Understanding Tube Power Ratings

Tubes watts are reputed to be louder than solid state watts. Of course this is not technicaly true. Tubes tend to overolad more gracefully than transistors. Jim Thiel told me that the recommended power given for his speakers were given with quality solid state amps, which could double down, in mind. He further recommended doubling those recommendations when using tubes, and to use the 4 Ohm taps on tube amps. Make sure the amps are up to the job at hand.

1) Gs: I've never seen anything in Ohm's law that states the following:

"While Ohm's law says this is inevitable, Ohms law also states that the voltage decreases with an increase in power (current). As the load is halved, the PS voltage drops and lessens its ability to keep up the current flow."

The only time voltage will sag or drop is if there is not enough current to be had within the circuit. The "pressure" ( voltage ) within the circuit will remain consistent so long as the "volume of flow" ( amperage ) is maintained. As such, an amp can act as a voltage source ( doubles down in power output as impedance is halved ) so long as the power supply and output stages are capable of passing enough current to meet the demands placed upon them.

When you find an amp like this, you will notice that the clipping power at 4 ohms is almost exactly twice that of what it is at 8 ohms. If it is really a "high current" design, it will produce twice the power output at clipping into a 2 ohm load that it does at 4 ohms.

2) Since there is SOOOO much "fudging" of power output specs and manipulation of numbers for marketing purposes, forget about "rated power" and start looking at what the amp does at clipping at various impedances. This will tell you a LOT more than what they rate the unit at. On top of that, forget about frequency response on a power amp. Look at the power bandwidth. Frequency response is spec'd at 1 watt of output ( typically ) whereas power bandwidth is the frequency response as measured at rated power.

3) As to tubes vs SS, tubes appear to be more powerful than an equivalently rated SS amp for multiple reasons. SS amps tend to clip much harder and sound much worse when running out of steam. Tube amps can be pushed into clipping without sounding nearly as harsh. In effect, this makes them sound like they actually have more headroom since they aren't getting "nasty" sounding yet they are being run at or above rated power output.

4) As to tube amps having a much higher headroom rating than SS amps, that is a pretty broad generalization that i can't agree with. Either design can have a lot of headroom. While most of this will have to do with the power supply of either unit, the transistors would obviously have to be able to pass enough power to do this. While the same is true for tubes, some tubes will do this far easier than others. Tubes with a high Mu are probably more suitable to "peak power" or dynamic headroom than those with a lower Mu with the same power supply / circuitry supporting them. At least that's what we've come to find out with RF circuits. 8417's are a perfect example of a tube that works excellently in an RF circuit and was also used in audio power amps. Too bad these tubes aren't available anymore. 6L6's are another example, but they are nowhere near the tube that an 8417 is.

5) Regardless of all of the above, it is harder to match a bunch of output tubes than it is to match output transistors. This is not to mention that tubes are "born to die" and replacing them at random as needed will surely create greater imbalances / poorer performance over time. As such, most "experts" agree that all of the tubes within one section of a circuit should be replaced at one time. Matched parts should be used when doing this if possible.

6) While i have nothing against tube amps or those that favour them, you should know what you are getting into should you choose to go that route. Tube power amps are SURELY higher maintenance than an equivalently rated SS amp and i don't think that anybody ( tube-head or not ) would argue that point. If you like the sonics of a tube amp enough to put up with the maintenance and expense that is required to keep it running at optimum performance levels, go for it. Sean
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I had more success running my 2.2's on 45 tube watts (C-J) than on 70 (Classe) or 100 (NAD) SS watts. Now I have them on 200 tube watts (VTL) and they're even better (this isn't a parable though - my SS amps weren't of comparable cost or quality to yours, or to my tube amps). The 2.3's are a little more demanding than my speakers, but for average listening levels in an average-sized room, I would imagine that good 100-watt tube amps could contend. Stick with beefy power supplies and output transformers, and leave maximum rated power to be influenced by how loudly you like to listen and your room size. (For an amusingly perverse take on the conventional wisdom concerning 'doubling-down' and SS vs. tube power, check out Ralph Karsten's white paper on the Atma-Sphere website.) Bottom line: Sound quality is difficult to directly correlate with power ratings (and that goes in both directions!) or choice of active devices (tubes or transistors) - better to trust your ears.

Sean: Since tube amps, as a rule, don't 'double-down' as some SS amps can, but are certainly capable of sounding just as good, why do people focus on this aspect of SS amp performance and not of tubes? Is the fact that this capability doesn't seem to reliably correlate with sonic quality indicative that in the real world, most good-quality and properly-matched amps are not being called upon by actual speakers and listening styles to test their ultimate limits in the current delivery department, thus obviating the distinction? (In other words, at listening levels comfortably below the onset of clipping, even tubed amps are putting out all the current being demanded of them into varying speaker impedances when playing music?) If true, pure voltage-source behavior into static loads using typical test signals at maximum rated power would seem to be a red herring of a specification for anything other than offering bench confirmation how over-built any particular big SS bruiser may be - not a bad thing, but maybe not the most important or telling of qualifications either.

Z: Tubes and transistors are required to do the same thing in an amplification circuit. The advantage that tubes have is that they have ( typically ) a much greater voltage potential than that of most conventional SS amp.

As mentioned, so long as one can provide enough current to drive the load as needed, the amp with the highest voltage potential will typically play louder and sound cleaner doing it. As such, a tube amp is ahead of the game so long as the power supply, tubes and output transformer are relatively "beefy". The only times that you might run into severe problems with tubes on the output are if the speaker is very low in impedance and the output transformers are puny with impedance taps that are not well matched to the speaker. Otherwise, you will have a harder time controlling the driver and you'll eat tubes up like there is no tomorrow.

My personal preferences in SS amps are for those that run very high rail voltages and can provide gobs of current. Most SS amps will never have the voltage potential of a tube, but for all practical purposes, a tube will never have the same amount of current. As such, tubes will work best with higher impedance loads that are non-reactive in nature whereas a well built SS amp will offer greater control on low impedance / high reactance speakers. These are both very "broad" generalizations at best, especially since there are so many varied types of tubes / ss output devices and the support circuitry and designs that go along with them.

Having said that, almost all of my speakers are low impedance and make use of multiple woofers. The one system that i do have that has a tube amp in it is of a higher impedance that is of very high efficiency and a single woofer. As such, the single woofer is not making long excursions to produce high volume levels due to the very high efficiency, so there is a minimal amount of reflected EMF to deal with. As such, the need for "muscle" or "high current" is not very critical.

As to what is required of most amplifiers during normal listening sessions, bare in mind that speaker impedance, reactance, loading characteristics, etc... all vary as spl is raised. Whether or not one needs "brute force" for their system will deal with their individual speakers, room and type of music listened to.

Besides that, it all boils down to personal preference and system matching. Does anybody REALLY need the power, handling, braking, etc.... of a Porsche or Ferrari ? Even if they don't "need" such things, you can bet that they are running the vehicle on high octane fuel, have good tires on the vehicle and go in for regular maintenance. That is, if they want the vehicle to perform at peak potential should they ever really "need" it. : ) Sean
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Tubes rule!