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?
Best Answer
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.