Testing the DIY triode from PWL

I was curious to see how well the PWL triode performed. So I made a little test to measure cathode emission and amplification.

The first step was to build a variable high voltage power supply, since I’m under-equipped enough not to have one laying around. So I used a 20V power supply transformer connected to a 120V variable autotransformer scavenged from a 70’s American-built X-ray machine that happened to have a permanent tap within the first turns, allowing it to step-up the voltage to a nice 180V. For turning it into DC, I simply took the switching power supply PCB from a discarded fax machine, cut it right after the filtering capacitor, and hooked it to the variac. There are easier options, but I did with what I found in my pile of junk :)

The grid voltage is provided by a 9V battery and a potentiometer, and the filament voltage by my (single!) lab power supply. I then used cheap DMMs to measure anode voltage, anode current and grid voltage.

This resulted in this little kludge:

Experimental setup

Schematics of the above little mess

Emission tests
The first test was to see how many electrons the hot filament is capable of sending into the tube. I connected together the anode and the grid and brought them to a 176V potential, while I varied the filament voltage.
This led to this plot:

Cathode emission versus filament voltage

Clearly, the tungsten cathode only begins to work at very high temperatures! Wikipedia lists an efficiency of 5mA/W for a tungsten cathode (oxide-coated cathodes, used in commercial tubes, are 100 times better). At 4V, the filament current is 280mA, which represents a power of 1.1W. The emission, however, is only 2.2mA. Perhaps it works better with more filament voltage, but I did not dare cranking it up for fear of damaging the filament.

The second test was to see how much the anode potential influenced the anode current. I set the grid to the ground potential, and varied the anode voltage. The plot shows a quite linear dependence:

Anode current versus anode voltage

Amplification test
Now, let’s see how good this triode is at doing its job: amplifying signals! With the anode potential set to 176V and a filament voltage of 4V, I sweeped the grid voltage and plotted the anode current:

Anode current versus grid voltage

Clearly, the tube is working! It is not the best triode in the world, as the grid apparently struggles to stop all electrons (and because of the low emission efficiency of the tungsten cathode). But it definitely does some amplification and it certainly is a usable tube. Impressive work!

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