0
\$\begingroup\$

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:

prototype

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?

\$\endgroup\$
4
  • 1
    \$\begingroup\$ The voltage labels (110...240) on the transformer in the diagram are the wrong way round. Before you get an unwelcome surprise. \$\endgroup\$ May 24, 2017 at 14:50
  • 3
    \$\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\$ May 25, 2017 at 4:15
  • 2
    \$\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, 2017 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, 2018 at 4:07

2 Answers 2

1
\$\begingroup\$

You can go MUCH simpler! A traditional "simple" tester merely ties heater to specified voltage, cathode to ground, and all other electrodes together to positive -- yes, even control grid; then a fairly low voltage is applied (say 10-100V), and a selected shunt resistor measures the current as a basic go/no-go test for emissions. The slightly fancier ones separate the grid, so it can be held near GND but measured for current flow (leakage). Only top-of-the-line testers set up representative voltages, bias the tube for class A operation and measure voltage, current, transconductance, etc.

If I were breadboarding a tester, I would do:

  • Filament voltage: gather a selection of transformers of suitable rating, with voltage more than needed. Use a variac to reduce voltage to meet spec.
  • Plate voltage: maybe a bench supply, maybe a transformer and rectifier. The V, I operating range is very different than for normal operation, so I might want to look up how a commercial tester does it (in terms of applied voltage and threshold current). (I happen to have a few testers of this type, so I could just measure one; if I didn't, I might look up the manuals for a few and see if I can figure out what they're doing based on the schematics. Nice thing about old hardware, they often provided full schematics!)
  • Measurement: DMM. AC volts for setting filament, DC amperes for measuring emission.

This is obviously a bit more labor intensive than using a proper tester, but that's no accident; they've literally gone through and done all that work for you, and tabulated it on the roll chart, or manual, or whatever. So a proper tester is quite good value in that respect. As a result, you'll be working a bit slower through your collection, and you'll likely make mistakes. Whether this is worthwhile to you, I don't know -- for example it's probably more economical to sell them as-is, if intended for sale. Or to put in a bit more effort (value-add) such as matching of common hi-fi types (usually by Ia at fixed electrode voltages, but matching gm can be additionally helpful).

\$\endgroup\$
0
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\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, 2017 at 14:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.