Why Power Tubes Sound Different

FractalAudio said:
Outdated technology. The future is here and those who refuse to embrace it will go the way of the horse and buggy. The boutique tube amp guys are hurting. I've heard anecdotes about sales figures and it isn't pretty.
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Interesting quote, as I was talking with a co-worker just yesterday about how companies that make and sell very expensive tube amps are going to start to feel the pinch as guitar technology is moving toward the future, and that future doesn't involve tubes.

More and more tube amp die-hards are hearing and trying my Fractal rig and going 'wow'. You can see the light turning on and the long held beliefs being challenged. Digital/modelling (when done properly) is definitely coming of age....

Great post too about tubes having 'no sound'; I've argued this with fanatics for years and the misinformation rampant on the Internet/guitar community always irked me a bit regarding this 'voodoo'.
 
FractalAudio said:
... People claim that preamp tubes sound different. They have no sound. They might have different characteristics though which cause the CIRCUIT to sound different.
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This is precisely what I think. But this meas "tubes sound different". If you put another tube, circuit sound different, in non-linear manner when tubes are overdriven.
 
Philip34 said:
Sales are that bad? I had no idea.
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Wouldn't surprise me. I've built a few amps, and even with a lotta training/planning/experience/etc., there's still a lotta labor in those things - they can't turn over many per year. That's why they're expensive. I could imagine the difference between making 15 or 18 of them per year could be significantly felt. Then, the more of the little rascals you get out there, the heavier the support demands become. End up in a death spiral.
 
Man wouldn't it be great if Cliff Notes were published with extensive supporting equations! :encouragement:

PS - it would be great - but I am more than satisfied with what Cliff gifts us. :)
 
FractalAudio said:
... 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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Is that part of the secret sauce in Beta 4? Various interchangeable preamp and power amp tube resonance values in a can, plus additional secret sauce ingredients x, y and z? :)
 
FractalAudio said:
Well, the fact is that power tubes do NOT sound different. They do not have any intrinsic tone.

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.
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Okay, I was hoping to shed some light on this. I am not claiming to be an expert, nor am I claiming that anybody is wrong. I am just trying to understand this better. I own an Axe FX II with Matrix GT1000FX, and many tubes amps. I don't feel as if one is tonally better than the other, just different.

I looked up the tube values in the RCA Receiving Tube Manual:
EL34%2Band%2B6L6%2BData.png


If we take 360 Volts / .088 amperes = 4049 Ohms for the 6L6GC. The EL34 is 450 Volts / 0.12 Amperes = 3750 Ohms. It would seem that the two tubes plate to plate resistance aren't that dissimilar(NOT clear if impedance is appropriate due to that fact we are talking about DC). This would also agree with the fact one does NOT have two change the primary to secondary ratio (ohms selector) when switching from 6L6 to EL34. How can impedance of the speaker to the secondary be the FUNDAMENTAL reason why tubes sound different from each other? Also, why is it then that different brands of the same type of tube sound so different?

I am just trying to understand this so please don't flame me.
 
If you look at Cliff's math in the OP, you'll see that he's looking at how the plate current changes value as the plate voltage changes. That means multiple measurements: setting the circuit to cause different plate voltages, then measuring the plate current at each of those voltages. The data you presented from the tube manual doesn't give multiple measurements that are within the useful signal range of the tube, so you can't apply Cliff's math to it, so you can't determine the signal impedance.
 
Rex said:
If you look at Cliff's math in the OP, you'll see that he's looking at how the plate current changes value as the plate voltage changes. That means multiple measurements: setting the circuit to cause different plate voltages, then measuring the plate current at each of those voltages. The data you presented from the tube manual doesn't give multiple measurements that are within the useful signal range of the tube, so you can't apply Cliff's math to it, so you can't determine the signal impedance.
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Correct, and that's where I am a little fuzzy on this (as I am a hobbyist at best). How can we apply impedance to this? Aren't we dealing with DC from the tubes to the primary? Wouldn't that mean no phase?

Also, that still begs the question as to why different brands of the same tube type will sound different.

Thanks for helping me understand this better.
 
It's not just DC resistance; that's just one component of the actual AC impedance—and audio is all about AC signals. The tube puts out an AC signal that's added to the DC from the power supply (part of what the power transformer does is get rid of that DC part so you don't fry your speakers).

What Cliff did here was measure how much the tube's output current changes when its output voltage changes by 200 volts. With that information, Ohm's Law lets you figure out the impedance of the tube.


The "same" tube from different manufacturers can sound different because of differences in the way the tube is manufactured (differences in element size/spacing, etc.). That can cause differences in impedance, among other things.
 
The values in those charts are typical operating conditions, they are not parameters.

The impedance of any component, active or passive, is the slope of the v-i curve (or i-v curve in this case). For a tetrode or pentode the slope is nearly zero which means this impedance is quite high. EL34s have a steeper slope than 6L6s. This means EL34s have lower impedance which means they look less like a perfect current source and more like a current source in parallel with a resistor.
 
Okay, that makes a lot of sense. Thank you both for sharing the knowledge. I feel kinda silly not understanding this to begin with.
 
Is it just me or does the different tubes sound/feel more different in FW18?
I hear more difference between them than before. Have not messed to much with them in FW17 though.

It's not like going from a Marshall to a Mesa, but it changes the amp. Nice to tune things to my liking with this.
 
Kriig said:
Is it just me or does the different tubes sound/feel more different in FW18?
I hear more difference between them than before. Have not messed to much with them in FW17 though.

It's not like going from a Marshall to a Mesa, but it changes the amp. Nice to tune things to my liking with this.
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They should. The power tube models are more advanced now.
 
Xrocker said:
All well and good , but which power tubes will get me laid?
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No such technology exists at this time.

This is all very interesting stuff, but I basically jumped off the forum board, right before we broached the universal question: Toilet paper - over or under? And that is as advanced as I'm willing to go, at this time.
 
So I understand that the data says that power tubes dont have or change the tone. I still can't wrap my head around it just based off of my personal experience in swapping out tubes in amps I've owned (i.e. 34's and 6L6's ) hearing and feeling the differences. If I am reading right on this thread its more of how the amp reacts to the tubes than the tubes themselves. So I guess my ultimate question is how do I dial in my Ax2 for the differences to be more noticeable. Like if I wanted to layer my Mark IV recorded track with some EL34 flavor?
 
stm113 said:
So I guess my ultimate question is how do I dial in my Ax2 for the differences to be more noticeable. Like if I wanted to layer my Mark IV recorded track with some EL34 flavor?
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A big part of the tube's interaction with the circuit has to do with the tube's impedance, which varies from tube to tube. Play around with XFMR MATCH to simulate that interaction.
 
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