[WEEZ]

Hey all,

I hope I'm posing this question on the correct circle-

Why is it that tube amplifiers seem to have power ratings that vary all over the place when comparing similar products?  For example:

I read one guy's site where he said that a pair of EL-34 tubes will dissipate 30-35 watts. Yet I've seen EL-34 based amps rated from 25 watts to 85 watts. Are some people "fudging" their ratings- or am I missing something. Maybe some amps are pushing the tubes to their limits while others are rating their's conservatively? Some are rated only at 1kz, while some specify no freq. tolerance at all. Are the standards different between tube ratings and solid state ratings?

WEEZ

[_scotty_]

WEEZ, The defacto standard for power vs distortion measurements has the definition of clipping set at a THD of 1%. An amps power rating is defined by how much power it will deliver from 20Hz to 20kHz before the 1% THD threshold is reached into a specified load, usually 8ohms.
Most tube amplifiers will not produce an equal amount of low distortion power at the frequency extremes compared to what they will do at 1kHz. Output tubes produce power as a function of applied plate voltage. More voltage equals more power.More voltage also means that the tube will age faster and fail sooner. Any output tube has a linear operating range of voltages, so depending on the voltage applied, the same tube can have different output power ratings.  People buy watts and think they need a lot of them to do the job. In some instances they may be right.
Manufacturers are aware of what people want to see for power ratings,
instead of a 1% limit on distortion they use 3% or 10% so that the available power looks more impressive or they will only give a spec at 1Khz where their amp looks ok. If you are interested in a particular amp call the manufacturer and try to pin them down on what
the amp does from 20Hz to 20kHz at a 1% distortion limit. Don't let them tell that because it is tubes that how much distortion it has doesn't matter
because it will be predominately 2nd harmonic distortion and it sounds musical. If you wouldn't  listen to a CD player with 3% to 10% THD
why should you listen to an amp with that much distortion.
     We can frequently get by using 1 to 3 watts continuous with average efficiency speakers in an average room, peaks may require 30watts to 300watts or more. As a result most of us live with a greater or lesser amount of distortion of all forms when attempting reproduce peaks in the program, insert alphabet soup here. When attempting to reproduce musical crescendos, a tube amplifier will
typically crap out sooner under these conditions and exhibit rapidly rising
distortion especially at the frequency extremes. To avoid this problem
buy as much power as you can afford and use higher than average efficiency speakers. If you can, buy the best sounding first watt you can find and the lowest distortion power rating that looks adequate for your
loudspeakers,room,and listening tastes. Sorry for the complicated answer to what seems like a simple set of questions, Scotty

[WEEZ]

Scotty-

Thanks for the response.

Yeah, I just wonder if those 50wpc El-34 tube amps aren't really 30wpc tube amps.

Maybe due to the "softer clipping" of most tube amps, the 50 watt ratings are peak ratings, not rms ratings.

WEEZ

[mgalusha]

WEEZ,

scotty's points are dead on. One addtional thing to remember is that more output tubes in parallel can provide more power since each tube has a limit of how much voltage can be applied and how much current it can pass. An example - the ARC REF600 has 8 pair of  6550C output tubes and is rated at 500W from 20Hz to 20KHz (16 ohms), the ARC REF300 has 4 pair of 6550C tubes and is rated at 280W from 20Hz to 20Khz (16 ohms).  

This is why you see some EL-34 amps rated at 30W and others are 50W or 70W. Depends on how robust the power supply is, how many output tubes they are using and how good (and large) the output transformer is.

As scotty mentioned, most listening at moderate levels is done at very low power levels. A good first watt is vital IMO.

mike g

[WEEZ]

Scotty-

So are you saying that a 30wpc "full-bandwidth" rated amp might be the same as a 50wpc rated amp w/ no tolerance given? (all else being equal)

Again, for example;

  Product "A" uses (1) 12AX7 and (2) EL-34's (per channel) 30watts

  Product "B" uses (1)  5687  and (2) EL-34"s (per channel) 50watts

mgalusha-

Your comments re: "first watt" is well taken, and understood. Average listening levels probably require only 1 or 2 watts (rms) to fill an average room. As you correctly point out, it's the drum-whacks and/or sudden crescendos that require more headroom or power reserves.

My point is, that 50 watts provides more than half-again the 'reserves' of 30 watts. So, if all we can go by is the "claims" of the manufacturers- we might be duped into buying the 50 watt amp instead of the 30 watt amp- and wind up with something that is really no better-(power-wise).

