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Test Cards for L1 and L3 tube testers (L3-3)
(Last updated: 02-Feb-2016 0:20 )

 

POWER TESTING OF RECTIFIER TUBES

Many Methods to test rectifier tubes excists. Not all are mentioned here. The L3-3 is fully programmable, and it has the components inside to set up a programmable rectifier circuit with the tube you want to test. So via the card plugs you can choose the capacitor, the load resistor, the transformer voltage, and single or double sided rectification. The "fun" part is, there is no handbook anywhere for how to make rectifier test cards, and I was able to find even a way to connect a dual wave rectifier circuit to the UX4 socket. As there is no such card with the original cards, and as the Russian did make UX4 rectifiers, also for the Chinese market, my guess is, they never had this idea themself. Well this is evaluation.

Note, it is not 100% possible to replace a real circuit rectifier test by something else. So when you have a tube like for instance 5U4G, capable of delivering 125 Watt DC power into a load, the question to ask is: 1) Will the 125Watt come out, and 2) will the tube spark or not. Specially this with the spark you may be going to see sometimes with questionable rectifiers, whereas any low power test circuit will not develop this spark. Yet, this IS what we want to know, so we want to make sure the tube does not spark in an amplifier too.

Methods to test:

  1. The hardest method is, use the rectifier in a real circuit, which produces high DC voltage, and load this voltage with a (variable) power resistor. As tubes like 5U4G are capable of delivering 125 Watt in such a resistor, both tube and resistor get very hot from this. This means you can not leave the tube under test in the tester for a very long time, as this will over heat the load resistor. So there is a maximum test time for each tube, and a minimum cool down time before another rectifier can be tested. This is written on the test card for each tube. Note, this test is really very hard for the tube, and in many applications the tube doesn't need to work as hard. If a tube fails the test, while you observe it seems to work still in the equipment, that is possible. Also the tube may spark during this test, which is a clear failure. This may blow the mains fuse in the tube tester deck, which is not something catastrophic, it is what this fuse is for. Replace the fuse, and the tester will work again, as this rectifier test is not using any of the delicate electronics inside. It uses only the mains transformer, then comes already the rectifier, and at it's output is a large resistor with a capacitor. So a very simple circuit, but very hard to drive, as we use maximum conditions here. P
  2. A less tough method is, measure the forward voltage of the rectifier, using the regulated internal power supply. Actually this test is also very reliable, but not the "real life" test in a real circuit. This method will let tubes pass, that will work fine in a less demanding circuit, which circuits are many around. Like 5U4G can be used at 300V 80mA, and sure many used tubes are still ok for this. However the same tube may fail at 500Volt 250mA. The forward voltage test is nice to check how tubes respond to a change in heater voltage.

Which methods is used, is written on the test card.

 

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