Why tube amps are rated so low, still don't get it

Hi Guys,

I am still very confused as to howcome tube amps are rated so low, and how to tell whether the amp will be able to power your speakers properly. In particular I am interested in the Shanling SP-80 tube amplifiers for my system. This is since I have the CD-T100 and I want to partner it up with the amp for the synergy. The amps are rated at 50W each (monoblocks). Is that enough? How do I compare it to a SS amp? I tell someone 50W SS and they laugh, but I say 50W tube and they call it a monster! I am still very confused!

Thanks for any help/explanation,
Since in general tube amps have a large low freequency slope bellow 70Hz, they seem to sound more loud than SS amps.
You may figure that 50W tube amp will do only half-power bellow 60 Hz and a quarter power @45Hz...50Hz.
So the meaning of "Monster" is realy a misconception since most of the power in this case is being spread from mid-bass to higher freequencies.
Music or audiable freequencies bellow mid-bass require the most amount of power... let's say 70...80%. SS amps can work from DC to a very high freequencies thus spreading the same amount of power over much larger freequency bandwidth than tube amp. In case with SS 50Wpc, 40W may go to the bass freequencies and only 10W will be left-over for the rest of audiable bandwidth while in tube case everything goes right-above bass freequencies and only 25...30% down-bellow...
Such slopes are due to the output transformer that is already itself acts as a high-pass filter. The quality of a low freequency performance and so is overall performance of a tube amp(non-OTL) largely depends on an output transformer.
The linearity of a tube amp is also being altered by speaker load characteristics that are in most cases very complicated.
Hi There,

Thanks for the info! Is there any way in being able to estimate if the amps (Shanling SP-80) will be able to drive the speakers? What factors must I look at besides the speaker power handling and sensitivity rating?

I think this is a more accurate explanation.

It has been reported for years that tube amplifiers seem to play louder than similarly rated SS designs.

Bart Locanthy (now deceased) a former president of the AES society, borough to my attention a test that validates this contention. The test method uses pink noise rather than a single sine wave as a test signal.

The rational is that pink noise, consisting of random amplitude and frequency components over the audio spectrum is a much closer approximation of music and speech than a sine wave of any single frequency. The test consists of comparing the maximum NOISE POWER (or music power) output between SS and tube amplifiers with similarly rated single frequency maximum RMS power output.

The test method is as follows:

The noise signal is applied to the first amp with the output connected to a parallel combination of a load resistor, the vertical input of an oscilloscope and a true RMS voltmeter. (Not an average responding meter calibrated in RMS).

The signal level is increased until the occasional clipping of signal peaks is noted. It is a matter of judgement to determine a rate of clipping over a unit of time that can be reasonably duplicated.

The reading on the meter is noted and the power calculated by the formula -Voltage squared over the value of load resistance. The same test is then repeated on the second amplifier. You will discover that a well designed tube amplifier will deliver 2 to 3 X more power than the solid state unit. The test described was with a Wolcott Presence in comparison to a well respected SS amp of 300 Watt rating, and revealed a 3 to 1 power advantage with tubes, delivering 75 watts to the SS output of 25 watts.

Another type of test was conducted by "Stereophile" magazine several years ago using tone bursts instead of pink noise, showing almost identical results.

Other advantages of tubes, particularly in the output stage, is that they do not exhibit slew rate limiting or reduction of gain with increasing frequency except for electron transit time considerations.

Transistor current gain falls off with frequency as their alpha factor approaches unity or zero gain. From a designers standpoint, tubes are easier to work with because they are characterized by only three main parameters:

Mu factor, Mutual conductance and plate resistance, whereas transistors require 22 parameters to fully characterize them.

This complication, in conjunction with their less desirable transfer function makes them prone to the generation of many more forms of distortion in music reproduction than are possible from vacuum tubes.
At the risk of asking a dumb question, exactly what speakers are you trying to drive? What is the minimum impedence? What is the efficiency spec? With that someone can give you very specific advise as to whether or not you should use tubes amps and how much power you need.
Hi There,

The speakers are not 100% certain yet. I am busy deciding (will decide this week) which ones I will go for exactly, but the top two candidates and their specs are as follows:

