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Why Power Tubes Sound Different

Lilarcor

Inspired
I'm glad there is someone like Cliff who can point on this things without getting assented as a jerk by some tube fanatics.
I guess he already is just because he dared to invent something to model tube amps accurately which is - as every real tube fanatic knows - impossible by definition. ;) /sarcasm off

This post is a great read again. Thanks Cliff. I'm playing a lot with all those parameters in the speaker tab at the moment trying to find the best match for my Celestion Century Vintage loaded cab. This has turned out to be quite hard as first of all Celestion doesn't publish all needed data to do this by theory. And secondly I find quite a few different settings which just sound so good that I can't decide which one to choose. The knowledge I can try to match certain power tubes doesn't make it any easier. :)
 
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craiguitar

Power User
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:)
Haha, classic. An added response to that might also include "sell the ENGL, and buy a Marshall like you should have done in the first place". Good grief!
 

Morphosis

Fractal Fanatic
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...... (...))
If i get it right, changing the tube types will affect the tone mostly (only?) because of their different "reaction" on the speaker load:

(...)A power tube has a very flat frequency response (...). 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.(...)

You can simulate changing power tubes in the Axe-Fx by simply increasing or decreasing the LF and HF resonance values.
So these differences will be there, but the complete electronic / circuit design around the tubes will affect the tone/dynamics much much more and in absolutely first priority ... So in first line the circuit design will define the Amps sound, not the used tube-type.

And if the different Tubes affects tone only because of different "reaction" to the speaker-load, it is for me absoluteley comprehensible now, why changing LF and HF resonances will simulate this (aside from Pentode/Triode, etc. discussion...)

This is, how i understand this now ... ?! Cool ;)

EDIT: "reaction" is the expression i use as stupid idiot to avoid "correct tecnical terms" i ever use in the wrong context ... lol ... :?
 
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PacoCasanovas

Fractal Fanatic
If i get it right, changing the tube types will affect the tone mostly (only?) because of their different "reaction" on the speaker load
Yepp! Correct....and as said before. Not every amp act the same, since there are differences regarding output transformer, operation points or influence by NFB, connected speakers (a low impedance cabinet introduces less influence, than a 16Ohm cabinet).
Not every tube is doing the same thing either....transconductance is the ratio of delta plate current / delta negative grid voltage - which is vary from tube to tube on their matched operation point, operation voltages in the amp and max. signal swing etc.! So there where for sure some special tubes that can "open" the sound and feel more than others will do.....
Glad I don't need to worry about it on my AxeFx and Q12a - got the very best of 160+ amps and can tweak them all to infinity:mrgreen ...and back!
 

gabriele ciullo

Experienced
Hi Cliff, I like the "Power" of the Power section. I put a Compressor Block at the end of the chain (after the delay/reverb but before the Cab block) to simulate the change of the impedance when I active the various effects (example, only AMP active has a different equalization among AMP with Chorus and Delay or AMP with Delay and Reverb,ecc ecc).So the Compressor can emulate this limiting/impedance like a real rig with power amp after all.
I tryed to use as power amp the Axe AMP Block after all effects but it has not a neutral frequence response (with its value Tube pre at noon position).
The request for a Power Amp BLOCK is been done plenty of times but why the Power Amp can not be separated from its preamp?
 

PacoCasanovas

Fractal Fanatic
I tryed to use as power amp the Axe AMP Block after all effects but it has not a neutral frequence response (with its value Tube pre at noon position).
if you want a neutral response - flatten the impedance curve in the SPKR page of the amp block and increase damping, slightly decrease transformer match. A compressor can just limiting your signal or "compress it" :)mrgreen) it does not of what a power amp can do and has no power amp simulation fanciness at all.

but for what reason you need a virtual guitar tube power amp if there is no interaction between load and amp? Power Sagging maybe? Then, you're right - you just need some kind of a bus compressor that compresses the higher signal peaks to get a lower peak/average signal ratio (some kind of tube compression)


btw. now when Cliff put his attention on the detailed simulation of the complete loadline, which makes the preamp sag more feel-able on fat attacks and higher mastervolume settings, why we need seperate power and preamps again?
 

