Have the tech change the new posts to the 8ohm taps.
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Normally the 8 ohm tap would be correct but:
The 8 ohm tap will output 2x the voltage but 1/2 the current as the 4 ohm tap. However speakers impedence is not purely resistive and depending on the impedence vs freq. profile of your speakers, the 4 ohm tap might be better. Try both (as long as you don't listen too loud) to see which sounds best to you.
Yes, it is a try and see kind of thing, but the 4 ohm tap will always have lower impedance which will translate into better damping of the speaker, no matter what impedance the speaker is. This will not always sound "best" and using a 4 ohm tap with an 8 ohm speaker will sometimes diminish the power output. I say "sometimes" because the interaction is complex and many 8 ohm speakers drop down pretty far at a given frequency, as mentioned above.
If you have an 8 ohm speaker, use the 8 ohm tap. If you use the 4 ohm tap, the transformer will not be loaded correctly and will 'ring' that is to say it will add distortion, which usually makes the amp sound brighter (though not always).
Conversly, if you put a 4 ohm speaker on the 8 ohm tap you just cut the load that the tubes see. For example if the tubes are seeing 3000 ohms 'plate to plate' the impedance just dropped to 1500 ohms. This will cut power (maybe by as much as 1/2) and add distortion! Transformers are called that because they transform impedance and that goes both ways.
Funny, but the construction manual that came with my Dynaco 70, which I built in 1970, indicates that putting 8 ohm speakers on the 4 ohm taps, while reducing the max power available due to less power transfer, will give LESS distortion and if max power is not a concern then that is really the best way to connect the speakers, if in doubt.
Don't mean to hijack this thread in any way, only hope to augment it because I have a similar question. I have 2Ce Signatures and I see in the product literaure that the nominal impedance is 7 Ohms and the minimum is 4 Ohms. So what really makes sense from a rational standpoint which taps should I use? I'm hoping to hear something other than, "whichever sounds best." Thanks!
Eldartford, you know of course that the secondary on the transformer has different taps, 4, 8 and 16 to match the speakers. Those taps simply have different amount of winding to 'transform' the primary winding impedance which is constant. The tubes themselves don't see the secondary taps or impedance.
Salut, Bob P.
firstname.lastname@example.org...You have it backwards. The different taps of the transformer secondary make it possible for the tubes to see the same (high) impedance with different loads.
Here is a way for you to test your theory. Short out the speaker terminals of your amp and drive it hard. If your theory is correct, the tubes won't see the short. After the fire department has left, post results of this experiment :-)
Eldartford, exactly - the tubes do not see the difference of what is connected to the secondary taps or to which taps the speaker is connected. The poster that said that connecting the speakers to the 'wrong' tap would make the tubes 'see' a different load and thus distort their output.
The reason that the amp overheats when shorting the speaker taps is due to the tranformer trying to drive a 0 inpedence load and thus drawing more power from the tubes on the primary side. It has nothing to do with the impedence changing on the primary (which doesn't), which the tubes 'see'.
Salut, Bob p.
email@example.com...How can you say that a zero ohm load is "seen" through transformer coupling, but yet loudspeaker loads are not?
In another area, when you use a step up transformer for your moving coil phono pickup it can be connected to a standard MM preamp input, 47K input load, and still present the proper loading of a few hundred ohms on the pickup. The loading applied to the secondary is "seen" through the turns ratio of the transformer.
Eldartford, I think that we are saying the same thing, but not understanding our explanations. I never said that the tubes 'see' a 0 inpedence. We both agree that the transformer effectively isolates the speaker load from the load imposed on the tubes, i.e. the tubes 'see' a constant (high impedence of the primary windings on the tranformer) regardless of the speaker load and regardless of the tap used for the speaker. If an 8 ohm speaker is attached on the 4 Ohm tap, however, the power transfer is not as good as using the 8 ohm tap, but the level of distortion should be lower and the damping factor higher. The tubes never 'see' this, which was the claim made by the original poster (Atmaspere, I think) that using the lower tap would cause the tubes to 'see' a different impedence and thus distort.
Salut, Bob P.
firstname.lastname@example.org...An 8ohm speaker on the 8 ohm tap, and a 4 ohm speaker on the 4 ohm tap will look the same to the tubes driving the primary. (That's why different taps are provided). If the 8 ohm speaker is on the 4 ohm tap or the 4 ohm speaker is on the 8 ohm tap the tubes will see an impedance that is not what the amp designer wanted. Whether this will be enough to cause significant distortion, and if so, how much, is a question I will leave to the amp designers like Atmaspere.
Eldartford, this is where we disagree. If the 'wrong' ohm speaker is attached to the wrong tap, the tubes DO NOT see a different impedence at the primary. The tubes continue to see the primary winding impedence, whose impedence remains constant regardless of what impedence speaker is attached to whatever tap. The power transfer from the transformer to the speaker, however, is affected and also the distortion because of the mismatch of impedence on the secondary windings to the speaker. Due to this mismatch of power, the volume might not be sufficient and the operator then increases the gain at the tubes, perhaps driving them into distortion, but it is not because of the impedence changing on the primary side.
Salut, Bob P.
email@example.com...You say quite plainly, "The tubes continue to see the primary winding impedence, whose impedence remains constant regardless of what impedence speaker is attached to whatever tap". It is not a question of misunderstanding. You are wrong.
How do you explain the MC phono pickup step up transformer example?
No, Eldartford, the person saying that attaching an 8 ohm speaker to the 4 ohm tap will cause distortion because the tubes will 'see' a different impedence than if the speaker were attached to the 8 ohm tap, is wrong! When in doubt, use the lower tap, the worse that will happen is a loss of ultimate power transfer, but with lower distortion.
Respectfully, Bob P.