What is a Space Charge Tube? Mike at MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! Space Charge tubes operate at "safe" voltages. Normally 12 to 24 volts. Just as Compactron tubes were the last gasp for tubes in TVs, the space charge tube was the last gasp for tubes in car radios. While the concept had been around since the 1920s, because of the poor efficency, the design concept was not used in tubes until around 1958 when Tung-Sol designed a line of space charge tubes for use in car radios. The large battery in the car ment that the higher power requirements of space charge tubes was not as much an issue. The primary advantage was that the tubes allowed the elimination of the vibrator circuit in the car radio. The vibrator was a device used to convert the DC power in the car into AC so it could be boosted to high voltages for use by the tubes. However being a mechanical device, the vibrator had a short life expectancy. The use of the space charge tube had a narrow window. To be usable, the space charge tube needed the development of the transistor to happen. There are no power output space charge tubes, so transistors were used for the output. Once low cost high frequency transistors became available in the mid 1960s, the space charge tube became obsolete. These days the big advantage of the space charge tube is that it operates at non-lethal voltages (12 volts to 24 volts). The main disadvantage of the space charge tube (besides there not being any high power output tubes) is that they can only handle a few milliamps of plate current at the most. Many of them require a much higher heater current to operate. That also means that they get hot. Most of the tubes can only handle a milliamp or two of plate current. These are usually regular tubes that have been recharacterized for use at low voltages. There are several ways to improve the plate current flow. One way to increase plate current flow is to reduce the distance between the cathode and the plate, however this also increases the capacitance which reduces the frequency response. Another way to increase the plate current flow is to encourage more electrons to come off the cathode. One easy way to do this is to increase the heater current. This can often double or tripple the plate current flow. A tricker way is to use a space charge grid. This is a positively charged grid near the cathode that draws more electrons off the cathode. Typically this grid is tied to the plate. Some of the electrons are drained off by the space charge grid, (thus bypassing the control grid) but the increased size of the electron cloud around the cathode means that more are available to get to the plate through the control grid, so there is a net gain. Jeff Duntemann has an excellant article on space charge tubes. http://www.duntemann.com/12vtubes/12vtubesindex.htm All tubes are space charge tubes. Without it, they could not operate. When the cathode is heated, electrons are boiled off of the cathode which creates a cloud of electrons around the cathode. Many of the electrons are repulsed by the electron cloud back to the cathode. This creates a balance of electrons in the cloud which is the space charge effect. If a positively charged plate is placed near the cloud, it will attract the negatively charged electrons. By placing a negatively charged grid between the plate and the cathode, the electrons can be repulsed, preventing some or most from reaching the plate, thus controlling the flow of electrons between the cathode and plate. This is the normal operation of a triode. Note: Tubes require a vacuum to operate because if there was air in the tube, the electron cloud would disipate via the conductive path of the air. In a vacuum, the electrons have no place to go, so they collect in a cloud around the cathode. Gas tubes such as thyratrons and gas rectifiers use special gases (such as mercury or neon) to optimize the conductive path between the cathode and plate. This works great for switching and rectification applications, but the conductive path of the gas makes it difficult to control electron flow which prevents them from being useful in amplification circuits. To run most efficiently, the tube plate needs a high voltage to attract the electrons (around 50V to 200V). By using a space charge grid, the number of electrons in the electron cloud around the cathode can be increased. The more electrons that are available means the more that can be attracted with a lower plate voltage. Still it does not approach the amount of electrons that can be attracted with higher voltages. Also the lower plate voltage means that there is a smaller dynamic range available to work with, reducing even more how much power a low voltage tube can handle. For high frequency radios, the amount of current required is normally small as is the dynamic range requirements for the voltage. They normally operate in the microvolt to millivolt range. So for a radio the low voltage space charge tube, does not really provide any significant limitations. The only real problem is the lack of high power handling required for the audio output section where watts are required instead of microwatts of power. While a normal