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Author Topic: Triode Emulation X-Y plots  (Read 6671 times)

Loudthud

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Triode Emulation X-Y plots
« on: December 01, 2011, 08:08:29 PM »
This is a different take on the emulating triode tube distortion theme. First, a few acknowledgements are in order. Findeton's thread, http://www.ssguitar.com/index.php?topic=1532.0 is some fine work but I find it a little complicated. Certainly the math gives me a headache. I didn't look at Dimitri Danyuk's paper, I just figured it would be more mind numbing math. The Runoff Groove article http://runoffgroove.com/fetzervalve.html is interesting but I need to see waveforms, not just equasions. The one problem I found in the article was the statement "The first valve stage of a Fender amp can withstand input voltages up to +/- 2.5V without noticeable clipping." We shall see that isn't true. Steve Conner's thread http://www.ssguitar.com/index.php?topic=2039.0 is more my style. I know Steve from Ampage, he is very knowledgeabe on tubes and solid state design. I hope to be able to present pictures as good as Steve's

The first method I will present is the opamp with diodes in the feedback path. As I mentioned at the end of Findeton's thread, the clipping stage of the TS-9 does not come close because in the non-inverting configuration, the input signal gets added to the distorted signal due to the +1 term in the gain equasion. That leaves inverting opamps and shunt clipping. A significant problem is that diodes clip at pretty low voltages so, to keep from having to attenuate the signal, you would need lots of diodes in series. I have overcome this obstacle with a circuit I call the Diode Multiplier.

When I was in high school, I was told that a silicon diode drop is 0.6 or 0.7 volts. That's true at around one milliamp. With a 100K input resistor, +/- 2.5V input signal, we would like to know what the voltage drop is at +/- 25 microamperes. I have curve tracer photos of a 1N4148, a germanium diode, and the gate of a J201. The germanium diode is interesting because it starts conducting right at zero volts. The J201 gate stays off for a significantly larger voltage than the 1N4148, that could be useful. Low voltage zener diodes are another surprise. They start conducting around 0.4V and don't reach full zener voltage until the test current of 20mA is reached. The 1N5221B is a 500mW zener rated at 2.4V (at 20mA).

The first circuit uses a TL072. One trimpot sets the gain, the other sets the amount of diode multiplication by attenuating the feedback appled to the diodes. This circuit does not address the grid current of a triode issue, that will be added later. The 10K trimpot was set to divide by roughly 17, 10.26K on the top portion and 724 ohms on the bottom portion.
« Last Edit: July 29, 2016, 06:17:32 PM by Loudthud »

J M Fahey

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Re: Triode Emulation X-Y plots
« Reply #1 on: December 02, 2011, 06:09:57 AM »
Thanks Loudthud, interesting.
I still don´t get this fully, I´ll have to read it a couple extra times, but I much agree on the basic idea that clever Op Amp circuitry can be made and used to get about any transfer curve we need.
After all, that´s what they were invented for !!!!
The general nonlinear Triode transfer curve should be "relatively" easy to emulate.
Very non linear diodes (it´s in their nature) can be used to help achieve that, designing whatever feedback net we need to couple them to said Op Amps.
Congratulations.  :dbtu:

EDIT: of course, "the proof of the pudding lies in the eating", we should compare what a signal does when hitting a standard Triode, to what it does hitting an emulator (obviously at typical SS voltages).
I mean comparing waveforms.
A couple MP3s wouldn´t hurt either.  ;)
And thanks again.

KMG

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Re: Triode Emulation X-Y plots
« Reply #2 on: December 02, 2011, 08:04:26 AM »
2 Loudthud.
You left aside one important aspect of tube stage emulating - grid detecting.
This lead to anisotropy of the stage input impedance.
Due to it dynamically changes the operating point of the stage.

phatt

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Re: Triode Emulation X-Y plots
« Reply #3 on: December 02, 2011, 10:35:38 AM »
Hey loud thud,, interesting.
Good to hear you don't like maths,, You are not alone :)

Hum headscratch,,,*anisotropy* Well I had to look that one up. LOL
Meaning:
Physics, "Having properties that differ according to the direction of measurement"

Yes I must agree that even with limited knowledge I know enough to realize that under load Valve circuits do strange things which forms part of the sought after sound.
But in my experience this is also part of the quirks of Hi Z circuitry,, the passives may also play a part. There was an indepth page about the large value R's used in classic Guitar Amps and how it affects the triodes.  (I have long forgotten where, sorry. A link might even be burried on this forum somewhere)

I used this quirk to advantage inside my *PhAbb FPV floor unit* (Circuit on this forum) which is just a tricked up Mesa Vtwin. The clean section is a dead simple 3 meg series resistor hung between 2 triodes> into a tone circuit and opamps output.

Go look at the signal path in fender reverb circuits (AB763 or similar) where one extra triode is needed to mix the dry and wet signal.

Even with no reverb that channel has a little more gain and develops a magical mojo effect which has a quite wide dynamic range. (i.e. find the sweet spot on the dials and you get a very touch responsive sound.)