PacoCasanovas

Fractal Fanatic
Haha, classic. An added response to that might also include "sell the ENGL, and buy a Marshall like you should have done in the first place". Good grief!
I wish I could (because this is the correct answer) - but as an employee at the swiss ENGL distributor, I can't - customers fund my food intake. Beside that, ENGLs were awesome tone-machines too....just different to marshalls. So we ended up doing something on V1 or V2 in the preamp or just pulled in a fresh new V2 tube (some of my fav mullard-style TAD 7025 S) mostly does what they expected......


PS: preamp tubes also have no sound - the circuit were you put them in make it sounding different! But since a slight change would be amplified many times+ from the following gain stages, so even slight changes became more notable.....
 
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PacoCasanovas

Fractal Fanatic
The Tube Pre included in the AMP is not flat ! It adds something!:-|
yepp it does....it's mainly related to the tonestack regardless of the tonestack position. So what you're asking for is a model of a poweramp itself, so you can use your desired tube preamp in 4CM and use the axefx ampblock just as power amp simulation. mmh....Great idea! :) Maybe Cliff will add something like this in a further firmware update. We never know - I can't speak for the man.
 

gabriele ciullo

Experienced
yepp it does....it's mainly related to the tonestack regardless of the tonestack position. So what you're asking for is a model of a poweramp itself, so you can use your desired tube preamp in 4CM and use the axefx ampblock just as power amp simulation. mmh....Great idea! :) Maybe Cliff will add something like this in a further firmware update.
Yes !!! :))))
 

Smilzo

Power User
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.
I don't get it. Tubes "see" the impedance thru output transformer. When you change tube, you change the ratio so that impedance is almost the same as 6L6. Why 6L6 sound different than EL34 than? ;)
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
PS: preamp tubes also have no sound - the circuit were you put them in make it sounding different! But since a slight change would be amplified many times+ from the following gain stages, so even slight changes became more notable.....
Yes, another one of my pet peeves. People claim that preamp tubes sound different. They have no sound. They might have different characteristics though which cause the CIRCUIT to sound different. They might have more or less gain, or more or less capacitance, etc. It's such a crap-shoot though. Switching around tubes until you believe it sounds better. I prefer a more scientific approach. What are you trying to achieve? Okay, then lets reduce this coupling capacitor a bit.
 

groovenut

Power User
Tube rolling is for those with lots of time and money to burn. I prefer modifying the circuit to get guaranteed results. Customers want what customers want. Most of the time I am happy to oblige, but I am quick to point out that most of what they read on the internet is hype and misinformation.

Thanks for the good read Cliff!
 

barhrecords

Axe-Master
yepp it does....it's mainly related to the tonestack regardless of the tonestack position. So what you're asking for is a model of a poweramp itself, so you can use your desired tube preamp in 4CM and use the axefx ampblock just as power amp simulation. mmh....Great idea! :) Maybe Cliff will add something like this in a further firmware update. We never know - I can't speak for the man.
IIRC the tonestack is neutral with everything at noon in the Tube Pre. If you leave everything default and turn Sag to zero. It only adds tube "color" or distortion depending on the input levels.
 

Stadanko

Inspired
I love Paradigm shifts like this. Nothing I ever contemplated because well, I'm clueless. Always thought the tone was in the tube, cause that's what they told me and different tubes do affect tone as most of us have heard with our own ears. Cool to know that its because of how they influence the circuit itself not because of how they sound inherently. An Easy concept to grasp on a surface level.

And now all us dummies are a little less dumb! Type on sir....Type on!

Thanks
 

geetarman

Inspired
Cliff, With the amount of knowledge you have about tube amps have you considered designing an all tube amp head from the ground up or perhaps a Fractal Audio/Carol Ann collaboration. That would be super cool. ;)
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Cliff, With the amount of knowledge you have about tube amps have you considered designing an all tube amp head from the ground up or perhaps a Fractal Audio/Carol Ann collaboration. That would be super cool. ;)
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.
 

Morphosis

Fractal Fanatic
(...)designing an all tube amp head from the ground up or perhaps a Fractal Audio/Carol Ann collaboration. That would be super cool. ;)
Don`t know, if this would be super cool, all i know: This woul be not eXtreme Digital ! And we know, FAS is the pinnacle of eXtreme digital ... hehe ... ;)
 
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