies dropped down to outlandishly low values, say 1 ohm or so. In that case the output impedance of the amplifier would become a significant determinant, and perhaps the major determinant, of how much current would flow from the amplifier. And since that output impedance is lower on the 4 ohm taps than on the 8 ohm taps, greater current might flow from the 4 ohm taps in that situation. However, the only speakers that I am aware of that have impedance curves dropping down that low are certain planar speakers like the 1980's Apogees. I am not aware of any box speakers that do that, and certainly none that are rated at 6 ohms nominally.
I suspect that those who may have asserted the contrary are misled by the fact that a lower impedance speaker, or a speaker that drops down to low impedances at some frequencies, which would typically be connected to the 4 ohm taps, represents a more difficult load than a higher impedance speaker, which would typically be connected to the 8 ohm taps. But here the speaker is not a variable -- we are talking about connecting the same speaker to one tap or the other.
Finally, take a look at the video Pacific Valve has about your amp: http://www.pacificvalve.us/MDMC34AB.html. Fwiw, it is described as an amp that "loves difficult loads."
Not to worry!
Regards,
-- Al