O.K. to use 8 ohm taps?

[analogman]

Hello,

Could anyone please tell me if running a pair of Heresy II's, which are the type rated 4 ohm nominal impedence, using the 8 ohm taps of my tubeamp could damage either the speakers or the amp? I have done so briefly at moderately loud volume levels to compare the difference in sound. However, I am concerned as to whether this is a 'sound' thing to do. If it is a bad idea and someone knowledgeable has the time please explain why.

Thank you for your time.

analogman

[JohnA]

It won't hurt a bit. Even the HII has a pretty wide impedance curve. My His vary from 7.5 ohms to 120 ohms in the midrange.

However, why would you want to? If the woofer runs at 4 and the squawker and tweeter are higher, running on the 4 ohm tap is a little safer for the amp.

[analogman]

Dear Mr. Albright,

Thank you for your response. I still have a question, however. What does "safer for the amp" mean in terms of negative results? In other words, what's the worst case scenario driving a 4 ohm load with 8 ohm taps? Could doing this, over time, readily degrade or compromise the amplifier's circuitry in any way that may not be readily apparent.

Thank you for your time.

analogman

[wrench_peddler]

I didn't sleep at a Hoiday Inn last night but do feel safe in telling you that the load an amp sees with 4 ohm speakers is twice the load of 8 ohm speakers. If you run 4 ohm speakers on the 8 ohm taps, the amp is wired and is looking for an 8 ohm load but getting twice the load it expects. I hope that make sense.

[lynnm]

Running a 4 ohm speaker on an amp rated as having an 8 ohm output impedance will do no harm to either the amp or the speakers. The only risk might be if more than one pair of 4 ohm speakers were connected in parallel in which case the impedance could drop too low and the amp would be presented with what amounts to a short circuit.

The impedance of a speaker varies widely depending on frequency and a "4 ohm" speaker could present an impedance of anywhere from 2-3 ohms up to 70 ohms at any given instant.

[Benesesso]

This thread makes me wonder about the reverse condition-running "8 ohm" Khorns on a 4 ohm SET tap. If I have power to spare, would the distortion be lower?

Think I'll give this a try tonight.

[analogman]

gentlemen,at the risk of beating a dead horse,I am still in the dark. My amplifier has both an 8ohm as well as a 4ohm tap. I understand that these represent different points on the secondary windings,thus presenting a more appropriate load for the output stage.My concern is when one deviates from the "assigned" taps what goes on with the transformer,power tubes etc. with the resulting change in current flow.My knowledge of tube audio circuits is extremely basic.I am unable to visualize what goes on upstream, so to speak.My amp is the JJ Electronics 828, KT88 based integrated.

Thank you for all your insightful feedback.

Analogman

[JohnA]

The output transformer of a tube amp is there to match the high impedance/voltage tubes to the low impedance/voltage speakers.

Using a favorite EE equation, Ohm's Law, V(oltage) = I(current) x R(esistance), you can see that as you reduce resistance, current must increase to balance the equation.

Your tube amp's output transformer sets the output voltage range to the values appropriate for the impedance shown on that tap. If you run a 4 ohm speaker on an 8 ohm tap, you are drawing more current from the amp than its designer intended (and therefore more power). This MIGHT use up the tubes faster, depending on the amp's design and the phase of the moon. An 8 ohm speaker on the 4 ohm tap is just the opposite.

Speakers NEVER have flat impedance curves. Al Klappenberger's crossovers come darn close. The original Heresy was all over the place. An H-II probably is, too. If the lowest impedance was a 3.5 ohm dip in the bass and the highest was a peak of 150 ohms in the bass and 120 ohms throughout the midrange, how would you rate the speaker? This is what an H-I does, as well as I can remember, and it is called 8 ohms.

Use the tap that sounds the best.

[Malcolm]

IIRC the manufacturers of tube equipment years ago recommended that you use the tap that matches your speakers' nominal impedance to ensure that you are able to transfer the maximum power and achieve the best frequency response. Other than that, I don't think there is much of a down side to using a different tap.

OTOH using the wrong tap on a solid state amp can sometimes be a problem. In certain cases, it could lead to destruction of the output transistors.

[WMcD]

I believe that the 4 ohm tap is the best.

This makes the amp more like a voltage source, meaning it most able to deliver current. It also means that there will be less noise.

Gil

[D-MAN]

I could be all wet on this, and it could just be a "folk tale"...

I have heard that mismatched output impedance results in excessive heat at the output transformer when driven at high volume (i.e., high current levels).

I would guess that the difference would be current levels at twice the expected (at the transformer) load. However most modern transformers can handle very high temperatures indeed. Also there is quite a difference between 4 Ohms and 16 Ohms vs. 4 and 8 Ohms respectively... probably not going to be much of a problem.

Whereas 4 vs. 8 Ohms mismatch is not much of a problem, any more than that and I would hesitate to mismatch the output on an OLD amp for fear of smoking an output transformer.

DM