Voltage Regulator Tubes (What are they and why use them?) 15 April 2013 Mike at MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! A voltage regulator tube is used to maintain a constant voltage in circuits that are sensitive to shifts in voltage. The tubes are primarily used in power supplies to maintain a constant voltage output from the supply. Unlike amplifier tubes, voltage regulator tubes do not require a heater. The tubes work by ionizing the gas contained in the tube. The ionization (called a plasma) color depends on the type of gas used in the tube. A common gas used is neon (the same gas used in neon indicators; aka Nixie tubes and neon signs). Other gases may be mixed in with the neon gas or another gas may be used instead of neon. The other gases are added to change the voltage drop or stability of the voltage regulation. The other common gasses added are xenon, argon, krypton and helium. Neon gas will have an redish-orange color glow. Xenon gas will have a violet glow color. Argon will have a bluish color glow. Krypton will have a lavender glow color. Helium will have a yellow-orange glow color. It is common for various gases to be mixed, including sometimes inert gases to provide the desired ionization voltage characteristics of the tube. Sometimes a small amount of radon gas is added to stablize the ionization voltage trigger point. Many things can affect the ionization trigger point and the nominal regulation voltage of the tube. This includes nearby electronic fields, temperature and ambient light level. However the primary effect on the ionization and regulation voltage is the type of gas(es) used, the pressure of the gas in the tube and the distance between the anode and cathode elements inside the tube. While voltage regulator tubes can last a long time (up to 10,000 hours). They do slowly degrade. The normal failure is that the voltage across the tube slowly rises as the amount of available ionizable gas in the tube is used up. The other effect this causes is a reduction in voltage regulation capability. A typical voltage regulator tube will have a defined range of current flow through the tube for which it will maintain a stable voltage across the tube. (The voltage will still change somewhat, usually 4 to 5 volts over the defined range of the tube.) As the tube ages, the voltage drop for a given current will rise and the operating range in which the voltage will be relatively stable for a varience in the current will be reduced. Also the amount of shift in voltage for the change in current will increase as the tube is used up. At some point the change in voltage across the tube will exceed the design specification for the circuit and the tube will need to be replaced. One thing to keep in mind is that the initial regulating voltage drop across the tube is different from tube to tube. Thus a tube that is rated for 150V regulation may have an actual regulation voltage anywhere from 140V to 160V. The important part is that the stability of the regulation voltage (how much the voltage changes depending on the current flow through the tube). Thus a 0D3 tube with a run voltage of 155V will stay stable between 153V to 157V with a current flow between 5mA and 40mA. Whereas a tube with a run voltage of 145V will operate between 143V and 147V for a current flow of 5mA and 40mA. (Other voltage regulator tubes will have other, but similar, characteristics.) It should be noted that voltage regulator tubes require special care to manufacture. They require a burn-in time to stabilize the operating voltage (typically 100 to 150 hours). If the tube has been sitting on the shelf for decades (as is the case for most voltage regulator tubes these days since they are not made anymore), the tube may require a repeat of the burn-in to recondition the tube. There is a fast burn-in precedure that can be used, and was done for a while, but the side effect is that it can cause metal migration to deposit metal deposits on the inside of the tube glass. The metal migration is also an effect that shows up as the tube is used. As a result tubes that were made with the rapid burn in method were sometime confused with used tubes causing complaints. Because of this, the fast burn in method was discontinued by most manufactures even though it takes much longer to use the slow burn-in method. The only real way to know if a tube that shows evidence of metal migration is new or heavily used is to measure it in operation. Typically a tube that has been used long enough to show metal migration will have poor voltage regulation and the run voltage will be excessively high. If the run voltage is within the acceptable range and the change in voltage is within tolerance with a change in the current through the tube, then the metal migration may be a result of the fast burn-in process and the tube is perfectly good. Note: Don't confuse the metal migration with the getter flash. They will both look similar. A getter flash will be a large splotch of metalization on the inside of the glass. It may be on the bottom of the tube, the top, or on the side. If you look carefully, you will usually see the getter device mounted inside the tube near where the getter flash is. (Look on-line to see what a getter looks