Tube Amp. Power ratings

Ralph,
Your comment would appear to make it uneqivocal, yet I must ask; does this hold true for tube amps with output transformers specifying 4 ohms?
John

>>In the case of tubes- *any* tube amp, the investment dollar in the amplifier will be best served by speakers that are at least 8 ohms rather than 4, all other things being equal, regardless of the technology of the tube amplifier.<<

Jamnesta, yes. The degradation in performance of any output transformer on its 4 ohm tap is audible and measurable. You can often loose an octave off of the bass end, and often too the actual power output drops as the transformer is less efficient.

Cincy_bob, the way out of the conundrum is to choose a speaker with a flat impedance curve. Such a speaker, if 8 ohms, will be about equally friendly to either technology.

Atmasphere, you seem pretty knowledgeable on this topic. I am not throwing out a trap or anything of the sort. I have tubed power amps (BAT VK-60 running at 120 wpc in mono). My speakers are Wilson Duettes connected to the 4-ohm tap and running pretty consistant at 4-ohms (as these speakers are known to do).

Both Wilson and BAT feel that this is a good match (discussed this with both company's prior to purchasing the amps). As you may (or may not) be aware, Wilson is demonstrating more with tubed amps and virtually all of the Wilson speakers (I believe) are 4-ohm speakers.

Again, this is not a trap and I am not contradicting you in any way.

But would you suggest when running a 4-ohm speaker with tubes it is better to connect to the 8-ohm tap? Connect to the 4-ohm tap? Interested in hearing yours and other's feedback on this question.

My Duettes seem to perform well (I have been extremely pleased with them and this combo, as well as with my MLevinson amp) and I am happy with the sound (although I have always run them with subs).

Atmosphere, I have a similar question to Ckoffend but with a “twist”. I am also running tube amps in mono (+ or – 70 wpc) with 4ohm (Totem Mani) speakers. The Mani’s are rated at 85db and also known to be notorious for radical dips in impedance. I run the Mani’s off the 8 ohm taps on the amps because they sound better that way. Contrary to conventional wisdom, I am very pleased with the results. My question is since I typically listen at low volume levels, what could I expect to hear (or not hear) with an 8 ohm speaker? At low volume levels could I also expect to loose an octave off of the bass end? What else? Can you be a little more specific? I continue to read in these forums that what I am doing is not recommended and I continue to wonder what I am missing.

Ckoffend and Jaffeassc, I would try it both ways but I would expect that the amp and its power tubes would be more comfortable on the 4 ohm tap. The transformer actually does what its named for- it transforms impedance.

Here's how that works:

Let's say you have a set of power tubes and they are expecting 3000 ohms plate-to-plate. Now if you load the 8 ohm tap with a 4 ohm load, you have just cut the impedance in half, so the power tubes are now seeing 1500 ohms plate to plate. The result will be higher distortion and likely significantly less output.

There are output transformers out there that are not that efficient on their 4 ohm taps, so with some amps the 8 ohm tap may actually be preferred, but most other tube amps will likely be set right on the 4 ohm tap. This is of course assuming that the match that the power tubes want to see is in fact satisfied by the windings of the transformer, which is often not the case as the combination is an approximation. So I would expect that there are also some output transformer situations where the 4 ohm tap is getting the tubes closer to the ideal.

If the 4 ohm tap is the correct tap, then loading the 8 ohm tap with 4 ohms will cause the power tubes to work harder, as more of the power they produce will be dissipated inside the tubes rather than the load. So there is a certain gamble in trying 4 ohms on the 8 ohm tap, although tubes are forgiving enough that its not a risk to try- more of a risk to keep it set up that way.

This can be a big topic so I am glossing over a few things, but that's it in a nutshell...