SPICE PARAMETERS

Last Update:

 

Spice parameters of some tubes:

EML PX25
EML 12B-AC-heated
EML 20A / 20B
EML30A-AC-heated.txt
EML 45B-AC-Heated
EML 45B-DC-Heated
EML 50-AC-Heated
EML 300B-AC-Heated
EML 300B-XLS

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 in detail. You can try it for instance with a 5k load line, and compare it with a 4k5 load line, or 450V with 400V, and things like that., But.... you have to make another and another....until you conclude something final, 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 on the bench. But you can not do this for tubes which you do not own.

Good simulation software is a time saver, and will 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 one way or another. So how to communicate tube curves to a program? For this, not a graphic is used, but S-parameters. With those, the software can calculate the plate current, at any combination of Grid voltage and Plate voltage.

There are three ways for simulation

1) LT-Spice simulation

This is the most difficult one. In LT-Spice, you need to set up the COMPLETE electrical circuit, including the transformers, tubes, and all parts. Then, run this circuit as is, in LT-Spice, and you get to see the output signal, and just anything you want to know. 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 a signal generator to the input, and other virtual instruments like voltmeters, current probes, 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. S-Parameters for signal transformers are hard to get, and often not accurate, but on the electronics forums, some good ones 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 you can't pull a specific Emission Labs tube from their data base, but you can still add it, when you have just any "directly heated triode" and you just change it's S-Parameters to what you need.

Spice is not possible to learn by doing. That will make you crazy. 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 it is better than just "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.

2) LT Spice "runtime" programs

Runtime is a spice program, combining data of electronic parts, with something like a "viewer" for LT-Spice. The most used is PSU Designer, by Duncan Amps. Neither the spice program, nor the viewer are accessible. Just just install the program and it works right away. You can only recognise it is Spice, by the typical presentation of the result.

3) Load line simulators

These use Spice parameters of a specific tube, and give you the output signal of the tube stage. You can change the input voltage, change the load impedance, and more things, and then see what is the result. So how many Watt can you get out of it, and what is the distortion.

List of good Links for SPICE PARAMETERS