Monitor Audio GR-60
Frequency Response +/- 3dB 28Hz-30KHZ
Sensitivity (1W@1M) 90dB
Nominal Impedance (Ohms) 6
Power Handling RMS (W) 200
Drive Unit Complement 2 x 6.5" RST® bass, 1 x 6.5" RST mid,
1 x 25mm Gold Dome C-CAM tweeter
Bass Alignment Triple chamber differentially tuned sealed mid-section reflex bass sections
External Dimensions cm approx. 106Hx20Wx33D

Elac 209 Jet
Dimensions H × W × D 1030 × 200 × 285 mm
Gross Volume 58l
Weight 19 kg
Principle 2½-ways, bass reflex
Woofer 2 × 180 mm AS cone
Tweeter 1 × JET
Recom. Amplifier Power
at Nominal Impedance 30-300 W / channel
Crossover Frequencies 2,500 / 2,700 Hz
Sensitivity 90 dB / 2.83 V / 1 m
Nominal Impedance
suitable for amplifiers (from ... to) 4 Ohm
4 ... 8 Ohm
Minimum Impedance 3.5 Ohm at 190 Hz
Frequency Range 32 ··· 35,000 Hz
Nominal / Peak Power Handling 160 / 200 W

Any suggestions? (on both the speaker choice and whether the tube amp will work well with the)

The Moniter Audio speakers appear to be a bit more tube friendly based simply on the nominal impedance. If you want truly tube friendly speakers that will work on a variety of tube amps (from the lowest power to higher power amps) I would suggest you consider the speakers offered by Audio Note. They come in three basic sizes; each size is available in different cabinet finishes and different levels of components/build quality.

If you hear a pair of Audio Note ANJK/L or ANESp speakers you might be quite surprised at how good they sound. All three are a basic two way design with a minimalist crossover. The key is matching of components carefully and the quality of the components. The more expensive models use silver wiring, Alnico drivers, and sound materially better than the entry models. The entry models sound fine, too; I am on my second pair having used a J and now a K (due to space limitations).

All three of the speakers will work fine with 8 watts of good single ended tube power.
Wow, Thank you Mr. Porter for the fine post!
There are lots of good speakers that will give you very satisfying results with 50 watts of tube power in a moderately sized room. Though I'm not familiar with either of the two speakers you are considering either appear to be quite adequate. I'm using 90db floorstanders at home running them with 10watts of PP (tube) power and that system can give clean sound loud enough to disturb the neigbors next door. The two speakers you mention also boast 90db efficiency. Keep in mind also that synergy is not just whether the speakers will play music at a satisfying volume with the given amp. The best thing you can do to determine whether you'll like it is actually listen to the specific combination of amp and speaker you are considering. If that proves difficult, buy used, listen, and sell if you don't like what you hear. Repeat this procedure till you find the combination that raises the hairs on the back of your neck and gets your toes to tappin' involuntarily, OR until your wife/s.o. throws you out of the house on your ass!

50W is pretty hefty for a tube amp. Nobody can look at a set of specs and tell you whether a particular amp can drive a particular speaker. (All right, a 200-watter can drive horns.) There are too many variables. The only way to know for sure is to get them both into your room, and play them as loud as you are likely to ever want to listen with some really demanding (think orchestral warhorse) material. If it sounds good, it is good.
Hi There,

I'l be honest with you - Shanling is not available here. I tested out the CD Player when I was overseas and fell in love with it, so I imported it specially. The tube amps I will thus not be able to test.
All we're trying to explain is delivered power to a visible by amplifier freequencies.
A noise contains all components of such audiable "visible" freequencies.

If we take a trivial sine wave 1kHz and feed to both 50W tube amp and 50W SS amp they will both deliver the SAME power to the dummy load(measured by true-RMS Voltmeter).

Speaking of the noise noise experiment we may assume all audio freequencies but far not all load combinations.
We also can apply the term of "visible" freequencies and in SS case they're much larger let's say at particular THD limit set for both experimented units.

Now, in your case when you're shopping for full-range speakers of medium efficiency... I'd have to say you'll only hear half of them with 50W of tube power. I'm not saying that you shouldn't shop for tube amp that HAS plenty of advantages to the SS but you'll have to shop for higher powered ones than 50Wpc.
You must also take a note that higher power tube amps have a higher-complexity power supplies and so with cost of parts and lower reliability.
I have a tube amp myself and it took me a while of research how to improve a reliability without loosing performance and I finally did it at once. I now prefere in bi-amp setup together with Sunfire Symphonic Reference unit fed trhough DIY balanced converter with adjustable gain -10 to unity.
It saves the tubes and brings me the best of both worlds of amplification FYI...