How the Eico 666 and 667 Tests Tubes 1 March 2024 Mike at Fourwater.com http://www.Fourwater.com While the Eico Instruction manual provides and overview of how it works, there are some details they don't get into. Partly because the information isn't needed for most users, and probably partly because some of the things they are doing are unconventional and likely would upset some purists if they knew what was going on under the hood. This article describes some of the deep dark hidden secrets of the Eico 666 and 667 tube testers. If you have an Eico Tester and have read the Instruction manual, then I'm sure some of this you already know. But since I don't know the level of knowledge of the person currently reading this article, I'm starting with some of the basics to provide the ground work for the description of what is going on under the hood. So please bare with me... No! No! I don't mean that! Keep your clothes on! All tubes are rectifiers. Some add grids to control the current flow through the tube so it can be used for amplification and other work, but at the core of the operation the tube is a rectifier. This aspect is key to the way the Eico tube tester works. A rectifier only passes current when the plate is positive relative to the cathode. The during the test, the meter on the Eico 666 is connected in series between the A/C power source and the plate of the tube being tested. The cathode is connected to the other end of the AC power source, which is the ground reference for the tester of all the voltages used. The tube only passes current when the AC signal is postive on the plate relative to the cathode. The tube turns off when the power goes negative on the plate relate to the cathode. Thus the tube gain is only actually being tested during 1/2 of the AC power. That means the meter is only seeing plate current for 1/2 of the time. Not only that, since the tube is being feed AC from the power source rather than the typical DC power that a tube such as a 6L6 would in a normal circuit such as a guitar amplifier, that means that the voltage between the plate and cathode, and thus the current through the tube is constantly changing and no current at all during 1/2 of the time. The meter is designed to allow for this, it averges the current flow through the meter coil so that the needle remains steady during the test. When a triode, tetrode, or pentrode is being tested, an additional voltage is applied to the control grid. The control grid normally has a negative DC voltage applied to it, called a bias to place it at the best configuration for operation of the tube. Most tubes are characterized for operation such that the grid remains negative relative to the cathode within the operating range of the tube. However, the Eico testers do not provide a negative bias to the grid. They instead drive the grid with an AC votlage that is the same phase as the AC voltage applied to the plate. What that means is that the grid voltage is positive when the plate voltage is positive. While it is unusal to operate a tube with positive voltage on the grid, it doesn't hurt the tube, and the grid will still control the current flow through the tube, although not as well as with the negative bias method. In fact, some special tubes that were designed in the 1950s made use of applying a positive voltage to a grid next to the cathode, that were called space charge tubes. (http://www.fourwater.com/files/spacecharge.txt) While a negative grid voltage repels the electrons back to the cathode, a positive voltage accelerates them toward the plate. The higher the voltage, the more electrons that are pushed towards the plate. When the plate voltage on the 666 is negative relative to the cathode on the other half of the AC waveform, the grid voltage is also negative since it has the same phase as the plate voltage. But since the tube doesn't conduct when the plate voltage is negative, relative to the cathode, the grid negative grid voltage isn't doing anything either. So while the Eico tester isn't applying the grid voltage that the tube would normally operate with, it still is testing that the control grid is able to control the flow of electrons between the plate and cathode. Another grid that is used in tetrodes and pentodes is the screen grid. The purpose of the screen grid is to decouple the control grid and the plate. Because the control grid and plate are close together, along with everything else, they act as a capacitor. The problem with this is that the amplified signal at the plate can be coupled back to the control grid resulting in instability of operation and feedback causing unwanted and uncontrolled oscillation. The screen grid is used to provide an electrostatic barrier between the control grid and the plate. It significantly reduces the capacitance between the control grid and plate without seriously impeading the flow of electrons. The screen grid also helps to accelerate the electrons towards the plate, which allows the tube to have a wider range of operation. The screen is normally connected at a voltage less than the voltage applied to the plate. Some test setups on the Ecio tube testers connect the screen grid to the same voltage source as the plate. Other test setups set the screen grid to the separate screen grid selection. The Eico doesn't really do much to test the screen grid operation, but I recommend that you use a later setup chart that sets the screen grid to the screen selection. The reason being to reduce the potential for the tube to go into parasitic oscillation. Although normally you will only encounter that when testing power tubes like the 6L6. There are some tests that set the plate pin to the screen grid selection. While in a few cases, it is a poorly configured test setup, in others it is actually important. Certain tubes like space charge tubes can't run on the lowest plate