While tube audio is gaining in popularity again, it is still difficult to compare different products against each other. Many of the smaller amp makers are not sold in showrooms (good price-wise; maybe not-so-good other-wise)-as far as a point of reference is concerned.

I have compared many solid-state amps- and, in general, when power is the only factor considered, 60 watts from one amp isn't much different from another. ( Of course, sound QUALITY varies greatly-and I'm confident this is true with tube amps as well).

It just seems that there is less "certainty" when it comes to tube amps. (i'm only referring to power,here)

Just seems like the tube guys live by a less precise 'standard' than the solid-state guys when it comes to power ratings.

Maybe I'm missing something..

WEEZ

[_scotty_]

I don't think you are missing anything. Also remember that twice the power equals only a 3dB gain in SPL which is just over the threshold of obviously louder. It takes 6dB to 10dB for something to be perceived as twice as loud. There is virtually no meaningful difference  between 30watts
and 50watts from a headroom standpoint.

[WEEZ]

Correct-

But 20w is 20w- particularily with lower power. 30wpc vs 50wpc should be noticable compared to 150wpc vs 170 w/ solid state, right?

I still can't help but think that there is more voodoo w/ tube ratings. However- there IS a certain magic w/glass! Just such a hassle....

I just wish that there were better comparible standards with tube power ratings- particularly with all the "new" players in the market.

It's just hard for me to believe some of the power ratings quoted from a pair of EL-34's these days. 75 watts from (2) El-34's?. I can't believe it.

WEEZ

[_scotty_]

If you hear a difference between 30watts and 50watts, you are probably hearing the power supply and not the power difference. Sometimes the 30watt amp will sound better than the 50watt. You can seldom predict the outcome of a showdown based on power alone,despite what people think.

[WEEZ]

scotty,

thanks much for your insight.

WEEZ

[Artcurus]

Here's the Specifications for the tube amps that I am using in my home theater setup. These came directly from the Manufactor, RCA.

RCA Tube Bridging Amplifier
These amplifiers are designed to bridged for higher pwr output.

Power Output  100 watts @ less than2% THD

From 50hz to 20,000 hz 110watts @less than 5% THD
From 50hz to 22,000 hz

Maximum Pwr output @400 hz- 175watts

Maximum instant peak pwr output-238 watts

Regulation 1.5db from no load to full load.

Gain Bridging Input to 50 ohm load- 59 ½ db

Output Voltage  25volts for 6.3 ohm load (8 ohm speaker connection)
                           12.5 volts  1.6 ohm load  (4 ohm connection)


Signal to Noise Ratio at Bridging Input- 93db



Audio input is balanced with transformer isolation. Audio output transformer is bifiliar wound.


Amp is designed for continous duty at full rated power.

Tube complement

6550 x2
12BH7
6EU7

[PhilNYC]

I'm not a technical guy, but here's a thought:

power = voltage * current

So depending on what a designer is trying to achieve with an amp, these things can vary greatly.  Voltage is the primary variable for volume, and current is needed to drive a load.

One of the amps I sell (the solid state BC26mkII) is rated at 200wpc.  Another one (the hybrid BC28) is rated at 125wpc.  Yet the BC28 "plays louder"...at the same position of the volume knob, the BC28 makes my system much louder than the BC26mkII.

So perhaps there is a factor here that tube amp designers are playing with the physical limitations of specific tubes to achieve a specific type of amp performance...using different voltage/current combinations that result in different power ratings given similar materials/components...

[JohnR]

I just looked this up, and much to my surprise, a pair of EL34s appears to be capable of producing up to 100W (at 6% THD), see

http://tdsl.duncanamps.com/show.php?des=EL34

It is starting to push the maximum ratings of the tube. I would have thought that 50W was about the limit.

Having said that, I don't think I would buy an amp rated for more than 50W from a pair of EL34s. Tradition only? Dunno ...  

[lcrim]

This article helped me get a better understanding of amplifier power and as a result, speaker efficiency.

http://www.smr-home-theatre.org/Power/

[Artcurus]

It depends on the amount of voltage on the plates. That 6550 amp that I posted the specs for, has 600 volts on the plates.

EL34 are capable of 100 watts, though they are usually in quartets.

EL84 (6BQ5) are capable of  36 watts mono when used in quartets.

I can tell you this about vaccum tube amplfiers and power ratings. I was a DJ for  Junior high school dances with about 200-250 kids usually in attendance. I was using 4 Dyanco MKII (KT88 or 6550) rated at 60 watts continous power, and 4 Audio Centron full range speakers with two horns and a 15 inch woofer.

This was in a typical school cafeteria.

I never had problems competing with the crowd, and it was LOUD on the floor without clipping.