I believe that the *large series resistance* forms a part of that effect, not just Valves.

Someone once stated that to emulate the classic sounds you need half wave clipping followed by square wave clipping, the rest is just tone shaping.  I tend to agree :)

A little side track;
A while back I got all excited when I watched a short demo on how implimenting a sustain pedal before a humble tube screamer could increase the sustain.

After a long and tedious flurry with different Compressor circuits I became aware of two things.
1/ compressors/sustainers are useless (unless you want that specific sound) so unless you spend big money most just add noise,, often in bucket loads.


So to cut this short I had one of those ARHH HUH moments which brings me too
2/
To achieve sustain the signal has to be kept big for as long as possible.
For Compressor to do that trick the gain has to be made increasingly bigger,, and heaps of gain usually results in good old noise. :grr

But in thinking about it, (and lots of coffee later) it dawned on me that distortion units with diodes (which covers most) Are just limiters which gives some sustain but the incoming signal is always small so they always never quite sound convincing.

Instead of using comp's Why not just send a bigger voltage to the clipping stage?
But of course this is close to impossible with only 9 volts to play with voltage swing. :'(

Either way, a comp to make the signal bigger as it dies away or a limiter with a much larger incoming signal xP   The result is the same,,, sustain.
Arrh,,, but limiters will make far less noise than compressors. ;)

So it becomes obvious to me that Diode circuits work much like a Valve power stage when it gets into square wave. Diode clipped opamps are just a small power stage working on much lower voltage producing a square wave output.

(Another famous comment that has always stuck in my brain is,
If you want to make a Tube Amp distort/sustain more just send it a bigger signal)

So simple stompbox diode circuits *can have* incredible sustain but most lack the ability to produce *Large Voltage swings* **before the limiting diodes**.

Like most, My clipper circuits can reproduce the *power stage square wave trick* but NOT the half wave triode trick. :'(
(This setup also lacks the all singing dancing sweet sustain of real valves)

Once I implemented the triode front end *Before the clipper stage* BINGO it sings.
The bigger the incoming wave into the clipper the longer it sustains.

So in my mind it is simple; Big swing into something with smaller headroom and you have more sustain. A compressor/sustainer has no hope of matching this. :lmao:

And Now back in context;
The added triode was the most obvious way to get a big swing and what was OK before is now to something truly stunning. <3)

So yeah If anyone can point me to a SS circuit that can reproduce that classic half wave triode preamp Keith Richards Rock 'n Roll Rattle I'm all ears. :tu:

It would be so nice to have it all done via SS.
Ha,, I know *Joecool* wishes it so,, winky.
Phil.

KMG

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Re: Triode Emulation X-Y plots
« Reply #4 on: December 02, 2011, 12:37:31 PM »
Under the anisotropy I meant that the tube stage input impedance is different for positive and negative half-wave.
And the ratio of the impedances is different for different input signal amplitudes.
This changes the DC voltage at the interstage capacitor.
This looks like applying to grid virtual DC bias depending on input signal amplitude & varying in time.
Static oscillograms
K1 - Plate
K2 - Input (before input decoupling capacitor)
K3 - Grid
K4 - Cathode









Dynamic response to "burst" input signal




...

& finally

Rest of oscillograms there:
http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/

Loudthud

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Re: Triode Emulation X-Y plots
« Reply #5 on: December 02, 2011, 07:06:18 PM »
The circuit in my first post was just the beginning. I was thinking of a simple JFET source follower input buffer for a 1 Meg input impedance and with a clamp to simulate grid conduction. The problem is that would need to be AC coupled to the opamp. You have to be carefull where the signal is AC coupled to avoid baseline shift where you don't want it. The long range goal is to make a complete guitar amp. The first stop being something like a Tweed Princeton. I have a couple of class A power amps built. One about 5 watts and another about 15 watts. Man, do they get hot. The 5W runs off of a laptop power supply. A good project for the novice. No mains to touch.

I kind of gave up on zener diodes, they just weren't getting the curvature I was looking for. Posted below is where the idea reached a pause. Getting the bend due to grid current had me stumpted for a while, then I just put a clamp on the input and it looked pretty good. I'll need to do some more experiments to see if a resistor is needed in series with the J201s. Maybe two or three 1N4148s will make a better clamp. The Ge diode gives a nice gentle curve from the zero signal point (middle of the screen in the scope photos) and the 1N4148 adds additional curvature near where the tube cuts off. The gain is increased to around 10 which is about as much as can be expected with +/- 15 Volt rails. The signal is a little too linear between zero and about -1V on the input. I might add another Ge diode with a series resistor to give a little bend there.

This circuit is intended as a first stage. Since I would like to hit the second stage as hard the typical tube amp, the input of that stage needs to clip at somewhere around 200mV to 300mV. That will be more challenging.

I like bullet proof inputs so I added a 1N4148 to the -15 rail from the input of the opamp (not shown in the schematic below).

I'm trying to get set up so I can compare tubes and my circuit in real time like Steve Conner did in his thread. The stack of test equipment is getting bigger.

 

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