like.) Typically metal migration will appear as a small silver spot (or sometimes multiple spots) on the glass. Typically on the top, or sometimes as a ring around the inside of the tube near the top. This same effect can sometimes be found in regular vacuum tubes. A few tubes are built with an arc process to clean up the tube as one of the final steps. The arc can cause the metalization effect. Typically though the metalization is an indication of the tube being well used. Even if the metal migration is because of use, it doesn't mean that the tube is worn out. Many tubes will still work perfectly normal even though there is presence of metal migration on the inside of the tube. It is just a side effect of the operation of the tube. However, it doesn't affect the operation of the tube. Care and feeding of voltage regulator tubes: Voltage regulator tubes are shunt regulators. The tube must be driven from a resistive source. The tube will change it's internal resistance to maintain the constant voltage. The rest of the circuit voltage is developed across the source resistor. The circuit that uses the regulated voltage is connected across the voltage regulator tube and receives it's current through the same source resistor. The regulator circuit is designed such that the voltage without the regulator tube is higher than the desired maximum regulated voltage. The voltage regulator tube then shunts enough current to cause the voltage to be reduced to the desired level. If the amount of current needed is more than the voltage regulator tube can handle, then an amplifier circuit may be used to amplify the voltage regulation to the higher current level. Voltage regulator tubes have a limited range of current that they can handle. Typically they can maintain voltage regulation between 5mA to 30ma or 40mA of current through the tube. The earliest voltage regulators were simple neon gas tubes. These were normal indicator lamps that were used as voltage regulators. While they work well for that use, they are not optimized for it. One of the first tubes designed specifically for voltage regulation is the 874 tube. It could maintain a 90 volt reference for 10mA to 50mA of current through the tube. The first 874 tubes used neon gas. Later versions added argon gas to improve the operation of the tube. Later on a group of four voltage regulator tubes were developed to provide a wider range of voltage regulation options. 0A3 = 75 Volts : 5ma to 40ma : 5 volt regulation range 0B3 = 90 volts : 5ma to 40ma : 8 volt regulation range 0C3 = 105 volts : 5ma to 40ma : 2 volt regulation range 0D3 = 150 volts : 5ma to 40ma : 4 volt regulation range The above regulators are all in ST style bulbs. Later on the regulators were redesigned to fit into a GT style bulb. While the target voltage is the same, the maximum current and voltage regulation range is slightly different. These regulators have the same number but with an "A" added to the tube number. 0A3A = 75 Volts : 5ma to 40ma : 6.5 volt regulation range 0B3A = 90 volts : 5ma to 30ma : 6 volt regulation range 0C3A = 105 volts : 5ma to 40ma : 4 volt regulation range 0D3A = 150 volts : 5ma to 40ma : 5.5 volt regulation range When the miniature tubes were developed, a set of voltage regulators tubes were designed to fit that form factor. Only three tubes were defined initially and they used the same number scheme as the ST style tubes but with the tag number being "2" instead of "3". Unfortunately they didn't map the numbers the same, so it can be a bit confusing. 0A2 = 150 volts : 5ma to 30ma : 2 volts regulation range 0B2 = 105 volts : 5ma to 30ma : 1 volt regulation range 0C2 = 75 volts : 5ma to 30ma : 3 volts regulation range Note: The voltage regulator numbers start with "0" (zero) because they do not have a heater. So there is no heater voltage for the tube. Over the years, other special voltage regulators were developed for various needs. The most commonly encountered one is the 5751 tube. Probably because it fills in the missing 90 volt regulator tube. Although the current rating for the tube is much lower. The difference is that the 5751 is less sensitive to environmental effects that can change the ionization trigger point of the tube. So it is used in circuits where that aspect is important. 5751 : 87 volts : 1.5ma to 3.5ma : 3 volts regulation range There are many other voltage regulators that have been developed for special uses. These are less likely to be encountered though because of the small volume production for the tubes. The solid state equivalent of the voltage regulator tube is the zener diode. Other special uses for gas tubes: TR tubes: There have been other uses that gas filled tubes have been used for. One use was in radar. A special version was developed called a TR tube. It is used as a switch to disable the receiver during the period when the transmitter pulse is being generated. This allows the receiver circuits to recover more quickly because they are not being overloaded by the transmitter signal. The tube works by being placed in the microwave conduit that is used to connect the antenna to the transmitter and receiver circuits. When the transmitter pulse is generated, the energy of the pulse ionizes the gas in the tube. This is used