It won't hurt anything except it will sound different, only slightly perhaps, on careful listening. Typically at 8ohms the highs will be smoother and the bass a little loose. On 4 ohms you will have a tighter bass response and it may or may not effect the highs.
Thanks, I will check it out.
The 8 ohm tap will output more volts but less peak current.
Power = Volts x Current
= Volts x Volts / ohms
= Current x Current x ohms
Cford, The voltage at the 8 ohm tap is the same as at the 4 ohm tap, but the transfer of power to the speaker will be lower depending on the speaker inpedence. You are confusing the power consumption of the speaker depending on its impedence. The speaker at 4 ohms will consume twice the power of the 8 ohm speaker for the SAME voltage, if attached to the 4 ohm tap.
Tube outputs use transformers to match the voltage and impedence to the speaker requirements. Attaching a 4ohm speaker to the 8 ohm taps usually results in reduced power transfer and reduced damping, thus 'losser' bass. The opposite, results in stiffer bass, again with reduced power transfer. I find, that when in doubt, put the higher impedence speaker to the lower tap.
Cford & Bob P. , you are both right depending on how you look at the issue. At a given sound level the nominal voltage output to the speaker is the same but you may run out of juice on loud transients & deep bass at 8 ohms. At a given level of the volume dial, the 8 ohm tap gives you a higher nominal voltage output. FWIW.
Djk: I have a Cronus at 55WPC and found that the 4 Ohm setting sounded better with my Vandersteen 2Ces, which are above 4 Ohms nominally but still have a minimum impedance dip to 4 Ohms. The bass is better and there is a better dynamic punch. Do you know what the minimum impedance is for your speakers? If it is less than 4 Ohms and it probably is, then I would go with those taps.
If you load the 8 ohm tap with a 4 ohm load, the plate to plate impedance that the tubes are supposed to be driving will be reduced. This will result in less power and increased distortion (the 'loosness' that others have mentioned).
This is harder on the tubes, as the loss in power is dissipated in the tubes themselves.
The transformer will also be over-damped, resulting in a loss of high frequency bandwidth.
Ultimately, if you are investing in a tube amplifier (*any* tube amplifier), your tube amplifier investment dollar will be best served by a speaker that is at least 8 ohms as opposed to 4, all other things being equal.
If you can go with 16 ohms things get even better, but some amps these days don't have a 16 ohm tap, which is really too bad.
The interface between the amp and speaker is paramount to getting the performance out of any amplifier, transistors included.
Thanks to all for the advice. The speakers are Dali Helican 400's at 4 ohms. I think I will switch to the 4 ohm tap and see what happens. It sounds like that would put less stress on teh amp and perhaps better sound?
DJK let us know the outcome.
firstname.lastname@example.org...If the 4 ohm and 8 ohm taps have the same voltage (per your comment) why do we have two taps?
Actually, the 8 ohm tap has twice the windings of the 4 ohm tap, and puts out twice the voltage as Cford noted. So, for a given setting of the volume control, the amp will play louder. But it will clip at lower power because the output tubes are not optimally loaded. This may also increase distortion.
Eldartford, must be my dyslexia! Of course the voltage is twice as high on the 8 ohms as the 4 ohms taps, I reversed voltages with current to speakers or power or something in my mind! My comments about power transfer to unmatched speakers to taps holds, I believe.
I switched to the 4 ohm taps and wow what a difference. It was like a blanket had been removed from the speakers. Much better nearfield and detail.
Hi Eldartford, actually the 16 ohm tap is twice the number of turns as the 4 ohm tap. The 8 ohm tap is in between. The impedance of the winding goes up by the square of the number of turns. So the 4 ohm tap is the center tap between 0 and 16.
Also, the voltage of the 8 ohm tap is higher but it is not double! The difference is about a factor of 1.414, assuming that you have the same amount of power into your speaker loads of 4 and 8 ohms and the taps are properly loaded.
If you put a 4 ohm load on the 8 ohm tap the tubes will not be able to make the right amount of power and it is possible that your output voltage may go down rather than up. This depends quite a lot on the way the output section is constructed.
Atmasphere...Right about the impedance ratio. (I should have known that).
yeh you are pushing the amp to hard thats why there are taps use the 4ohm taps for 4ohm speakers and it will sound better because you will regain your headroom