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Author Topic: A 19OpAmp-2FET approach to the Triode question  (Read 33523 times)

rodriki1

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #15 on: March 23, 2010, 12:05:26 AM »
take a look at this. one amzing discussion about triode
simulation..

http://milas.spb.ru/~kmg/index_en.html
http://gtlab.net/gtlab4/archives/216

it is in russian language what makes things little bit hard to understand.


mensur

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #16 on: March 23, 2010, 10:45:36 AM »
We can make high voltage SS tube replacement, by using cascode. high voltage mosfet will be used as a voltage regulator from the HV supply, to a safe level 24V(actually 20V, since mosfet will use 2-4V to conduct),and do the heavy lifting(amplifying the gain):
SS 12AX7.pdf
The gain of this device is very high(in my example,around 570!!!),the gain is around 570 with mathematical approach.(gm is 5.5679 mS(Av= gm x Rl = 5.5679mS x 100K = 567.9 = 55.7dB).
The input capacitance is very low, less than 2pF(due to cascode connection which reduces the input capacitane,and rounds up the clipped signal),identical to 12AX7.With nicely set feedback, we can outperform the real thing,with ss devices, and what is the best, NO MICROPHONICS, NO MORE HEATER TO CATHODE ELECTRON EMISSION, WHICH CAUSES HUM, NO MORE CHANGING TUBES...
Tone Grinder,aka FETRON
Tone Grinder is probably old FETRON, in nowdays production.I doubt it has characteristics of a real 12AX7, more likely version that I posted.

Findeton

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #17 on: March 23, 2010, 11:38:28 AM »
We can make high voltage SS tube replacement, by using cascode. high voltage mosfet will be used as a voltage regulator from the HV supply, to a safe level 24V(actually 20V, since mosfet will use 2-4V to conduct),and do the heavy lifting(amplifying the gain):
SS 12AX7.pdf
The gain of this device is very high(in my example,around 570!!!),the gain is around 570 with mathematical approach.(gm is 5.5679 mS(Av= gm x Rl = 5.5679mS x 100K = 567.9 = 55.7dB).
The input capacitance is very low, less than 2pF(due to cascode connection which reduces the input capacitane,and rounds up the clipped signal),identical to 12AX7.With nicely set feedback, we can outperform the real thing,with ss devices, and what is the best, NO MICROPHONICS, NO MORE HEATER TO CATHODE ELECTRON EMISSION, WHICH CAUSES HUM, NO MORE CHANGING TUBES...
Tone Grinder,aka FETRON
Tone Grinder is probably old FETRON, in nowdays production.I doubt it has characteristics of a real 12AX7, more likely version that I posted.

I think those things are called transistors... What we want is not 12AX7 replacements with more gain,but to emulate the distortion that tubes give (and it looks like the Tone Grinder/FETRON) doesn't give that.

mensur

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #18 on: March 23, 2010, 12:09:55 PM »
I think those things are called transistors... What we want is not 12AX7 replacements with more gain,but to emulate the distortion that tubes give (and it looks like the Tone Grinder/FETRON) doesn't give that.
I'm only sayin we can incorporate triodizer and cascode and your tremendous knowlege and make real SS 12AX7, with the same characteristics and more reliability.Tone Grinder has samples on website, hear to them(they sound almost identical to the real thing).Do you think maybe they use deplatation mode HV MOSFET's and sell them like that?(I doubt that, cause MOSFET's will sound dark and cold due to high input capacitance)

mensur

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #19 on: March 24, 2010, 11:25:15 AM »
here is the spice simulation of Fetron/Triodizer:
TrioFetron
Is there any thing I can improve with this design?

Findeton

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #20 on: March 24, 2010, 01:29:20 PM »
here is the spice simulation of Fetron/Triodizer:
TrioFetron
Is there any thing I can improve with this design?

You can try this:



It has the same plate current characteristics, and a grid current like this:



It would start to create distortion at about Vgs=0.5V, when it should do that at about Vgs=-0.5V, but modifying that can be a little bit complicated... A way to overcome this is to lower Vk 1 volt, by reducing the resistor connected between ground and the cathode of the stage where the sstriode is used.

Also, the RDISTORTION is, obviously, a resistor that adjusts the level of growth of the distortion. Grid current depends on both Vpk and Vgk, which in the end means that, depending on the plate load line, the grid current curve will be more or less steep. I would use a 10k log pot in series with a 50ohm resistor in the place of RDISTORTION and calculate a good value of that resistor for each load line. I won't show you right now how to calculate this value but it's doable.

As you can see, I am using ground and a Vcc on this ss12AX7. 10V as Vcc would suffice. You could connect ground and Vcc to the ss12AX7 using the heaters pins.

