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From DATA to SIMULATION (Part One)

by Alessandro Galavotti L.R. 6.8.1999

You may distribute this section freely, provided that no commercial use is made.

1. Which data should we measure?

We'll focus on the most useful data: the anodic caracteristics of the tube.

Theoretically this can be directly done by measuring for each anode and grid voltage the resulting anode current.
At the end all the data can be collected in a common spreadsheet. The table shuold look like this.

Note that a good practice is to keep vg constant while varying Va. This eases the data collection.
Do do this We need a minimum instrumentation:

A common DVM

A Variable power supply for the anode.

A variable power supply for the grid.

2. Doing it !

The disposition of the elements is this:

As the grids add no current, we can measure the current putting the A-meter in the cathode instead of the anode. So the DVM are all "grounded".

GRID SUPPLY
To keep the grid at the choosen voltage every variable supply is good.
The simplest circuit I've used is based on the LM317 IC.

Please note that the "+" must be grounded so this circuit needs a separate supply.

The LM317 has the defects that Vout can't be lowered beyond 1,2V.
Vg value's in the region of 0-1,5 volts can be simply obtained by a 10K potentiometer connected to the LM 317 output or by a simple partition network made with two resistor.

ANODE SUPPLY
The simplest circuit I can imagine is this:

Easy isn'it?.
Almost every Hi-Voltage Mos can be used. Depending on the MOS your'e using this circuit can deliver a fair 100ma current that are more than enough for our scope.
The output resistor is necessary to draw a couple of mA to increase the LM317's output stability.

3. Tricks

Do collect measure quicky two DVM are recommended; one for the Va, One vor the Ia. The expenses is little and well worth the increase in speed and safeness

The measuring process is simple:

Fix (and measure) the Vg

Then vary Va

and measure Ia.

Beware not to reach the maximum ratings of the tube .
To obtain this the best strategy is to start from the maximum Va (and Vg) minus a 10% safety margin then DECREASE Va (and Vg).

About 20-30 point are more then enough for good graphical results.
Point can be evenly spaced. But if you are interested in a particular point in the anode plan, (...maybe around the bias point you're using... ;) ) the best strategy is to near the measuring step in that region.

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