Thoughts on the difference in sound between tubes and solid state

[phatt]

Thanks Teemu,
          I really appreciate the non mathamatical explanation.
Phil.

[Findeton]

This sounds like the theory behind the Fetzer Valve article on runoffgroove.com. I might play with the Fetzer Valve design using your figures for calculating Rs. BTW, JFETs are more like triodes than Enhancement MOSFET's, but not more triode like than depletion mode MOSFETs, which can be pushed into enhancement mode.

I've been thinking more about the source resistance in FETs used as feedback in order to achieve the 3/2 power law. Of course, it is a bad approximation to the behaviour of a triode: the drain to source current of a FET depends on Vgs, whereas the Plate current depends on the grid-cathode voltage and on the plate-cathode voltage. The "Fetzer Valve" has a 2 dimensional function and a triode is a 3d function, that is, a surface.

According to this paper  ("SPICE Models for Vacuum-Tube Amplifiers"), a very used model of a triode in pspice is:

Ip = m*(u*Vgk + Vpk)^1.5

Where m and u are constants and p=plate, g=grid, k=cathode. If the source resistance approximation to the 1.5 power law was good enough, we could model the triodes just like it's done in pspice: using, for example, OpAmps , we could easily add Vds to Vgs and we'd have a pretty good silicon triode.

But that "silicon triode" would only simulate well a triode with the cathode connected to ground! Many, if not all, valve guitar amplifiers, actually use resistors and or capacitors for feedback, which breaks the 3d 1.5 power law and transform the output into a very different thing (perhaps something similar to a 1.X power law). That's a problem because the source resistance we used to model the triode is connected to ground: if we add another elements in the path from the source to ground we'll modify the 1.5 power law in a different way than a triode does.

I don't know any way of solving this in a simple "universal" way. You can always adjust the value of the Rs in a case by case basis, but it's not a very neat solution, is it?

[Koreth]

When speaking of this 3/2 law or 1.5 law, are you referring to the spacing, or the slope/curvature of the grid curves in a triode, or both?

[Findeton]

When speaking of this 3/2 law or 1.5 law, are you referring to the spacing, or the slope/curvature of the grid curves in a triode, or both?

I'm referring to the output current, which is modeled as a function with exponent 1.5:  

[1] Ip = m*(u*Vgk + Vpk)^1.5

Whereas a FET has an output current with exponent 2. For example, the current of a MOSFET in Saturation mode follows this equation:

[2] Ids =k*(Vgs - Vth)^2

BTW, I have in mind a design that I think can get us close to the triode's formula [1] . I'll post the design on the forum when I finnish it so you can review it or give me new hints. I don't expect it to be the ideal silicon replacement for getting  the tube sound, or a very innovative method, but it will be my try!

[Steve Conner]

Hi all... I've been a long time contributor to the MEF, but this is my first post here, and I've been thinking about this subject a lot lately.

You can copy the transfer function of a tube as closely as you like, with circuits of ever increasing complexity, but what I'm wondering is: How close does it really need to be?

It seems to me that all a "tube simulator" needs is a gradual, soft clipping characteristic that starts with low-order harmonics, moving to higher-order as you push it harder. It shouldn't hang up from saturation effects, and it should draw "grid current" so that you can have duty cycle modulation.

I don't think any finer level of detail than that really matters. It might change the "flavour" of the tone a little, but it won't make the difference between caviar and cr@p.

Thoughts?

[J M Fahey]

Let us not forget that curve or slope cloning emulates a *clean* triode.
The *clipping* triode emulation is a whole other game, and then we will be able to emulate Preamp distortion only.
We still need to emulate power pentodes, both clean and overdriven, transformer nonlinearities, the works.
Just as a side note: it can and *has* been done.
A very intelligent guy I know from a Brazilian Forum ("Amplificadores Valvulados" in Orkut) did an amazing work as his final Project to become Electronics Engineer there.
AFAIK what he did was to create this *huge* Spice model of a Mesa Boogie, a DC-5 or something like that, including transformer nonlinearities, speaker response, power supply sag, everything.
I told him he was crazy, a Champ would do to prove his point.
End of the story: he reached a point (after 1 year work, involving almost all of his class and a couple Professors) where he could play 10 seconds of Blues with his trusty Strat into his soundboard, and hear them some 30 seconds later, undistinguishable from his miked Mesa whatever.
Almost real time.
Mind you, it was not DSP work, but fully a Software solution.
Yea, I know, you have Guitar Rig and others there who *claim* to emulate 500 classic amps. Do they?
This one did emulate just *one* .... faithfully.
Unfortunately he became Engineer and now he's working full time in a completely unrelated field, say Electrical utility company or something like that.