EDIT: added simpler circuit. btw changing C2 to a higher value would remove high frequencies of the distortion.
« Last Edit: March 24, 2010, 01:50:13 PM by Findeton »

mensur

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #21 on: March 24, 2010, 07:49:48 PM »
Thanks Findeton,

But can it be done without this many components, or with discrete components, and how can we increase input impedance since there is no 100Meg resistor?

Findeton

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #22 on: March 25, 2010, 03:09:12 AM »
Thanks Findeton,

But can it be done without this many components, or with discrete components, and how can we increase input impedance since there is no 100Meg resistor?

There are no 100meg resistors, you're right! The important thing is that R2 is 100 times R1. Let's say R1=10k and R2=1meg. In order to increase input impedance (which is about 1meg for negative values of Vgk in a real triode), you could disconnect the end of R1 from the grid and connect it to the OpAmp's output, but you may need to connect a negative voltage to Vee's supply of the OpAmp for it to stay lineal.

EDIT: If you just use R1=10k and R2=1meg, it will consume about 300uA of grid current for Vpk=300, and that's a lot more than it should be (for negative values of Vgk). Therefore I reccomend connecting R1 to the output of the OpAmp rather that connecting it to the grid directly. As I said you will need to feed about -1V to the negative power supply (instead of ground) of the OpAmp to maintain it's output lineal. This way the input impedance will be the one of the OpAmp, and that's large enough value :P
« Last Edit: March 25, 2010, 08:39:46 AM by Findeton »

Findeton

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #23 on: March 26, 2010, 07:40:36 AM »
Including the changes I said in the last post, and some more to refine the mix, here is the schematic for my solid state 12AX7:



The circles there point out that there are lines in the schematic that are passing one above the other without being actually connected. As I said, RDISTORTION controls the amount of distortion for the load line (which is given by HV and the resistor connected between HV and the plate). RCURRENTGAIN controls the cathode to plate current of the device, and is set at 220 ohms to get the same current than a normal 12AX7 tube would give. Vcc should be about 4 volts higher than the highest grid voltage we want to obtain and 10V will suffice. Vdd must be at least -1V. I'm thinking of designing a voltage multiplier and current pumper that could generate those DC voltages directly using the +/-5VAC heater pins so you have that problem solved :p Ah, and those two new 1n4148 are there for correct biasing. The plate current is:



If you compare it to the graph of a real 12AX7, it is pretty much the same:
 


The only big difference is that for our SS12AX7, Vgs must be set up 1V higher, because grid current will start to flow at Vgs=0.5V instead of starting at Vgs=-0.5V (and that's why I added those two 1N4148). This is the graph for the grid current for RDISTORTION=9k. As I said before, RDISTORTION should be a 10k log pot:



Again, Vgk for our SS12AX7 must always be between 0.7 and 1V higher than for a normal 12AX7, so you need to rebias cathode voltage 1V lower when you connect the SS12AX7. Taking that into account, this grid current is pretty close to the real deal (if we want a more steep curve and more distortion we just have to lower RDISTORTION):



So that's all for now for the 1:1 solid state 12AX7. On the next post, I'll describe a solid state triode that works with 0 to 10V and works up to about 170mW of power dissipation instead of 1W.  Ideal for stompboxes!
« Last Edit: March 26, 2010, 07:47:27 AM by Findeton »

Toney

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #24 on: December 20, 2010, 02:32:58 AM »

 Hey!
 I joined up just to find out more about this fascinating old thread.

 @Findeton did you ever make the low volatge version of this?
 I would love to know more.

 

dimitri

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #25 on: May 02, 2011, 10:12:01 PM »
Hi Findeton, any update on this? ;)

mensur

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #26 on: July 24, 2011, 11:21:37 AM »
Hi Findeton, any update on this? ;)
Heres te simpler version for the SS triode:

Findeton's idea is good although it's a bit complicated , so quick solution to that problem is to make a buffer which will not change the input signal, and which will isolate the input from the feedback DC voltage, and set the right amount of current.

Triode MOS

Triode JFET
Triode MOS.jpg is high voltage counterpart of real triode(with amplifcations factor of 100-130 times), Triode JFET.jpg is low voltage counterpart of a triode(with amplifcations factor of 22 times) for pedals, and low voltage preamps.

Loudthud

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Re: A 19OpAmp-2FET approach to the Triode question
« Reply #27 on: November 23, 2011, 06:13:26 PM »
This is an interesting topic but I'd like to offer a different point of view. Although the plate curves of a 12AX7 reveal lots of information, what is really of concern is the transfer function. An X-Y display of input verses output. The attached photo is of a generic Chinese 12AX7, 100K plate resistor, 1.5K cathode resistor with a bypass cap, and a 68K grid stopper. The B+ was 300V.

Note that the diode clipper stage of a TS9 can't emulate the curve correctly (with different diodes for + and -) because the input signal is added to the output by the +1 in the gain equasion. You need to use an inverting stage to get rid of the +1.
« Last Edit: November 23, 2011, 06:29:19 PM by Loudthud »