power output tube like a 6V6 could be used, it requires a high voltage on the plate to be able to function. For a battery operated system like a car radio, that means using a voltage converter (the old mechanical vibrator). So the low voltage space charge tubes provided no real advantage...until the invention of the power transistor. In the early 1950s, transistors were only really useful for low frequeny use such as audio and they were unreliable. This was largely due to the crude method of forming the transistor, which ment large capacitance between the transistor elements. One of the large research efforts was to create a high power transistor to replace the power output tubes. Once the power transistors for audio work became available, it allowed a short period of hybrid use in radios where the power transistor was used for the audio output stage and the vacuum tube was used for the high frequency radio section. This only lasted a few years. Low cost reliable high frequency transistors were also a large research effort and they became available in the early 1960s. Once the transistors became available, there was no need for tubes any more, and they quickly disappeared from the car radio designs. For table-top radio use, the tubes continued into the 1970s. This was mainly a cost issue. Low cost tube radios don't required power transformers which is one of the most expensive (and heavy) components. Also by that time, tube radio designs and components had become very standardized allowing large scale mass production. It would take many years for table-top transistor radios to beat the tube radio at the cost level. And of course there was the slight problem that most radio design engineers understood tube design, but did not know how to design with transistors. A part of the transition was to wait for the older engineers who could make the change to learn how to work with transistors and for the new engineers to come on board who were schooled on transistor designs. In the mean time, the low cost battery powered transistor pocket radio quickly became popular in the 60s. That in conjunction with the TV largely killed off the tabletop radio market. Although it has survived on in the transistorized version in the form of alarm clock radios. Many normal tubes can be used in low voltage applications. In fact, some of the low voltage tubes are just regular tubes recharacterized for low voltage operation. To find a normal tube that will work best at low voltage, look for minature tubes with higher than normal heater current requirements. Several examples would be the 12AV7 tube which is a high current heater version of the 12AY7 and the 12AZ7 which is a high heater current version of the popular 12AT7 tube. Some of the older tubes with larger distances between the cathode and plate can be used too, but they will be less efficent and more finicky to get them to work. Another way to operate a normal tube at lower voltages is to use the first grid as a space charge grid instead of the usual control grid. The second grid then becomes the control grid. This of course requires that the tube be a tetrode or pentode tube to be able to "steal" the control grid for use as the space charge grid. With the reduced grid compliment, the tetrode then operates as a triode and the pentode operates as a tetrode. Most hexodes have the second and forth grids tied together which makes it difficult to use them in the space charge grid configuration. Pentagrid converters could possibly be used, but they are difficult to work with even in their normal configuration. Also look for tubes with low maximum plate voltage. A 150V rated tube is more likely to work well at lower voltages than a tube rated at 500V. If you want to stay true to the low voltage tube power, but want some output, consider the 26A7 tube (GT style tube). This tube is rated for 26 volts, but can easily run at 24 volt (for which there are a lot of cheap transformers available) and can produce 165 milliwatts with 26 volts on the plate. That is enough to easily drive a small speaker. Another tube is the 28D7 (loctal tube) can produce 100 milliwatt with a 28 Volt plate voltage. The down side is that these tubes are rare and hard to find. Also, don't forget the subminiature tubes. Most were designed for battery operation, so many are actually intended to work at low voltages. Another place to look is compactrons. The high power compactrons used in sweep circuits generally have high current heaters and are designed to operate at low voltages for more efficent operation. You will not be able to get the maximum rated power from the tube, but you may be able to get enough for your application. For low cost, consider using one of the odd size voltage tubes, such as 8V, 11V, 13V, 15V, 25V and 26V. Keep in mind that compactron heaters are robust and can operate at +-20% of the rated voltage. You can easily run an 11V, 13V or 15V compactron tube at 12V, or a 25V or 26V tube at 24 volts. To use an 8V tube, just put a resistor in series with a 12V supply. If you run the heater "hot" (10% to 20% above rating), you can get even more electrons from the cathode giving you a greater chance at low voltage operation.