[Findeton]

Let us not forget that curve or slope cloning emulates a *clean* triode.
The *clipping* triode emulation is a whole other game, and then we will be able to emulate Preamp distortion only.
We still need to emulate power pentodes, both clean and overdriven, transformer nonlinearities, the works.
Just as a side note: it can and *has* been done.
A very intelligent guy I know from a Brazilian Forum ("Amplificadores Valvulados" in Orkut) did an amazing work as his final Project to become Electronics Engineer there.
AFAIK what he did was to create this *huge* Spice model of a Mesa Boogie, a DC-5 or something like that, including transformer nonlinearities, speaker response, power supply sag, everything.
I told him he was crazy, a Champ would do to prove his point.
End of the story: he reached a point (after 1 year work, involving almost all of his class and a couple Professors) where he could play 10 seconds of Blues with his trusty Strat into his soundboard, and hear them some 30 seconds later, undistinguishable from his miked Mesa whatever.
Almost real time.
Mind you, it was not DSP work, but fully a Software solution.
Yea, I know, you have Guitar Rig and others there who *claim* to emulate 500 classic amps. Do they?
This one did emulate just *one* .... faithfully.
Unfortunately he became Engineer and now he's working full time in a completely unrelated field, say Electrical utility company or something like that.

I'd like to read more about his project, perhaps I can learn something. Could you tell me more about it?

[J M Fahey]

Hi Findleton.
I'll try to get some links for you, maybe even some way to contact him presonally, problem is it's all in Portuguese, although he personally speaks perfect English.
Stay tuned.
PD: anyway online translators can do an acceptable job with technical-type text; and you can fill-in the voids because you know what you are talking about.

[phatt]

Hi all... I've been a long time contributor to the MEF, but this is my first post here, and I've been thinking about this subject a lot lately.

You can copy the transfer function of a tube as closely as you like, with circuits of ever increasing complexity, but what I'm wondering is: How close does it really need to be?

It seems to me that all a "tube simulator" needs is a gradual, soft clipping characteristic that starts with low-order harmonics, moving to higher-order as you push it harder. It shouldn't hang up from saturation effects, and it should draw "grid current" so that you can have duty cycle modulation.

I don't think any finer level of detail than that really matters. It might change the "flavour" of the tone a little, but it won't make the difference between caviar and cr@p.

Thoughts?

Hi Steve,, Hold onto that thought,, as it's a faily good grasp of reality.
Yep it's all just so over rated.
I've spent years building my own gear and quite fankly it's more smoke and mirrors than fact.

There are just so many viariables that it becomes damn near impossible to even establish what
even constitues a good amp Sound/Tone in the first place.

Whether you build with Glass or Sand if you are chasing exact Sound /Tone /Dynamics then there is a lot you need to know about how circuits work to extract the effect you want.

IMHO, whether it's Valves or Transistors
Tone shaping all the way through the circuits becomes super critical if you want stunning results.
This includes Pup's and speakers as well.

Having built some reasonably complex circuits systems even a soaked tubepoweramp running back to line and reamped via a SS poweramp.
It's a toss up between my very simple SS setup and the vastly complex tube hybrid setup.
When asked which one sounds better. I use both depending on how much gear I wish to carry to a gig.

If you are interested my old school analogg SS system is here;
http://www.ssguitar.com/index.php?topic=1446.0

The demand now is for every possible sound all in one neat box but it's all just gimik stuff
if you happen to be a *Real Working Guitar player* you end up realising you only need 2 maybe 3 basic tones/sounds and the rest is how good you can play.

What may sound stunning in the gararge at home may fall well short when used live at a venue.

Half the time my fellow musicians don't know if I'm playing through valves of SS gear.
In a Live stage gig very few in the audience would even know.

You asked for thought's on the subject well that's mine.
BTW, What's *MEF* stand for?
Cheers, Phil.

[Findeton]

Let us not forget that curve or slope cloning emulates a *clean* triode.
The *clipping* triode emulation is a whole other game, and then we will be able to emulate Preamp distortion only.

I've been searching more info on the saturation of a triode. On one of the links of this this webpage, I found the complete graph of a triode.

This is the normal, incomplete graph of a triode:



This is the plot of a triode's output current, including the saturation effect:

[Findeton]

I've got a simple question: is Ik=Ig+Ip or Ik=Ip?  Of course I need to know this to properly emulate grid current saturation.

[Findeton]

I've got a simple question: is Ik=Ig+Ip or Ik=Ip?  Of course I need to know this to properly emulate grid current saturation.

I mean, when the grid voltage becomes positive, the grid current starts to rise. Does that current flow to the cathode, like in a BJT or not? Of course there's a reason why we put capacitors in parallel to the cathode resistors (it isn't grid current though) but as I'm trying to emulate this grid current, it makes all the difference because, among other things it would mean being able to simplify the circuit as we wouldn't need this high power OpAmp I'm using.

[GB]

Hi people... I'm aware that this a very old thread but I just wanted to acknowledge the very many good thoughts in it... particularly from Phatt and Teemuk (whose publication I have read from to cover!) Many thanks for that. 
I have also read Danyuk's paper on FET triode emulation... along with a lot other related stuff.
I could now probably write a book of my own!  ... but I built an Amp instead... also emabracing many of the thoughts in related posts.

(Very) Breif Summary:-
* The signal chain is very closely modelled on VOX AC-30
* It uses the Danyuk FET biasing method - done correctly it works
* It uses Power MOSFETs AND an Output Transformer.

I am NOT a musician... but several who are... have used it - and like it! I have sound clips if anyone is interested?

Below is a pic... I'll wait to see if any interest / questions???

Cheers... GB

[HENK]

Hi GB.
I am very interested to see more.
So, a Schematic says more then thousend Words...

[GB]

A schematic might be revealing just a little too much at this point. How about a brief sound clip instead?