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Power Amp Power Tube Type Selection

finalformdc

Member
I am having a very difficult time deserning between the power tubes. I have a preset with the 1987X Treble and a looper block up front playing and as I switch between the tube types I don't hear any difference. Not even a subtle difference. I am not well versed in tube types but my expectation would be to hear some type of difference. Who has some advice they could offer (other than getting my ears checked :)) ?

Thanks.
 

666was999

Power User
Crank the volume further up? Get better monitors?
When you don"t hear the tube types now you also might miss some other problems with your sounds. It's better to notice issues before you crank it up on stage and send it to the FOH.
 

yyz67

Inspired
The differences are subtle but there.

If you read the manual/wiki Cliff outlines why the differences are subtle as different tubes don't have a very distinctive "tones" as such, even if they have different operating characteristics. It is more the circuit context and interaction with other components such as the output transformer and speaker impedance that can make tubes in actual amps sound very different. The Axe "normalizes" these relative circuit components based on tube type so the differences are smaller.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
The Axe-Fx automatically adjusts the output transformer to match the power tubes so the difference isn't nearly as apparent as it would be if you were to change the tubes in a real amp.

In other words a pair of EL-34s wants to see about 3200 ohms whereas a pair of 6L6s want to see about 4000 ohms. The Axe-Fx III automatically increases the transformer matching if you change the tubes from EL-34 to 6L6.

I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
 

Raab90

Inspired
The Axe-Fx automatically adjusts the output transformer to match the power tubes so the difference isn't nearly as apparent as it would be if you were to change the tubes in a real amp.

In other words a pair of EL-34s wants to see about 3200 ohms whereas a pair of 6L6s want to see about 4000 ohms. The Axe-Fx III automatically increases the transformer matching if you change the tubes from EL-34 to 6L6.

I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
Why not making an “authentic/ideal” switch or something that controls how parameters such as transformer matching or HF/LF speaker resonance are adjusted to the tube type?
 

joegold

Fractal Fanatic
The Axe-Fx automatically adjusts the output transformer to match the power tubes so the difference isn't nearly as apparent as it would be if you were to change the tubes in a real amp.

In other words a pair of EL-34s wants to see about 3200 ohms whereas a pair of 6L6s want to see about 4000 ohms. The Axe-Fx III automatically increases the transformer matching if you change the tubes from EL-34 to 6L6.

I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
So, w/o introducing a new control or switch, is there a way to mimic the output transformer adjustment using the current control in the AxeII?
Or is this easier in the AxeIII?
Or is it just not possible currently?
 

unix-guy

Legend!
So, w/o introducing a new control or switch, is there a way to mimic the output transformer adjustment using the current control in the AxeII?
Or is this easier in the AxeIII?
Or is it just not possible currently?
Do you mean when you change tube types? As far as I remember the same automatic adjustment occurs on the Axe Fx II.

Otherwise I believe you use the Transformer Match parameter - might be called XFRMR MATCH or something like that. I don't have a II any more.
 

VegaBaby

Fractal Fanatic
Why not making an “authentic/ideal” switch or something that controls how parameters such as transformer matching or HF/LF speaker resonance are adjusted to the tube type?
I've asked for this as well two updates ago. would be great to have authentic/ideal tube behaviour via a switch, especially when you could hear what exactly some tube changes would do to an amp without necessary adjustments...

worth to add as a wishlist item. I could imagine a lot of users would like that.
 

joegold

Fractal Fanatic
Do you mean when you change tube types? As far as I remember the same automatic adjustment occurs on the Axe Fx II.

Otherwise I believe you use the Transformer Match parameter - might be called XFRMR MATCH or something like that. I don't have a II any more.
What I mean is...
After I've changed the pwr tube type parameter in the Axe, what other parameters do I need to adjust to make it sound more typically like when I select different pwr tubes within a real-world amp that allows for various types of power tubes, e.g. like many of the Mesa amps offer.