to change the impedance of the wave guide at the receiver to appear to be a short thereby preventing the transmitter signal from getting into the receiver. Actually this is far from perfect. It takes a bit of time for the gas to ionize so the initial pusle still gets through. Also, the "short" is not perfect, so even when in full operation, some of the transmitter pulse gets through. Still, there is a significant reduction in the amount of the transmitter signal that gets through that makes the TR tube a valuable addition to the circuit. There is no direct solid state equivalent for the TR tube, however other design methods of providing a similar action of blocking the receiver input during the transmitter pulse are used. Spark Gap tubes: Another application was use as a lightning or arc supressor. This was used by the telephone companies to protect the equipment attached to their phone lines. When the lighting stike hit the phone line, the voltage surge would cause the gas in the tube to ionize thereby redirecting voltage to earth ground instead of into the sensitive telephone equipment. The advantage of the gas tube is that it can be triggered many times with little or no degredation. The other typical method of lightning suppression is a spark gap. This is just a device with a small gap between the electrodes. The ambient air becomes the ionization source. The problem with this method is that the contacts can get dirty and cause a short under normal use. Also it is highly sensitive to atmospheric conditions (high humidity vs. low humidity, smoke or other contaminants in the air). Also, a single trigger of the spark gap can leave behind a carbon trail which can cause the device to be degraded or shorted. Using the gas tube means a much more reliable operation. The solid state equivalent of the spark gap tube is the Transorb. Gas Triodes: A voltage regulator tube requires a much higher voltage to be applied than the voltage it normally runs at. This is required to trigger the ionization in the tube. The ionization voltage may be 10% to 30% higher than the regulation voltage. How much higher depends on the gas(es) used and the construction of the tube. One of the discoveries that was made with voltage regulator tubes was that with the addition of another element the ionization of the tube could be triggered at a lower voltage than it would normally occur. The extra element is a wire that is placed closer to the cathode of the tube. Because it is closer, the voltage needed to trigger the ionization is lower. Once the ionization is triggered, the plasma spreads to the rest of the tube and the current can flow between the anode and cathode of the tube even though the voltage between them was not high enough to trigger the ionization by itself. Since there is normally a large difference between the voltage needed to trigger the ionization event and the voltage that the tube runs at once ionization occurs, the difference can be used to trigger other circuit elements. In other words the tube becomes an electronic switch. The first tube that used this capability was the 0A4 tube. Like the voltage regulator, the gas triode doesn't require a heater. Later on, the ability to trigger the ionization with a separate element was optimized, and it was found that putting a heater in the tube allowed for better control. A gas tube switch that has a heater in it is called a thyratron. They can have a single trigger element, or several additional grids to improve the operation of the tube. If it does not have a heater, then it is referred to as a gas triode (or gas pentode in the case of the 0A5 tube). If it has a heater, then it is referred to as a thyratron regardless of the number of elements it has. The most common thyratron tubes are the 2050 (large ST style) and the 2D21 (miniature). The solid state equivalent of the gas triode is the SCR device. Strobe tubes: Another use for gas filled tubes is to use them for high output strobe lights. A strobe light is a device that emits a high intensity light that can be triggered. This works just like the gas triode except that the tube is optimized for maximum light output from the tube. Typically the tube power is provided from a storage capacitor. The capacitor is used to store a specific amount of charge that is dumped through the tube. The higher the voltage, the brighter the light. The larger the charge available from the capacitor, the longer the period of time the light is emitted from the lamp. One of the first strobe tubes made was the SN4 made by Sylvania (sometimes called the 1D21 or 631-P1 which was the military version). The SN4 used neon gas. It was commonly used as a marker light in airports and other places. It was also commonly used as a timing light because the strobe could be carefully controlled and the light was bright enough to be seen in daylight conditions. Later strobe tubes were designed using Xenon gas which is brighter and has a wider spectrum of light output. This was the technology that was used for the camera strobe flash. It has only been recently that solid state LED technology has reached a point where it can provide equivalent light output to the xenon strobe tube. Almost all cameras these days use a LED for the strobe light.