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Why Power Tubes Sound Different

FractalAudio

Administrator
Fractal Audio Systems
Moderator
You'll often read that 6L6's sound "full" whereas EL34's have more midrange and other colloquial descriptions of the tone of a power tube. These myths are perpetuated by forum dwellers, uninformed tube "experts" and even amp manufacturers as marketing tools.

Well, the fact is that power tubes do NOT sound different. They do not have any intrinsic tone.

"But I can hear the difference when I change to a different type of power tube. How can that be?"

A power tube has a very flat frequency response and they all clip roughly the same. If you put a resistive dummy load on a tube power amp (assuming it doesn't have any intentional frequency shaping) it will measure very flat. However a speaker is not a resistive load. A speaker is a highly reactive load. As I've mentioned in the other threads in this forum section a speaker has an impedance that is sort of scooped at the midrange frequencies.

It is the impedance of the speaker that affects the tone of the amp and different types of power tubes react differently with that impedance. As I've mentioned before a power tube is nearly a current source. The operative word here is "nearly". No power tube has an infinite plate impedance and that's why power tubes sound different. A current source has infinite output impedance, an actual power tube has a finite output impedance.

The output impedance of a power tube (or any active device for that matter) is defined as delta V / delta I which is the change in voltage vs. the change in current.

Let's take a 6L6 for example. Let's assume that the tube has a quiescent operating point of 300V and let's assume we swing +/- 100V around that point. If we look at the plate graphs for a 6L6 at a bias of -10V we see that the plate current at 200V is 95 mA and at 400V it's 105 mA (roughly). Using our formula for impedance we get 200/0.01 = 20 Kohms.

Now let's take an EL34. At 200V the current is 130 mA and at 400V the current is 150 mA. The plate impedance is therefore 10 Kohms which is half that of the 6L6.

This lower output impedance "de-Q's", or flattens, the speaker impedance. Essentially the EL34 has a higher damping factor than a 6L6. This higher damping factor reduces the mid-scoop due to the speaker impedance. This makes the tone have more midrange.

There's a little more to it as the output transformer plays a role as well and 6L6 power amps typically have a slightly higher impedance ratio. There's also different operating voltages and bias points but I'm trying to keep this simple.

You can simulate changing power tubes in the Axe-Fx by simply increasing or decreasing the LF and HF resonance values.
 

funny_polymath

Fractal Fanatic
I was amazed by how much more 'open' and how much more 'compressed' different sets of tubes could sound in my Princeton. I took it to a 'tube guru' about 10 years ago, who pulled out several 'sets' he'd assembled from various NOS sources, totally changed the character - from a really compressed sound I couldn't stand, to something much more (to my ear, anyway) 'natural' and 'open'. How is that accomplished?
 

PacoCasanovas

Fractal Fanatic
I was amazed by how much more 'open' and how much more 'compressed' different sets of tubes could sound in my Princeton. I took it to a 'tube guru' about 10 years ago, who pulled out several 'sets' he'd assembled from various NOS sources, totally changed the character - from a really compressed sound I couldn't stand, to something much more (to my ear, anyway) 'natural' and 'open'. How is that accomplished?
Well, to be honest - no tubes were exact the same as others in various amps. Tubes itself were often matched for ideal operation points aka plate idle current and transconductance values. Not every tube has the same specs....not every tube works best in every amp. As Cliff mentioned before, there are many amps using 6L6 tubes - but all on various operational points. From there you get some data from the tube spec sheet - characteristic curves. Most important are transfer characteristics and plate characteristics here - there you also read and calculate all (three) tube constants and the output impedance as described by Cliff above. There were even more reasons why tubes (same type or different) may sound different than the output impedance in various amps. Cliff just mention this in relation to the usual blahblah regarding "why tubes sound different" in general. I'm glad he did.....
NOS tubes were just "new old stock" tubes. Remember that tubes were made specially for military occasions some years back, they often had smaller tolerances and were matched to reach best possible performance - some said earlier tubes would have that special sauce because of better selection, lower tolerance in making. I believe that good quality and carefully selected and matched tubes from a today production can offer similar or equal quality to these so called "special sauce/magic mojo blahblah" tubes.
Maybe I'm a bit off here - it's so much easier to explain this in my native language... ;)
 