E.g.
How would one adjust the power amp setting of the USA Clean model (which defaults to 6L6/5881) to better approximate a real MKIV running EL34s in the outer sockets?
[I know this is their so-called "Simul-Class" mode which is a bit trickier to emulate in the Axe and probably also involves adjusting the Axe's power tube bias parameters somehow.
And since the Mesas run both 6L6s and EL34s in so-called "Class A" mode, to do this accurately probably requires two Amp Blocks in parallel as well.]
Or just EL34s in their so-called Class A mode?
[Again, requires adjustment of the Axe's pwr tube bias parameters in some way.]
Or all 6V6s?
[Might also requite re-biasing the model.]

I.e. What else can we do to make it more authentic sounding to changinmg pwr tubes in real world amps when we change the pwr tube types in the Axe models?

Put another way, with another example.
How would one modify a Double Verb model to sound like its preamp was being run through a Marshall power section running EL34s?

Etc.
 

joegold

Fractal Fanatic
I've asked for this as well two updates ago. would be great to have authentic/ideal tube behaviour via a switch, especially when you could hear what exactly some tube changes would do to an amp without necessary adjustments...

worth to add as a wishlist item. I could imagine a lot of users would like that.
Hmm.
Every amp that I'm aware of that allows for swapping out pwr tubes has a modified electrical path with modified circuitry when each pwr tube type is inserted/selected.
It might all be under the hood, but you can't just put EL34s into the sockets for 6L6s on any amp and expect it to work well if at all.

So how would Cliff define authentic vs ideal behavior then?
Well, the only way I can think of is to allow for swapping virtual pwr tube types only on amp sims where the real-world amp also had this feature.
Then Cliff might be able to find a way to add the relevant parameters (user adjustable or under the hood) and just model the exact behavior of these particular amps.
Changing the virtual pwr tube type in an amp sim for an amp that never allowed for tube swapping might still behave the way it does now.

Admittedly, this is probably a lot of work for a feature that most users won't use.
But it might be cool.
 

VegaBaby

Fractal Fanatic
Hmm.
Every amp that I'm aware of that allows for swapping out pwr tubes has a modified electrical path with modified circuitry when each pwr tube type is inserted/selected.
It might all be under the hood, but you can't just put EL34s into the sockets for 6L6s on any amp and expect it to work well if at all.

So how would Cliff define authentic vs ideal behavior then?
Well, the only way I can think of is to allow for swapping virtual pwr tube types only on amp sims where the real-world amp also had this feature.
Then Cliff might be able to find a way to add the relevant parameters (user adjustable or under the hood) and just model the exact behavior of these particular amps.
Changing the virtual pwr tube type in an amp sim for an amp that never allowed for tube swapping might still behave the way it does now.

Admittedly, this is probably a lot of work for a feature that most users won't use.
But it might be cool.
at least the parameters that are not "normalized" when switching tube types. Cliff explained it a bit more in detail in a different thread. pretty sure that some users would like it as you can tell by repeating threads talking about not being able to tell a huge difference as in real amps...
 

Randalljax

Fractal Fanatic
The Axe-Fx automatically adjusts the output transformer to match the power tubes so the difference isn't nearly as apparent as it would be if you were to change the tubes in a real amp.

In other words a pair of EL-34s wants to see about 3200 ohms whereas a pair of 6L6s want to see about 4000 ohms. The Axe-Fx III automatically increases the transformer matching if you change the tubes from EL-34 to 6L6.

I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
Cliff can you explain this one a bit more
So your saying currently if you change a tube 6l6GC to KT77
Things like bias impedance etc all stay the same at default for the 6l6
Is that correct ?

What you are thinking of would that actually remove the defaults
And have those parameters like bias
NFB plate impedance all change to “match “ the selected tube

If yes i would think this would be a pretty big step FWD would it not
Create a much more audible and
Felt change say if one changed
Uber from 34s to the KT88 the Twin Jet would use

If this could.be a switch
For Default/ tube match

I would think it would be pretty next level stuff
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Cliff can you explain this one a bit more
So your saying currently if you change a tube 6l6GC to KT77
Things like bias impedance etc all stay the same at default for the 6l6
Is that correct ?

