Simple vacuum tube tester design

Question

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?

Answer 1

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).

Answer 2

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.

Answer 3

this is a type of project I can respect & appreciate, as I've done it many times before; building/fabricating "bulk/speed" general purpose "quick" or "speed" testers is a bit of an art form in itself. I'm looking at the post date here, and wondering if this is still an active endeavor? If this still an active pursuit?, I'd like to offer some suggestions...

I'm not seeing any meters, or notable test/metering points marked in your diagram? I'm not certain if your intent is to have fixed meters, or to use a separate multimeter? the later of the two requiring requiring break points (often by installing more switches).

Tubes often need to be organised into groups, for a speed tester as well: socket type, function type, cathode voltage, and shared/common pinout configurations... For example, one dedicated tester with a mini9 pin base/socket can be set up for testing all 6A7 & 12A7 tube types the only major change being the filiment voltage. I would like to send pictures or schematics, unfortunately I haven't figured out how to do that here yet.

I think your transformer may need to be reconfigured, to make better use of it? though I don't know the manufacturer or model type of the transformer (I'm actually intrigued)? another aspect of adding another switch, rectifier (probably a 1N400x or a 1N540x type), condenser, resistor or two, and probably a filter/hash choke type coil...

hope this helps, provided it may still be an active project?