Impendance taps setting on a tube amp

The transformer transforms the 4 ohm impedance to the higher impedance that the tubes have to drive. If you load the 4 ohm tap with 6 ohms, the impedance that the tubes see plate to plate will also go up. You will get slightly less power, but the tubes will make less distortion. OTOH, the transformer will not be loaded quite right and so will ring (distort) a bit more. So you get 6 of one/half dozen of the other. It will not hurt the amp.

Bottom line is you just have to try it. I can't think of a good reason to not have the taps available on the outside of the amp. The last thing a manufacturer wants is for the customer to have to open up the unit and mess with it!

I believe that the amount of power capability that would be sacrificed by "light loading" will vary as a function of the amplifier's output impedance, and in many cases will be considerably more than 20%.

Assuming the amp is designed to present the same loading on the primary side of the transformer when an 8 ohm speaker is connected to the 8 ohm tap as when a 4 ohm speaker is connected to the 4 ohm tap, then as the amp's output impedance approaches zero the reduction in power capability into an 8 ohm speaker as a result of being connected to the 4 ohm tap will approach 50%.

That is because the voltage step-down ratio of the transformer to the 4 ohm tap equals 0.707 of what it is to the 8 ohm tap (0.707 = (1/square root of 2)), so for a theoretical output impedance of zero, and based on P = (Esquared)/R, only half the power will be dissipated by the 8 ohm speaker impedance if the voltage across it is reduced by a factor of 0.707.

With a non-zero output impedance the power sacrifice will be less than 50%, with the per cent sacrifice decreasing as the output impedance of the particular design increases. Of course, placing a 6 ohm speaker across the 4 ohm tap instead of an 8 ohm speaker will reduce that sacrifice as well.

Regards,
-- Al

From Roger M:

"The RM-10 is very tolerant of short circuits and will not be damaged by them." "The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material." "At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes' rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB."

Thank you all for interesting responses. I'm not sure why RA decided to keep the taps inside, but I assume there must have been a good reason for it. Here's an excerpt from the Cronus manual regarding the impedance setting:

The Cronus also provides options for either 4 or 8 ohm speakers. The impedance selected will depend on the speakers used. Consult either the owner's manual for your
speaker or your local dealer to determine the correct choice. The amplifier is factory set to 8 ohm.
To switch impedance setting:
1) Using a #2 phillips screwdriver, remove the top cover or deck.
2) Using a 5/16” wrench, remove the outermost nut from the positive (red) binding post.
3) Remove the 8 ohm tap (green wire marked 8) from the binding post. Be careful to leave the black wire with heat shrink on the post.
4) Remove the polyurethane tubing from the unused 4 ohm wire (yellow) and lug.
5) Place the 4 ohm wire onto the binding post and replace the outer nut.
6) Place the polyurethane tubing on the 8 ohm wire and lug.

Not looking forward to fiddling with this, I must say. I just hope it sounds more complicated than it actually is...

Another point I wanted to make and see what you guys thought about it is the fact that most speakers' impedance is not linear but varies across the frequency spectrum. Out of curiosity I went through a few copies of Stereophile yesterday and checked the graphs in the tech specs section. All of the speakers reviewed including ones in the $30-40K price range exhibited impedance that was both above and below the specified nominal impedance. If that's the case, wouldn't the arguments about "light loading," considerations about losing power or halving power be rather moot in practice, given the fact that the amp is presented with different loads depending on the frequency?

Wouldn't the arguments about "light loading," considerations about losing power or halving power be rather moot in practice, given the fact that the amp is presented with different loads depending on the frequency?

My feeling is that those considerations are not made moot by variations in impedance vs. frequency, they are just made less predictable, and the need to judge by listening is given greater impetus.

Since maximum power levels are typically required in the bass region, though, I suppose that some sense of the degree of power sacrifice that would result from "light loading" could be had by assessing the degree to which the speaker impedance in the lower part of the spectrum is greater than the impedance that the tap the speaker is connected to is nominally intended for (if in fact it is greater!).

Another point to keep in mind, though, related to that part of the spectrum, is that for a given amp design output impedance is lower at the 4 ohm tap than at the 8 ohm tap. Everything else being equal that will result in better damping of back emf from the woofer, and tighter control of the woofer, as Phil indicated in the first response above. So that is one more tradeoff that is involved, along with the others that have been mentioned -- reduced maximum power vs. improved woofer control.

Regards,
-- Al