What you are thinking of would that actually remove the defaults
And have those parameters like bias
NFB plate impedance all change to “match “ the selected tube

If yes i would think this would be a pretty big step FWD would it not
Create a much more audible and
Felt change say if one changed
Uber from 34s to the KT88 the Twin Jet would use

If this could.be a switch
For Default/ tube match

I would think it would be pretty next level stuff
All the tube models are "normalized". IOW if you change tube types the bias current stays the same, the transconductance (mu) is the same, etc. All that changes are the tube parameters that determine the shape of the I/V curves.

The reason for this is you would never but EL-84s in an amp designed for 6L6s. First of all they wouldn't fit, but more importantly the bias circuit would be all wrong and the transformer impedance ratio would be way off. It would sound completely wrong.

What you mostly hear when you swap tubes in a real amp is the change in the transconductance and concomitant transformer mismatch. The curves aren't really all that much different. In fact the parameters for 6L6s and EL34s aren't all that different aside from the mu. A JJ 6L6GC has roughly the same parameters as a Mullard EL34. kvb for both is around 30V, the big difference is mu which means a pair 6L6s want a primary impedance of 4K whereas a pair of EL34s want around 3.2K.

If you put 6L6s in an amp designed for EL34s the OT will be undermatched which will make the amp sound more "open" and the power amp won't distort as much. You also run the risk of redplating the tubes because you might go outside the SOA.

So, all this being said, the biggest tools in your arsenal are the Transformer Match and Speaker Impedance parameters which can be used to simulate different matching.
 

Randalljax

Fractal Fanatic
All the tube models are "normalized". IOW if you change tube types the bias current stays the same, the transconductance (mu) is the same, etc. All that changes are the tube parameters that determine the shape of the I/V curves.

The reason for this is you would never but EL-84s in an amp designed for 6L6s. First of all they wouldn't fit, but more importantly the bias circuit would be all wrong and the transformer impedance ratio would be way off. It would sound completely wrong.

What you mostly hear when you swap tubes in a real amp is the change in the transconductance and concomitant transformer mismatch. The curves aren't really all that much different. In fact the parameters for 6L6s and EL34s aren't all that different aside from the mu. A JJ 6L6GC has roughly the same parameters as a Mullard EL34. kvb for both is around 30V, the big difference is mu which means a pair 6L6s want a primary impedance of 4K whereas a pair of EL34s want around 3.2K.

If you put 6L6s in an amp designed for EL34s the OT will be undermatched which will make the amp sound more "open" and the power amp won't distort as much. You also run the risk of redplating the tubes because you might go outside the SOA.

So, all this being said, the biggest tools in your arsenal are the Transformer Match and Speaker Impedance parameters which can be used to simulate different matching.
[/QUOTE

But could you for the less educated
Have an auto match that would adjust
The above mentioned transformer
And impedance parameters based of the power tube the user selected

Would that make a big difference or not much that the user would notice
Let take something like Uber from 34 to KT88 like the Jet for instance
 

Stadanko

Inspired
The Axe-Fx automatically adjusts the output transformer to match the power tubes so the difference isn't nearly as apparent as it would be if you were to change the tubes in a real amp.

In other words a pair of EL-34s wants to see about 3200 ohms whereas a pair of 6L6s want to see about 4000 ohms. The Axe-Fx III automatically increases the transformer matching if you change the tubes from EL-34 to 6L6.

I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
Maybe you could make it a switchable feature? Maybe a global parameter?
 

Randalljax

Fractal Fanatic
Maybe you could make it a switchable feature? Maybe a global parameter?
I don’t know if you would want a global
What about a per preset
So if you were not changing anything
It we be Default
If you switched it to Matched then it would change the Transformer and impedance to match the selected tube

I do wonder though how much of this we would hear or feel
Its could be a ton of work for a minute audible result
 

Joe Bfstplk

Power User
I've been debating removing this "mu normalization" but it would break existing presets where people had changed the tubes from the default type.
Is it something that could be a choice, defaulting to existing operation but allowing the OT primary impedance to follow the amp model instead of the selected tubes?
 
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