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PacoCasanovas

Fractal Fanatic
Well, the fact is that power tubes do NOT sound different. They do not have any intrinsic tone.
THANK YOU for posting this! I wish all my customers could read this.....(and stopped me asking the silly question if it would be possible to change the 5881/6L6 on their ENGLs to EL34s to get more of that typical british marshall tone......stupid sh** %&ç*"++&% :devilish:)
 

R.D.

Power User
O.K., but then for the "ignorant" amoung us ( ie; me ) .... How and how much do we adjust the LF & HF resonance ?

Is there an "app" ( calculation ) for that ?
 

miketheman

Experienced
You can simulate changing power tubes in the Axe-Fx by simply increasing or decreasing the LF and HF resonance values.
It would be cool to see an list of general LF and HF values "to get different sort of tubes": EL34, 6L6, KT88 and so on...
I have no clue on this matter, but it is fun to experiment... especially when you can do it safely within the AxeII.
 

PacoCasanovas

Fractal Fanatic
O.K., but then for the "ignorant" amoung us ( ie; me ) .... How and how much do we adjust the LF & HF resonance ?

Is there an "app" ( calculation ) for that ?
Simple....just slightly decreasing the LF and HF gain if you want the EL34, leave it or increase the LF and HF gain for 6L6, 5881, KT88 - set the values to your own taste....


Also use the correct Power Tube Type: Pentode = EL34, EL84 / Tetrode = 6V6, 6L6, 5881, KT66, KT88, 6550 etc.
 
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Funeral

Fractal Fanatic
O.K., but then for the "ignorant" amoung us ( ie; me ) .... How and how much do we adjust the LF & HF resonance ?

Is there an "app" ( calculation ) for that ?
The app is built in to your head. Use your ears bruda.
 

xpenno

Power User
Wow! Thanks for taking the time to educate us. The way you explain things is superb, you make complex subject matter understandable by the masses.

Thanks

Spence
 

roodboy

Inspired
O.K., but then for the "ignorant" amoung us ( ie; me ) .... How and how much do we adjust the LF & HF resonance ?

Is there an "app" ( calculation ) for that ?
I would be interested in this also.
A general ballpark kind of thing would be great.
 

pima1234

Fractal Fanatic
This is true. I've owned a number of Emery Sound amps that are cathode biased and accept a wide range of tubes (I enjoyed experimenting on some Maven Peal amps as well).

Yes, tube changes make a little tonal difference, but it's mainly in their breakup characteristics and headroom. EL34s are actually a little brighter than 6L6s (the Fender circuit made some of those amps sound bright).

The circuit makes the biggest difference. Then speakers and tubes. We get to experience it all in one great box.
 

PacoCasanovas

Fractal Fanatic
and stopped me asking the silly question if it would be possible to change the 5881/6L6 on their ENGLs to EL34s to get more of that typical british marshall tone
to explain this: you can change the sound for sure, but it wont get instantly british or american just by changing power tubes! You can't change the complete tonal goodness of any amp into another just by changing power tubes, but the internet or most companies said so...... On these modern higain concept amps, sound is mostly coming from the preamp - most of the people were unable to hear any tonal differences just because of the brootal loudness when turning their highgain amp up to the last possible 10%. So we often ended up to a slightly change in the V1 section, which has the most notable effect for these people - if they hear a bit more brightness, they invest in my food intake (which becomes essential for me :lol )

I'm glad there is someone like Cliff who can point on this things without getting assented as a jerk by some tube fanatics.
 
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