Picture 1. The slide switches of the METRIX U61B.

This is a French machine, but here is the German Front panel version of it. The horizontal numbers 1....9 indicate the tube socket pins, the same way they are numbered by a tube data book. Then by means of the slide switches, you connect those to the tube tester inner circuits. Like this any "new" tube can be tested, provided of course you have a socket adapter for it.

What you see is:

So take quick reference datasheet (click here for example) and you can quickly connect and test a random tube, because normal datasheet pin numbering is used here!

Before you start you do need to test for shorts and leakage. Here is shown ho that is done. With all switches to "M" as you see in picture1, all pins are connected to ground of the tester.

Picture 2. Press each slide down, and see if the neon lamp burns.

Function: Shortage or open filament.

All tube pins are connected to ground (M) and now you press one by one, each switch to the position "kS". This connects the corresponding tube pin to an AC voltage source, and the neon lamp is placed in series with it. When you let go the slide, it moves back to the M position. When the tube is ok, you find the connections of the filament like this. Suppose the filament is connected to pin2 and pin7. You start to press all slides down, one by one. You press 1, 2, 3, 4, etc. The moment you press slide switch 2, the lamp will burn. That is because switch 7 is connected to "M" (ground) still. Then when you press switch 7, the lamp also burns. So the filament is obviously connected to 2 and 7. This means you have no shortage, because the "shortage" you found between 2+7 is the only one, but this is the filament of course. If there is no connection to be found at all, you have an open filament. If the lamp burns at three positions, you have a shortage. An exception is of course tubes with three filament connections like ECC81. This tube must light up the neon lamp at three positions. So this is a simple, logical and efficient way.

Picture 3. Apply filament voltage.

Function: leakage test

Assumed there are no shorts in the previous test, and the filament is not open, we continue with the leakage test here. This must be done on a warm tube, because the leakage normally increases when hot, and may even fully disappear when cold.

With the tube we have here, we found the filament between pin 2 + 7. To connect the filament transformer to those, pin 2 is connected to "F" and pin 7 is left connected to "M". Between F and ground (M) the filament voltage of the tester is connected. Now the filament will light up, and the neon lamp is off. To double check this, you press the left button from picture Nr. 4. When you press it, the lamp burns, indicating a good filament. So we really know everything is ok. Now let warm up the tube for at least 5 minutes, and then the leakage tests can be done. For this, all slides are pressed one by one, to the "kS" position, apart for the filament connection, which is left at "F".

What you might see now, is:

• Nothing: Meaning there is no capacitive load recognised, and leakage resistance is above 4 Meg ohms.
• A flash: Meaning the tube pin has some capacitance. This normal with many tubes.
• Some small light effect, the lamp is barely on. This is normal and indicates a very high isolation resistance, even under high AC voltage, which is the Metrix U61B method here.
• Medium strong light effect: Suspicious tube
• Strong light effect: Leakage is detected. BAD Tube.

Picture 4	Value 1 Meg Ohm. Limit to where lamp starts to burn. Look at the filament voltage on the meter, it is 6.3 Volts.
Picture 5	Value 470k. Lamps is burning at 25%. This is the maximum limit for a good tube. This tube is still considered good.
Picture 6	Value 100k. Lamps is burning at 50% . This tube is bad
Picture 7	Short circuit. Lamps is burning at 100% brightness. Resistance is below 50k. This is more than a leakage, better call it a short.

Picture 8. This is how the effects of a leakage resistance was pictured.

A variable resistor was connected between pin 3 + 4. So when pushing slide 3 or 4, the lamp would burn. By this we could picture the neon lamp, and say what the leakage resistance was that caused a certain brightness.

Conclusion: It works amazingly well, and quick.

Do not think this "lamp" method is primitive. You get very reliable results with it. When you use an AVO or any other tester having an analog meter, you will soon notice the meter is insensitive. So if you have a tube that you know has some leakage, you will try to measure the leakage. So you have to say something about a movement of 4 mm on the needle scale. So the scale gives you a number, but what does that number mean to you? Probably nothing, unless you're an ultimate expert of course.

What you want to know is the leakage too much or not. It's just yes or no. For that you might just as well have a look at a neon lamp, provided it indicates "good or bad". What is nice about the neon lamp is, that it starts to burn short before the critical area of cathode leakage. (The most important test, because that is the kind of leakage you'll see, and that's the one that causes a tube to hum)

A reasonable value for cathode leakage is 470k Ohms. The neon lamps burns at 25% with that value. The beginning of the light effect is at 1Meg Ohm, which is a good tube. Note: You must measure this with a hot filament. With a good tube, you'll measure no leakage when cold, and often some minimal (normal) leakage when hot.