voltage setting (45 volts), they need a lower voltage. By setting the plate to the screen grid selection, the lower 15 volts selection for the screen grid (when the "V" lever is set to "1") can be applied to the plate instead. If there is a screen grid present in the tube, then normally it should also be set to the screen grid selection, but since some test setups have it set to the plate selection, that is ok. In fact, on space charge tubes, the normal configuraiton is to connect the screen. There is another grid in pentodes call the supressor grid. It's purpose is basically to allow tubes to have a wider range of operation. Electrons striking the plate can end up being bounced back to the grid, especially if the tube is using a large number of electrons with high voltage on the plate. When the screen grid is added to the the tube, it agravated the problem. Much of the wider range was not much use because of non-linear effects caused by the bounced electrons. The supressor grid captures the stray electrons and sends them (typically) to the cathode, although some tubes are configured to allow the suppressor grid to be tied to ground outside the tube. By capturing the stray electrons, the linear range of the tube is much wider and allows pentode tubes like the 6L6 to have a lot more power output than a triode of similar size and with similar voltages applied. The "V" selector on the Eico testers selects the voltages that will be applied to the plate, screen grid and control grid during the test. The "S" selector selects which shunt is to be applied to the meter to set the current measurement range of the meter. The lever switches map the tube socket pins to the proper grid, plate, screen, cathode and filament and ground reference sources inside the tester. There is one special lever selection, "6" which is used for small signal diodes. It sets the plate to a 5 volt AC source through a 24K Ohm resistor. Since small signal diodes are used for things like RF detectors, they expect small voltages and currents and can potrntially be damaged if tested as a regular power diode. When you press one of the latching buttons located above the pin selector levers, it disconnects the associated pin from being directly attached to the selection on the lever and inserts the test meter in series between the socket pin and the lever selection. When the merit lever is NOT pulled, the leakage test circuit is connected to the pin through the meter which allows measurment of the leakage current through that pin when the H-K Leakage button is pressed). All the other socket pins are tied to the ground reference (the frontpanel is the ground reference). The leakage test is done by placing the negative 70 volts DC voltage supply to the negative connection of the meter and the positive connection of the meter is connected on the tube pin selected by the latching push button. This places the negative 70 volts on the latching push button selected pin thourgh the meter. The selected Plate, Screen and Grid pins are held at at the ground reference. If the tube has an indirectly heated cathode and the cathode is selected with the latching push button and with the tube warmed up, the tube will conduct and the meter will usually deflect the needle to full scale, although a weak tube may not conduct enough to drive the meter to the full 200uA deflection. When the H-K Merit button is pushed, the ground connection is removed from the Plate, Screen and Grid pins selected by the lever switches, which allows the pins to float. Any leakage, including a short circuit, between any of the pins, including (and especially) the cathode will thus be measured by the meter. If a push button other than the cathode selected pin is pressed, then the cathode will remain connected to the ground. The other pins (except the heater) are connected to ground except for the pin selected by the latching push button, which is connected to the negative 70 volt supply through the meter. So any conduction, including a short will cause the meter needle to deflect by the amount of conduction detected which is read off on the Inter Element Leakage scale. If the H-K button is then pushed, the Plate, Screen and Grid pins are disconnected from ground and allowed to float. The exception being the pin selected by the latching push button which is already connected to the meter instead. This doesn't really tell you much, although it could be used to determine if there is a multiple element short. Generally, the only time the H-K button needs to be pressed is to check for the leakage or short between an indirectly heated cathode and other elements (filament, plate, screen, or grid) selected by the pin selection levers. When the merit lever is pulled, the selected socket pins on the pin selection levers are connected to the associated Plate, Screen and Grid supplies and the meter is switched to be in series between the selected latching push button and the source selected on the associated lever switches. So while normally the Plate pin is the latching button that is pressed to measure the current through the tube (ie the emission), you can alternately put the meter between the screen or the grid pin to check current flow through that tube element by pressing it's associated latching push button. For more information about tubes and testers see our companion files: http://www.fourwater.com/tubeinfo.htm http://www.fourwater.com/files/666-667-mod.png http://www.fourwater.com/files/eicotesting.txt http://www.fourwater.com/files/eico666-667-repair.txt http://www.fourwater.com/files/mutualconductance.txt http://www.fourwater.com/files/eico666meter-power-notes.txt http://www.fourwater.com/files/eico666tester-meter-check.txt http://www.fourwater.com/files/how-eico666-works.txt http://www.fourwater.com/files/eico666parasitics.txt http://www.fourwater.com/files/testertypes.txt