Tube Simulation method (WIP)

[valdiorn]

This is just a note on some technology I'm trying to develop, I hope someone will find this interesting.


After some personal revelations and shit I realized that the JFET is actually a very close approximation for a triode, but there are ways to make it even better. I did some simulations with a simple 12AX7 spice model I found here. The Cutoff curve seems very close to the real deal, but it's lacking in the saturation region (I hope these are the right words I'm using, english is not my first language).

Mission: Create a voltage transfer function that emulates the response of a tube.

Anyway, here is a comparison of the two curves, one JFET, one 12AX7 (models !!!)
http://dl.getdropbox.com/u/1342726/Forums/Tube%20Sim/tube_model.jpg
You can see there is a slightly larger "hump" near the cutoff region of the triode, while the JFET is more gradual.
I thought to myself, maybe I can alter this curve using some diode clippers... and yes, I could

Here is the original test circuit for the JFET and the altered circuit with 5 diodes and a resistor:


And the result:
http://dl.getdropbox.com/u/1342726/Forums/Tube%20Sim/tube_model2.jpg

Pretty close, eh?
I increased the voltage from 10 to 10.7. This is because the diodes can only cut away voltage, so in order to end up with the same range I had to increase it (also modified the drain res a bit). Because of the increased voltage there is more voltage gain. Once you have the curve you want, you can use opamps in front to set the bias and gain (using only a drain trimpot (as in fet4tube) will affect BOTH bias and gain)

Basically, this is a proof of concept: You can take what is already a pretty close alternative and improve it. I still have to try this in action, but the idea is solid.
note, the circuit is highly dependent on voltage (zener diode is 8.2v).

I got the idea from a digital emulator, if you guys know the ReValver mkIII software, it's one of the best simulators, and it also lets you tweak everything, even the tubes, bias, etc. I looked at the transfer functions and saw that they simply use a curve / look-up-table for the basic tube simulation, and then do some math around it to alter bias and gain, very simple actually.

That's it for now, I'll update once I get more progress. hope this was interesting to someone...

[Ripthorn]

Let us know how it works in reality (is there a noticeable sonic difference?).  Interesting idea.  I also wonder how it would work in the context of a whole circuit.

[frequencycentral]

Very clever. But:

1. Can you burn your fingertips on it?
2. Will it eventually go microphonic?
3. How does it smell?

Have you even considered these essential factors?

[Brymus]

Rick you always make with the funny HA hA 
Thanx for sharing the info Valdiorn

[Ripthorn]

Rick, I think I have the olfactory package for spice, let me see what I can do and get back to you on item 3.

[WhenBoredomPeaks]

necrobump

[Venusblue]

Very clever. But:

1. Can you burn your fingertips on it?
2. Will it eventually go microphonic?
3. How does it smell?

Have you even considered these essential factors?

Don't forget about the best part, having to fix the whole thing if you drop it by accident!

[ubaid88]

interesting update us on how that sounds.

[brett]

Hi
a solid state "triode" would very handy.
If you can bias them off-centre and for plenty of gain (like the Real McTube triodes), that will be even more terrific.
Tetrodes and output transformers next?
cheers

[Eb7+9]

with this circuit we finally see a more complete and correct approach to establishing a basis for triode circuit emulation -  first, by portraying it in the DC Transfer realm, and second, by avoiding the diode turn-on edge that other "traditionally biased" jFET circuits can't avoid ...

the dynamically varying harmonics that such a transfer function puts out will give a big part of the signature response of certain tube circuits - that is, Common-Cathode Triode and Single-Ended Pentode output circuits - by virtue of the similar transfer curve shape ... and more - wait till you try this circuit in front of a tube amp or other jFET stage ... this interactive component will produce the other aspect of tube circuits - namely, dynamic compression ... the real deal, and it can be controlled (exaggerated/moderated) through pre-gain

yours is dissimilar to the two methods I developed - well done ...

~jc

[cctsim]

This circuit doesn't look properly biased to me. With the current arrangement it will work more like a on/of switch.  A from resistor from gate to ground seems to be missing.