Telefunken RGN1064
Test card Nr 038
Description
Sparking of tubes. RGN1064 was designed for low voltage medium current. Those who treat tube rectifiers like silicon diodes, will have observed the sparking in the tubes. Not realizing these are not "bad" tubes at all, but we have here a wrong use. Though indeed it is possible to find such tubes of the same kind which don't spark in a wrong use application. Are these the "better" tubes perhaps? No, probably not. These apparently are not capable of exceeding the maximum limits in circuits where a good tube would probably do so, and spark.
What causes sparking? For this we have detailed technical bulletins on the Emissionlabs website, but it is generally caused when the tube is switched on, and being forced to produce full current while only half warm. For the "half warm" tube, which can not do full current yet, this is an overload condition, and quite a bad one. And now it becomes difficult, as maximum current is calculated by a formula, and depends in the AC voltage which is supplied to the tube. So at high AC voltage, you need to use low current (DC mA), and also a high impedance load (So, a small capacitor). Many designs, not worth the word "design", use HIGH Voltage, HIGH current, and BIG Capacitors. For more details, read the Application Notes on EmissionLabs.com. For the cards here, we are using the 350V AC winding, which is high voltage for an RGN1064. This means we need to use low current, and a possible small capacitor.
HOWEVER... What we can not do, is comply with the RGN1064 requirement for a certain (minimum) transformer AC windings resistance. This is another reason why the test current is only 100mA. This doesn't mean the test is too soft or something like that. A good tube will give 100mA, and a bad tube us below 83mA. Still because we do not use the minimum transformer AC windings resistance, a small, mild spark at start up MAY occur, and this does not mean the tube is bad. If not repeated very often, this will not harm the tube.
The L3-3 Schematic has an error in the connection of the B4 socket for this tube. So you can't go by this schematic. Luckily the wiring of the L3-3 is good. We choose not publish this error in the form of drawings, since there is already enough confusing with this, and people damaged the testers with self-made cards because of this error. So just be informed, this part of the Russian Schematic is wrong.