I recently acquired a bunch of vacuum tubes from ebay and I'm in the process of trying to design a simple tester to check the functionality.

Below is what I'm currently thinking about:


I managed to scrape a transformer that was used to output 6V from various input voltages; so I'm inputting 6VAC to get different HVs on the secondary (from 110 to 240VAC).

This would connect to a full bridge rectifier filter capacitor (the switch to the rectifier tube to be tested is an impromptu idea that I got right now). The rectified voltage then goes through a potentiometer to get 200V (on the picture 220 is wrong), 0V (ground) and -20V for the polarization. The rails that come out of the first potentiometer go through other another potentiometer to set the variable voltages (0 - -20V and 0 - 200V). This latter rails will go to pins to be connected to the D.U.T..

I used the voltage dividers because I would like to have at the same time 200V to have a decent plate voltage, a variable bias voltage for the control grid and a variable voltage for the control grid.

I know that the voltage dividers are not exactly the best option but I would like to keep it as simple as possible and is not essential that the device is power efficient. Would this be a feasible idea?

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    \$\begingroup\$ The voltage labels (110...240) on the transformer in the diagram are the wrong way round. Before you get an unwelcome surprise. \$\endgroup\$ – Andrew Morton May 24 '17 at 14:50
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    \$\begingroup\$ I think you have the transformer backwards - you have to put your 6 V AC input into the original secondary to get the 110...220 volts out of the original primary. \$\endgroup\$ – Peter Bennett May 25 '17 at 4:15
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    \$\begingroup\$ There is a whole chapter about tube tester design in the Radiotron Designer's Handbook, Langford-Smith. There's nothing simple about them. \$\endgroup\$ – user207421 Sep 22 '17 at 23:23
  • \$\begingroup\$ You only show a single filament voltage (6.3 vac) although, depending on the tube, the filament voltage can range from 1.3 to 110 vac. I think you are better off buying a used tube tester from Ebay. It will save you a lot of time and probably a lot of money, too. \$\endgroup\$ – Barry Jan 1 '18 at 4:07

The short answer to your question is "no". A voltage divider has too much output impedance to feed the dozens of milliamp tested vacuum tubes will need to be tested accurately. The current drawn by the tube will change its pin voltage hence change the measure.

Even a basic tube tester is not a simple project but an interesting one, especially regarding the power supply. IMHO you would better use a FET regulated power supply (in French unfortunately but google will translate that for you). Maybe the negative power supply used to set the grid 1 voltage will not deliver current and can be adjusted using a voltage divider.

  • \$\begingroup\$ I see, so you would put a potentiometer instead of the fixed resistance R in your link, so to get a regulated and adjustable supply? I thought the problem would be the changes in voltages but wouldn't it be possible to readjust the voltages once the tube is connected? Anyhow the point of the device so far is just to connect a tube and get an ampere reading, I'm not going as far as to properly test the current draw and so on. The tubes I got are untested so I don't even know if they still work, let alone if they work properly. \$\endgroup\$ – Luca May 24 '17 at 14:30

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