Using Other Magic Eye Tubes in a Circuit Mike at MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! With the supplies of eye tubes growing ever tighter and the prices going ever higher, it is becoming more desirable to find ways to use other readily available (ie cheaper) eye tubes in place of expensive hard to find eye tubes. =============================================================== Using a 1629 Eye Tube in place of a 6E5 Eye Tube One relatively easy conversion is to use the 1629 eye tube in place of the 6E5 eye tube. There are two differences between the 6E5 and 1629 eye tubes. The 1629 uses an octal socket and has a 12.6volt 150mA heater. The 6E5 uses a six pin socket and has a 6.3volt 300mA heater. The rest of the characteristics are the same. This makes installing a 1629 tube in a circuit that uses the 6E5 eye tube relatively easy. To make the change, you will need a 12.6 volt 150mA source for the heater. If the 12.6volts is not available, you can install a small 12.6volt filament transformer some place handy to provide the voltage. The next thing you will need to do is replace the 6pin socket with an octal socket. Many eye tube circuits use a free-standing socket at the end of a group of wires tied together. This makes replacing the socket relatively easy, just remove the old socket and replace it with an octal socket. Be sure to properly insulate the wiring to prevent a shock hazard (so that accidental contact with exposed connections is not possible). You can get octal sockets from various radio supply places such as http://www.radiodaze.com or http://tubesandmore.com Radio Daze has back covers available for the octal sockets to help protect the connections from exposure and accidental contact. The following shows how to change the wiring to change the six pin 6E5 socket to the octal 1629 socket. Leave pins 1 and 6 of the octal socket unconnected. Alternately, you can connect pin six to the chassis (as long as it is not a "hot" chassis). 6E5 -- element -- 1629 ---------------------- 1 --- heater --- 2 2 --- plate ---- 3 3 --- grid ----- 5 4 --- target --- 4 5 --- cathode -- 8 6 --- heater --- 7 S/NC ---- 1 NC ---- 6 ===================================================================== Replacing the 6E5 eye tube with a 2E5 eye tube or a 2E5 tube with a 6E5 tube The 2E5 tube is the same as a 6E5 tube except for the heater. The heater on the 2E5 eye tube is 2.5volts at 800mA. The heater on the 6E5 eye tube is 6.3volts at 300mA. To swap these tubes, you simply have to provide the correct heater voltage, which can normally be done by installing a filament transformer some place handy. Except for the heater difference, the two tubes are pin for pin and electrically the same. Warning: Do not plug a 2E5 tube in a 6E5 circuit if you have not changed the heater voltage you will blow out the heater on the 2E5 thereby turning a valuable eye tube into antique junque glass. Pluging a 6E5 tube into a 2E5 circuit is not as likely to destroy it, but it won't work in the circuit unless the heater voltage has been changed. ===================================================================== Replacing the 6AB5/6N5 eye tube with the 6E5 or 1629 eye tube Replacing the 6AB5/6N5 eye tube is a bit harder as it's characteristics are different than the 1629 or 6E5 tube. The 6AB5/6N5 eye tube was designed for lower power use. The 6AB5/6N5 eye tube has a 6.3volt heater, but operates at 150mA while the 6E5 tube uses 300mA at 6.3volts. Depending on the circuit design, this may or may not be a problem. Of course if you will be using a 1629, the heater difference is no longer an issue since you will have to supply the 12.6 volts from another source. The other issue is that the 6AB5/6N5 has a slightly different bias voltage than the 1629 or 6E5 tube. This may require tweaking the bias voltages to fix. There is also an associated greater dynamic range of the grid voltage for the 6AB5/6N5 tube (10 volts compared to 8 volts). Again depending on the circuit this may or may not be a problem. One nice thing is that the 6E5 tube uses the same socket and connections as the 6AB5/6N5 tube. If you have a 6E5 tube, the easiest way to find out if it will work in the 6AB5/6N5 circuit is to plug it in the circuit and see if it works. The only aspect of concern is the heater current. If the heater is driven in series with other tubes, the 6E5 will not work as a replacement. If the 6.3 volts is directly provided from the power transformer, then it may work. This will cause the transformer to be driven harder because of the extra current, although the difference is relatively minor for most tube based circuits. It is not recommended going the other direction (putting a 6AB5/6N5 eye tube in a 6E5 circuit). The 6AB5/6N5 tube has a much lower maximum plate voltage rating and it is likely that the circuit will exceed that rating and potentially damage the 6E5 tube. To rewire the 6AB5/6N5 circuit for the 1629 tube, see the section "Using a 1629 Eye Tube in place of a 6E5 Eye Tube". You may want to consider using an adapter cable instead given the higher probability of the 1629 tube not working in the 6AB5/6N5 tube circuit. =========================================================================== Replacing the 6U5/6G5 tube with a 6E5 or 1629 eye tube The 6U5/6G5 tube has the same pinout and heater voltage as the 6E5, so in that aspect they are plug compatible. The big difference is that the 6U5/6G5 tube has a higher bias voltage than the 6E5 tube. Depending on the circuit, plugging 6E5 tube in a 6U5/6G5 circuit may work, but usually there will be problems because of the bias voltage difference and less gain. In some cases you may be able to get the 6E5 to work by changing the bias circuit to reduce the drive voltage from -22volts to -8volts. To use the 1629 tube, as well as dealing with the bias drive voltage difference, you will have the same issue as replacing the 6E5 eye tube with the 1629 eye tube. You will need to provide the 12.6volt heater voltage and replace the six pin socket with an octal socket. To rewire the 6U5/6G5 circuit for the 1629 tube, see the section "Using a 1629 Eye Tube in place of a 6E5 Eye Tube". You may want to consider using an adapter cable first given the higher probability of the 1629 tube not working in the 6U5/6G5 tube circuit. =========================================================================== Replacing the 6E5 or 6U5/6G5 eye tube with a 6AF6 eye tube The 6AF6 looks similar to the 6E5 and 6U5 eye tubes, but it is different electrically. The circuit will need to be redone if you want to put a 6AF6 tube in the circuit. Where the 6E5 and 6U5 tubes use a negative grid bias voltage, the 6AF6 tube uses a positive voltage. The 6AF6 tube target is usually connected to the plate output of the AVC circuit and the 6AF6 plate is connected to the B+ supply. Given the significant difference in the way this tube works, knowledge of tube circuits in general and the specific circuit it is being applied to is needed to make this substitution. It is not recommended if you don't know what you are doing. =========================================================================== Replacing the 6T5 or 6U5/6G5 eye tube The 6T5 tube is electrically similar to the 6U5 tube, but the target pattern of the 6T5 is circular while the target pattern of the 6U5 is the more common "cat's eye" pattern. Generally you can swap between the two tubes with no changes. =========================================================================== 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.