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

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?

Thanks,
X

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)

Cheers,
X