SPICE PARAMETERS

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How to use S-Parameters

Spice Parameters, in short: S-parameters, are used in simulation software.

A first beginning. Perhaps some of you have experience with PSU designer freeware, from DUNCAN AMPS. If not, try this nice, self explaining program first, to design a power supply. It works with S-parameters, though you can not see that, or need to know that. However users of LT-Spice by Analog Devices, will recognise the similarity.

Why simulation software?

Construct load lines by hand, is a basic engineering skill, but it is very time consuming when you want to do it is detail. You can try it for instance with a 5k load line, and compare it with a 4k5 load line, and compare 450V with 400V, and you have to make another and another....Finally to conclude something where you say: this is it. But is this really so?

Alternatively, when you have adjustable power supplies, an adjustable load resistor, and an Oscilloscope, you can just try it out, which is much less time consuming. But you would can not do this for tubes which you do not own yet.

Good simulation software is a time saver, and it can quickly tell you if a load of 5k or 6k makes a difference, and what is the change in distortion or output power. Also it tells you if any "crazy" ideas are perhaps good, or just crazy. However, the program needs to be informed about the tube curves in detail, on order to get a real, true result. For this, the S-parameters are used.

When simulating a circuit, most of the time is needed, to draw the schematic with the simulation program itself. This is however needed, to tell the simulation software how the circuit is build. Such schematics look nice and professional just as well. Once a simulation is working, 95% of the work is done. From here it is a real pleasure. You can change anything whatsoever with a mouse click, and not just that. You can attach virtual voltmeters, current probes, and digital oscilloscopes to any point of the circuit.

The most used software is LTspice, by Analog Devices. The "light" version is freeware, but already this version is extremely powerful. Their strategy is, to provide free spice parameters for their transistors and ICs. For parts of other vendors there are many data bases in the internet, almost anything is available. Just not for all tubes. For this reason, we supply the S-Parameters of some of our tubes here. Also S-Parameters for signal transformers are hard to get, but on the electronics forums, some are available. You can combine any electronic part in the database. Like feed a tube heater with a linear regulator IC, and you can see EXACTLY what the signals are doing. Not just in steady state, but also during start up. You can combine tubes with an analog or digital ICs, or anything you like. All results you will see are correct, as long as the parts you use, are specified correct. For this Analog Devices has their data base, but missing parts can be added from your own data. So no, you can't pull an Emission Labs 12B tube from their data base, but you can still add it, when you have a "directly heated triode" and you just change it's S-Parameters to what you need

So far, for how to make it look easy. However, Spice is not possible to learn by yourself, without tutorial. After installing this freeware, best thing is to go through some of the many tube tutorials first. After that, try to set up something simple all by yourself, like an RC high pass filter, for instance. That will explain you how to run a frequency curve. Which is in the frequency domain, so the horizontal axis is "frequency". However the DC bias, and all current and voltages, and signal in and out, is checked in the time domain. So the horizontal axis is in "seconds". Like this you can see for instance, how the coupling capacitors charge up, after switching on, and if that causes perhaps positive grid current in one of the tubes. Which is quite a devastating effect, which you should be very interested to learn about, in the simulation. Once you are at this level, you will agree how silly it is to "try things out" only. You can't beat good simulation software. First, it should pass a good simulation, and then you can try it out.

Still, to learn it from scratch, try to set up something very simple at first. Try to serialize a 1V 1000Hz voltage source with a 1V 1002Hz source, and of course the result is an interference. So at some moment you have 2Volt, and some moments zero, and that changes with the difference in frequency. So here, 2 Hz. You could set this up with a real scope, and two real frequency generators. Try to set this up in LT-Spice, an you should see just the same.

Such things should have no secrets for you, and then you can set up tube circuits as well.

For using a specific tube, you only need it's Parameters in a data format which LT-Spice expects. You can even read into the S-Parameter file of a tube, because it's just "text" version. With this file of S-parameters, LT-Spice can calculate the plate current, at any given grid voltage and plate voltage.

List of good Links for SPICE PARAMETERS

=> Free Download of Spice Simulator Program. Download only the original!

http://www.dmitrynizh.com/ This man is a genius.

Duncan Amps.

Youspice.com

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SPICE PARAMETERS