RECTIFIER TUBES GENERAL INSTRUCTIONS

WARNING:

The safety warnings apply in particular when you test rectifiers in the power test mode. Please read it again here.

Description

There are two ways to test rectifiers with a tube tester. That is power output testing, with the tube in a real rectifier circuit, or parametric testing, where the tube is tested for some parameters, from which you can derive it's condition. The L3-3 can do both, though original cards for parametric tests were not issued by the Russians.

  1. You can only plug in and plug out a rectifier when the tester is switched OFF. Otherwise, it may cause a spark in the rectifier socket, or even inside the rectifier, or transformer, due to unlucky order of the tube pins that make contact first. Most people know, you can not pull a rectifier tube out of a working amplifier, or you risk severe damage, For the same reason, you can not pull a rectifier out of of working power test in the L3-3, because the tube is indeed used in a real circuit here.
  2. 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. The card plugs can choose the capacitor, the load resistor, the transformer voltage, and single or double sided rectification. There is no handbook anywhere for how to make rectifier test cards, and the possibilities of the L3-3 with the original test cards are extremely limited. I call this a pity, because L3-3 can do almost anythingn you want! Even so, I found a way to test rectifiers with the original UX4 socket, whereas the makers of the L3-3 did not find out this way. You can try, but you will soon find out, there is a problem. Well, I have solved it :) So I can test with my cards the 274A, 5Z4, 80, 83, etc.
  3. Gassy Rectifier RGN1064Check if you see a faint, dark blue haze, around the heater wires of directly heated tubes. If so, the tube may still work, but it does have some gas, and should be replaced and not further tested.
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  4. With some tubes, maximum test time is limited, and cooling down between tests is required. If so, this is written on the card. This is because the output power of the rectifier (up to 125 Watt with some types) is dumped into those large, green ceramic resistors inside the L3-3, which are located at the back of the tester. They look more like heater elements, than like resistors, and they do need to cool down after each test, or too much heat will be radiated on the wiring and other inside parts.
  5. Many Methods to test rectifier tubes exists. With circuit design many people consider a tube rectifier just like a silicon diode, and in case of problems blame the tubes. With such problems you will have to do, when testing tubes. Keep apart a functional test from a qualilty test. The functional test will prove if the rectifier works. These are the cards for POWER TEST.

  6. The parametric test will give a better idea of the expected lifetime. This is called FORWARD VOLTAGE test. These cards indicate if the tube is used or not. The more use a tube has, the higher the forward voltage will be, though current capability is often still good. Such a tube, though appearantly working (with the POWER TEST card, or in an amplifier) has much higher heat developement inside. This is due to higher forward voltage at the same current. This additional heat is not anticipated in the thermal design of the tube, and so it begins to self-destroy at an unexpected moment, when heat gets too much. How far out this moment is in the future, can only be estimated, but there is a direct relation to forward voltage going up. Note, this is an avalanching effect. So a used tube may appear to be working, but unexpectedly begin to over heat and self destroy, due to this. It is obvious, this happens rather to rectifiers used at or close to maximum current. Moreover, the so called "derating" curves for rectifier design are not respected today by many designers, they don't even know what that is. As well as maximum capacitor values are regularly exceeded. Not to mention the minimum Copper resistance a tube power supply transformer MUST have. This summary makes that failing rectifiers have become a more common problem now. So good testing is important.

  7. There are no fixed rules for this, but generally it can be said, when the tube forward voltage is too high, it shows a much stronger relation to heater volatge change. Best is, to compare this between known good and known bad tubes. Like when a "just" good tube, rapidly increases it's forward voltage at 90% heater voltage only, this is a very bad sign.
  8. Sometimes a rectifier can have so much emission, it is almost "too" good. Such a tube will test outside the green zone. Of course, such a tube is not bad, but in some rare cases this still may give problems, like too much hum. In extreme cases this can even saturate the choke at switch on, while the capacitor is still empty, which many even blow out the fuse. Most of the time though, such a tube will work normal
  9. BURN IN. NOS tubes, specially indirectly heated tubes, may need a short burn in, to bring them in good condition again. Initially such a tube may fail the test, and by just using them they will get better soon. However, normal use, or a POWER TEST (which is the same) is not a good way to burn them in, since using a pulsed current is not a good way for that. Use the forward voltage test for this, which is a plain DC test. Often within 20 minutes the tubes have recovered, but a longer burn it may be beneficial still, before the tube is used the regular way (which is in pulsed mode). Continiously monitor the plate current, and keep in constant, by changing the plate voltage. What value to take for this, is printed on the card. Doing this carefully, can change an over-stored, "quite bad" testing NOS tube into a strong tube. When you have such virgin NOS tubes in your collection, burn them in is a good way to preserve them safely for another 25 years. On the other hand, failing to do so, will further decrease the quality, and at a certain, too low level, burn in may take very long, or even fail to bring the tube back to full 100% again. Such a rectifier has permanent damage, which is pity, because it could have been avoided easily.