Tube Amp. Power ratings

What is it about tube equipment that intrigues you?

The pwr. of tube amps decreases rapidly once the spkr. impedence deviates from the output tap being used. Beaware of the heat in summer unless you live in the polar regions. Bear in mind that matched prs. of output tubes can be pricey w/ some amps needing retubing after 2000 hrs.

>>The pwr. of tube amps decreases rapidly once the spkr. impedence deviates from the output tap being used<<

Not true. In the case of a 12 ohm load connected to an OTL amp, for example, the output power actually increases.

Conversely, the power output of a solid state amp decreases as the impedance increases.

The door swings both ways.

In general you will be noticing more detail with a more relaxed presentation with tube gear. As you noticed, power is harder to make with tubes so they are often less powerful for the money. You have to use a speaker that is designed for tubes- tube amps tend to make constant power with respect to load rather than constant voltage of most high quality transistor amps. This appears to be a matching issue, but it is more than that, take a look at

http://www.atma-sphere.com/papers/paradigm_paper2.html

If you use a speaker intended for tubes with a transistor amplifier, you will get tonal aberrations just like you will if you use a tube amp to drive a speaker intended for transistors. But overall, if you have a setup where the speaker is optimized for the tube amplifier, the presentation will be more lifelike and musical, as tubes in general obey more of the rules of human hearing than transistors do. Its something that you hear right away- its not subtle.

I've seen 20 year old amps running on original tubes. Heck, I've seen 40 year old tube amps running on the original tubes. In any case, I wouldn't worry too much about tube costs as a miantenance issue, but if you're coming from the SS camp, it should be a consideration.

I've seen sytems with SS pre's and tube amps that were great. That being said, if you like tubes you will likely end up with a tube pre and tube amps as they have similar attributes. Tube pre with SS amps is both popular and helps with tougher speaker loads while adding some tube magic.

In any case, if you buy used to try, you can almost always get the same money back out that you have into the piece.

My persoanl preference is to have tubes everywhere, but also remember not all tube amps sound the same and some amp/speaker combinations will be a problem as mentioned earlier. But oh, the fun of the journey!

And, of course you can listen in the dark and enjoy the
tube glow.

RFG

what speakers per se would be "tube speakers? and then tube digital etc?....

OTLs are an exception to the rule being designed for 8 ohms or greater. These amps are virtually useless w/ most 4 ohm spkrs of which there are many more on the mkt than 14/16 ohm spkrs.

Whether tube amps reveal more detail is highly debatable. Most white papers need to be carefully analyzed as the mfg. isn't always objective.

given a reasonable speaker match, to my ears push-pull tube amps sound much more powerful than ss of the same ratings, partly because the distortion artifacts from tubes are more benign than from ss. I moved to tubes when I realized I had lost interest in listenting to music, and identified part of the issue as the somewhat irritating sound produced by humanly affordable ss (digital was the other factor). The reviewers seem to think that extremely expensive ss amps (eg Boulder) do not suffer in comparison to tubes, but that is academic to many of us. Single ended triod amps are another category entirely, and sound more lifelike in the midrange than anything else, but require extremely efficient speakers without low impedence dips and most suffer in the highs and lows compared to other designs.

>>OTLs are an exception to the rule being designed for 8 ohms or greater<<

Exception or not, it invalidates your initial assertion.

Furthermore, there are OTL amps that handle 4 ohm loads with ease.

Sorry.

How many OTL amps are there which will easily handle 4 ohm loads that are inefficient w/ a wicked phase angle? I'd be willing to bet these amps are stratospherically priced, & one can count these amps on one hand & not use all 5 digits.

Inefficiency, phase angle, and price were not parameters in your initial argument so feel free to change the rules as we play the game.

One more time and then I'm out there are OTL, even push-pull and SET, amps that will easily handle 4 ohm loads.

Hasta la vista baby.

Why deal w/ real issues? Enjoy your OTLs, SETs & have a nice holiday.

Porziob, the fact that there are OTLs that can handle four ohms is not a matter of debate. Some 20 years ago, your assertion was common knowledge, but the succeeding decades have produced amplifiers that fly in the face of that. Also, that tubes are more revealing is a demonstration issue, but one so apparent that it is easily settled in 5 seconds flat- its not subtle. If you've not been exposed to such technology yet, perhaps its time?

I should point out that if you are considering a tube amplifier, it really is a matter of the right tool for the job- you wouldn't use a hex wrench to tighten a pipe, you use a pipe wrench for that. 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.

IOW the point here is not the ability to arc-weld, as that has nothing to do with hearing- its all about delicacy and authority manifesting in the same system at the same time. This is the pervue of tubes, which is lost on 4 ohm speakers ( which really did not appear until transistors came in). 16 ohm speakers are starting to appear again, now that there are more tube equipment manufacturers in the US now than there was in 1956...

Wake me when it is over. So much has been written. Should we have to suffer???

I would be interested in some commentary regarding how the fairly recent breed of solid state amps known as "chip amps" or "gainclones" fit into this discussion. As I understand it, chip amps perform best with a "friendly" load (i.e., efficient speakers with a benign impedance curve, no impedance dips below four ohms, and no difficult phase angles to negotiate). So I understand that, despite the fact that these little amplifiers are solid state circuits using semiconductor chips in lieu of transistors, the amps are generally best paired with speakers that were designed to pair with tube amps. I should note that I have minimal understanding of the more technical aspects of circuit design and amp/speaker interface. (I probably already made that clear with some gaffe in my narrative above.) I am interested in some perspective from the technical set on how best to choose speakers to pair with this new breed of amplifier.

Cincy_bob, 'chip amps' have been around in one form or another since the late 70's. The issue with them is that the form factor limits the amount of heat they can dissipate and the amount of current that they can pass. So you can damage them if you try to operate them on 4 ohm speakers. They require feedback for operation, a fundamental difference between them and tubes, which can be designed to operate without feedback.

Negative feedback enhances (slightly) high-order odd harmonic distortion, a form of distortion that the human ear can detect in levels of 100ths of a percent. This we hear as harshness or at best an unnatural sheen on top.

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.<<

Thanks for the clarification, Mr. Karsten. One follow-up question: You mentioned in your post above that pairing a speaker intended for tubes with a transistor amplifier will result in tonal aberrations. Given that chip amps are generally not suited to driving difficult loads and seem best suited to pairing with speakers that were most likely designed around tube amplification, does this pose a quandary when trying to match these amps to speakers? In other words, does this leave you with one of two less than ideal choices: (1) You can pair the amps with speakers designed for solid state amplification (causing heat and reliability problems due to low impedances - not a very practical alternative) or, (2) You can pair the amps with speakers designed for tube amplification (causing tonal aberrations due to the solid state nature of the amplification).

Am I missing something here? Is there a type of speaker that is ideally suited to pairing with a chip amp?

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...