Testing of dual triodes with L3-3 tube tester

Most tube testers I know, test double triodes separately, so you need two test cards, or repeat the settings every time via the roll chart. Not all testers are doing so! Like for instance the AVO testers, and Neuberger allow to swap from Anode1 to Anode2, just by one switch. This is done by putting the tubes in parallel, and connect only the significant parts of one half tube.

 

A warning when using these cards:

When plugging or unplugging card pins, or tubes, the tester should be off. This is the only safe way. When the anode plugs of double triode are removed while testing, nothing will get electrically defective by this, but there is full anode voltage on the pins, and the risk to touch the metal of those with your finger tips is high. When leaning with one hand on the metal handle, and then touch the metal of the anode plug with your other hand, is dangerous. This is written multiple times in the Russian manual, which you have carefully studied of course, so I write this here only in addition to it, and not as a replacement.

The advantagess of dual system testing with one card.

First of course, there is no need to swap the test cards, which is very annoying when doing series testing of tubes.

Another advantage is, both triodes can be heated up simultaneously, and then after 2...3 minutes, only one triode is finally tested. This reflects realistic use, unless of course there is an application, where only one system is used, and the other is not. Specially with used tubes, it will often be so, that System1 responds better, if System2 is under plate current also. That is because the whole tube gets more warmer from this, and not just a little bit. Knowing the heater power is only 20% of the total tube dissipation, we don't have to wonder where most of the heat is coming from. Of course thermal balance of any tube is optimized at normal use conditions. Meaning the tube is much hotter than "ideal" if anode is heated at full power. The same applies when the anode is not heated at all. Or even worse, a double triode is tested, and neither if the anodes is warmed up. We call this cold testing. More tube testers (mis)perform by this method as one would expect, because it stays invisible often for the user. When the tube tests good anyway, and precise results are not needed, it doesn't matter much indeed. However, the weaker a tube gets, the more it reacts negative to this form of under heating, which makes the test results worse than they really are. Many tubes which test "just in the red", suddenly test "just in the green", if heated up properly before testing. Sometimes after burn in, but with weaker tubes, there is always quite a difference. So to say it in a few lines, you should heat up the tube with both anode plugs in, and then test each system separately, for more reliable results.

Is there a disadvantage to this method?

I never found one.

  1. Heater to cathode leakage. This should be low, in order to have no hum via the heater. To test this realistically, a high test voltage should be used, because this type of "resistor" will get lower at higher voltage. It is the reason why so many E88CC are humming, because this tube is often used in cascade. Sometimes the schematic is the blame and not the tube. So leakage can be below minimum, and still there is tube hum. Also DC heated tubes can begin to hum from this kind of leakage! That is because there is still the primary to secondary capacitance of the transformer heating winding. This AC current flows, regardless AC or DC heating. You can do some extended tests, when you have a tube which shows to be somewhat leaky, and you will soon see, the leakage only appears when you are at, or close to maximum anode dissipation. So again to test this realistically on double triodes, the whole tube (so two systems together) should be heated to maximum, and then very rapidly do the leakage test. Then while doing so, you will already see the leakage go down rapid fly after 3...5 seconds, since during this test, the anode current is off. Then to repeat the test with only one anode heated at maximum, add up both results, and you will see it is significantly less than the result for both tubes together. The reason is found, as now repeated a few times, in the different temperature of the whole tube.
  2. Grid leakage. There are two kind for this. Resistive leakage, which is generally low, and often socket related. Grid (G1) emission "leakage" which behaves and tests electrically like leakage, but as the word says is grid emission. L3-3 is a great machine, and measures this DIRECTLY, and it is the only tube tester I know, which can do this. All other tube testers, put a resistor in series with G1, and the voltage drop resulting from this will offset the tube anode current, indicating leakage. It is very typical for grid leakage to disappear fully when the tube has no heat dissipation from the anode. The heater can not heat up the grid far enough for this, but the anode can and will. So when the anode current of system2 is off, we can precisely measure the grid current of system1, and vice versa.

With the above in mind, these cards were designed.