Why Tubes Sound Different....

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Why Tubes Sound Different....

Postby the boogie man » Sun Mar 22, 2015 6:11 pm

What you guys think of this? I pulled it off the Fractal Audio forum.

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.
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Re: Why Tubes Sound Different....

Postby dai h. » Mon Mar 23, 2015 1:17 pm

hi, (I'm not an expert by any means, but) my understanding is that if there is, it has to do with topology. More "how the tube is used", than a "tube having an inherent sound audible no matter how it's used". So single ended vs. differential. Amount of feedback (more can "nuke character" i.e. reduce distortion). From what I vaguely understand, lots of distortion reducing techniques are used in something like an op amp. Sometimes parts that are supposedly the same aren't (such as Sovtek 12AX7WA--a rebased tube close to a 12AX7/ECC83 but isn't exactly, or some Japanese 12AX7/ECC83 which apparently have slightly lower gain and might need tweaking of the operating point to get them to sound like other ones). IIRC, R.G. Keen wrote on this (things that might factor in perceived differences between tubes) on this over on Music Electronics Forum.

For my self (my ears) I have perceived differences between different makers of 12AX7/ECC83s and power tubes so I don't think it's crazy to hear differences, but IME it wasn't enough to use a tube I liked the sound of. That is, I found I still had to do a lot of tweaking 'til I was happy.
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Re: Why Tubes Sound Different....

Postby dai h. » Wed Mar 25, 2015 6:09 am

found it (the post by R.G. Keen mentioned above) :

http://music-electronics-forum.com/t18844/#post154865
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