A Note About Gas Rectifiers 10 Oct 2019 Mike at MDBVentures.com http://www.MDBVentures.com - Great prices on great tubes! Gas rectifiers such as the type 83 tubes (such as the ones used in Hickock tubes testers) have different characteristics than a vacuum rectifier like the 5U4. A vacuum rectifier is essentially a resistor which only passes current in one direction. Thus as the load increases, the voltage drop across the tube increases. For most applications this is not a major issue, and in some aspects it can be a critical part of the overall characteristic of the device in which the tube is used, such as a guitar amplifer where the distortion characteristic of the amplifier can be an important part of the sound output. The overall tube resistance is also an important part of the rectification circuit of which the tube is only one part. Too little resistance can actually increase unwanted distortion or even generate hum as the power may not be properly filtered. It can also shorten the life expectancy of the filter capacitors if they are not designed for the higher current surges. The lower resistance also means the power supply voltage will be higher, which can also negatively affect the circuits. In extreme cases it can even damage the circuits. Using a tube with a higher overall resistance has similar although somewhat different issues. It will take longer for the filter capacitor to charge, which can result in an increase in hum if the needed charge time exceeds the duration of the A/C sinewave time. It also means that the power supply voltage will be lower. This will not be as damaging as too much voltage, but can still negatively affect the circuits, and as before the sound characteristic of an amplifier will be different. Gas rectifiers bring in another set of issues that have to be considered by a circuit designer. It has no real output until the ionization threashold is reached, then almost all at once (in microseconds), the tube turns on. The gas in the tube ionizes and begins passing current to the circuits. Since it uses an ionization process, as the load increases (more current demand), the tube's internal resistance drops because as more current flows through the tube, more gas is ionized providing more paths for the electrons to travel though the tube. The end result is that the voltage drop across the tube remains relatively constant over the load range of the gas rectifier. This has some advantages and disadvantages. One obvious advantage is a more stable power supply voltage under current load changes. One of the many negative aspects is that since it turns on almost instantly when the ac voltage rises above the gas threshold, that causes added surge current loading on the filter capacitor which can damage it if it is not designed to handle the current surges. It can also increase circuit hum. In most designs where power usage is moderate, the gas tube offers no improvement over the vacuum rectifier as the other critical part of the filter circuit, the capacitor, still has to provide power to the circuits during the time the rectifier is not providing power (during the off period of the rectification cycle). And since the circuits need to account for the current surges caused by the tube turning on and off on each partial A/C waveform, it can actually sometimes be more expensive to use a gas rectifier. Where gas rectifiers work best is in circuits where high power is needed or the more stable voltage during the on-time of the rectifier, is useful and the current surges as a result of the ionization characteristics of the tube is not as important. An example use would be in a car battery charger. Before high power solid state rectifiers were developed, the mercury gas rectifier was used in car battery chargers. Another example at the other end of the scale was the use of the gas rectifier in tube testers. In particular the Hickock testers used them for many years before finally switching to a solid state circuit. The stable voltage regardless of the load was useful in the tube tester. Especially since the gas rectifier will tend to keep a stable voltage drop over the life of the tube, whereas a vacuum rectifier will slowly increase in resistance causing the voltage drop across the rectifier to increase resulting in a lower voltage out of the power supply, especialy when testing high power tubes. So what would happen if you were to replace the type 83 tube in a Hickok tester with a type 80 or 83-V tube? For starters, those tubes do not have the current rating of the type 83 tube. While this is not an issue for most tubes tested, it may be a problem when trying to test a high power tube, such as a sweep tube. It may result in lower reading test results than expected due to the higher voltage drop across the vacuum rectifier tube. For low power tubes, it can have the opposite effect as the power supply voltage will be higher with low currents when using a vacuum rectifier instead of a gas rectifier. Since the voltages used to test tubes with basic tube testers like the Hickok testers remains relatively stable for the given test setup and load, you could probably get away with using the vacuum rectifier instead of a gas rectifier as long as the target tester readings were corrected for the test conditions. However since most people don't have any way to adapt the test results to the changes, it is best to keep the same tube type in the circuits. So what about using a solid state rectifier instead? Like gas rectifiers, solid state rectifiers maintain a relatively stable forward voltage across the operating range of the device regardless of the current load. However, the solid state rectifier has a very much lower forward voltage (around 1V) compared to a gas rectifier. The gas rectifier voltage drop can be anywhere from 15 Volts to 45 Volts (or more for specialty rectifiers) depending on the type of gas(es) used in the tube. The type 83 tube has a forward voltage drop of around 15 Volts. That means the power supply voltage will be 14 Volts higher with a solid state rectifier. Note: The type 83 tube is used to drive the plate (and screen grid if present) of the tube under test. The grid voltage is derived via a different circuit. Since the Hickock uses around 200 Volts (240 V peak) for the plate supply voltage, that would seem to mean you would expect up to 10% higher readings. However, the real-life reading error will be lower. Rectifier tube test results will be higher because a simple emisisons test is used for rectifiers. Tubes used for amplification however will not read as much higher because the Hickock testers use mutualconductance testing. Specifically that means they measure the tube's output as a result of it's amplification rather than just it's overall emission. That means the error will typically be only a couple percent off instead, an exception would be if the test settings are operating the tube near the outer limits of it's linear range, in which case you will potentially see a greater error. Mostly though, the error is going to be minimal. The test results will be more than adequate for most users. So, what about adding a resistor to try to make the solid state rectifier have a similar voltage drop and base resistance? While that works when replacing a vacuum rectifier (and in fact may be desirable in that case), when replacing a gas rectifier, the resistor could end up making things worse as it means the circuit will no longer have a stable voltage drop relative to the load current. So what to do since it is becoming very difficult to find type 83 tubes (they haven't been made for a long time). The simplest answer is to go ahead and use a solid state replacement and accept that you may see slightly higher test results compared to a gas rectifier. Note: the Hickok testers don't use filter capacitors in the power supplies that use the type 83 tube, so there are no worries about current surges damaging filter capacitors. If you feel it is critical to have exactly the same test results, then it is best to shell out the money to get a real type 83 tube. Another solution would be to make a solid state rectifier circuit that more closely emulates the gas rectifier. To do that you would need two 2 Amp or better rectifier diodes and two 12 volt 10 Watt Zener diodes and two 5 Ohm 10 Watt resistors. The diodes are used to block the reverse voltage, the zeners are used to emulate the voltage drop across the tube, and the resistors are used to emulate the base resistance of the tube. Warning: Use extreme caution when working with tube circuits. Do not do this work if you are not qualified to do it. The voltages in tube circuits are lethal and can kill you. However, in most cases, the above would be an exercise in overkill. For the vast majority of situations, the slightly higher readings by using a simple solid state rectifier circuit is more than adequate, and given that tubes operate with tolerances of 20%, a 1% error in test results is not going to be even noticable to most users, and it allows you to simply buy an off-the-shelf solid state type 83 tube replacement and save yourself a lot of headaches and potential damage to the tube tester because you got the circuit wrong. Another consideration to remember about mercury gas rectifiers is that they are considered hazarous waste and must be disposed of via a hazardous waste facility. You must tell them that it contains mercury (if they don't understand what it is as they probably have never even seen one before, just tell them it should be disposed of like a florescent light). Also see the companion files: http://www.fourwater.com/files/rectnotes.txt http://www.fourwater.com/files/voltreg.txt http://www.fourwater.com/files/fullrect.txt http://www.fourwater.com/tubeinfo.htm