The evolution of the 6SN7 tube - 28 Dec 2011 (revised 1 Aug 2018) Mike@MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! The elements of the 6SN7 made their first appearence in the 6F8. The 6F8 like the 6SN7 uses an octal base, but the grid of the first triode section is brought out to a cap on top of the tube. The heater of the 6F8 uses the original 2/7 pin wiring for the octal base. While the 6F8 was useful for radio work where high gain/high frequency circuits benifited from having the grid cap, there was a call for a less cumbersome tube to be available. Thus came the 6SN7. The 6SN7 took the 6F8 elements and crammed them into a smaller bottle and got rid of the cap. Bringing the grid out a pin on the bottom of the tube. To minimize problems with cross talk between the two sections and the heater, the pin-out was also changed. A 12 volt version of the 6SN7 was also created for use in systems that used 12 volt supplies for the heaters. Both the 6SN7 and 12SN7 became very popular tubes in many different systems, including radios, TVs and amplifiers. Much later, the 5692 tube was designed for the military which had a more rugged design, longer life, and better overall construction. Some of the best 5692 tubes used a red colored material for the tube base and became known as "red base 5692" tubes. They were popular in high-end designs. However, the high price limited the quantities that were made. Because of this, the 5692 has become very rare and commands very high prices when they are offered for sale. The 12AU7 was originally created as a 9 pin miniature tube for use in circuits where the 6SN7 tube was used. However, the 12AU7 didn't have the exact same characteristics, so it required different circuit designs to be used. To address this problem, the 6CG7 was created which simply put the 6SN7 elements into a 9 pin minature tube. The main difference is that the plates of the two sections are slightly closer together because of the narrow confines of miniature tube. They did add a twist though, since there were 9 pins available, they decided to add a shield between the two triode sections and bring it out on the extra pin. This was used to keep the two sections isolated, which can be especially useful for high frequency use. This made the tube slightly more expensive to make, so later the 6FQ7 was created which eliminated the internal shield. Soon after the 6FQ7 appeared, many tube manufacturers started calling the 6FQ7 by the combined name of 6FQ7/6CG7. Technically this is not a valid designation since the combined designated tube lacks the shield that a true 6CG7 has. When the loctal tubes came out, the first designs included the 6J5 and 6SN7 versions. These were called 7A4 and 7N7 respectively. The 14A4 and 14N7 tubes were the 12V heater versions which were targeted for cars with 12V batteries. There was never a Compactron version of the 6SN7 made. Presumably the smaller 6FQ7 version of the tube satisfied the demands where it was needed. Since there are times when the circuit design does not need a tube with two triodes, there were single triode versions of the 6SN7 tube made as well. The 6J5 being the first one made. As the loctal and miniature tubes evolved, the single triode version was migrated to the new tube envelopes along with the dual triode versions of the 6SN7. While the 6J5 tube was popular for the octal based tubes, the 12G4 and 12H4 miniature versions never really caught on. The 6C4 was commonly used for single triode applications where the 6J5 had been used previously. The 6C4 has slightly different characteristics and lower power handling than the 6J5, but it is in a small 7 pin minature envelope and used half as much power to operate. As time went by, the demand for lower cost tubes resulted in methods to reduce the cost of the tubes. Particularly for TV and radio tubes where cost was a critical aspect of the designs. Lower cost tubes were designed using more automation and less expensive components. For many applications, the reduced cost tubes were perfectly adequate. 6BC8 (35x), 6BK7 (40x), 6BQ7 (35x), 6BS8 (36x), 6BZ7 (36x), 6BZ8 (36x) and 6EU7 (100x). In other cases, high quality tubes were demanded, such as the 6DJ8, 12DJ8, 12DT7, 12DT8 and a multitude of industrial and military tubes such as the 5814 and 6201. Sometimes the medium gain (20x) of the 6SN7 is not enough for the needs of the circuit. The basic 6SN7 triode has been adapted over the years with a multitude of other gains. Sometimes using smaller elements such as the popular 12AU7 (20x), 12AV7 (40x), 12AT7 (60) and 12AX7 (100) tubes with the associated reduced current handling. For those applications that still need the higher current handling of the 6SN7 style elements but with different characteristics or heater voltage, other similar tubes were designed. The octal version 6BL7 and 6BX7 tubes were popular in early designs. The smaller miniature tubes were popular in later designs such as 6GU7 (17x), 12BH7 (17x), 6EV7 (60x), 12BZ7 (100x) and 6414 (42x). Note: 12BZ7 and 6BZ7 are completely different tubes. This is one of the few cases where more than just the heater is different between the two tube numbers. Over the years, the internal construction changed as well as the tube envelope itself. Even within a particular tube designation the designs changed (although the electrical characteristics remained largely the same). One area of change was the plate design, which is the subject of many heated debates on which are the best tubes. The original 6F8 tubes used a round plate construction. A few of the first 6SN7 tubes used these same round plates from the 6F8. These tubes can command very high prices as they are very rare. The first redesign of the 6SN7 plates changed them from round to rectangular with some ribbing to stiffen the plate and create a little more area to disipate heat. Another design used square plates. These were used as it helped to reduce the cost of making the tube by making it easier to build more consistently (few production failures). Another debate about plates is the plate material color. The material color does not have any real impact on the performance of the tube. While some argue that the black plates are better because they can radiate heat more effectively, the reality is that the tubes are not operated at a level where this would have any impact on the performance of the tube. A better reason for paying attention to the plate material used is to get an idea of who made the tube. Black rectangular ribbed plates are most commonly found on RCA tubes (although they produced rectangular gray and square gray plates as well). Square plates are found in later (cheaper) constructed tubes, which is why they usually don't command as high a price as the rectangular or round plates. The round plates don't have as much maximum power handling as the rectangular or square plates, but the round plates have a more even distribution between the cathode, grid and plate which improves the amplification quality of the tube. The tube supports were another area that changed a lot. The most common supports were simply a mica washer at the top and bottom of the plates to hold every thing together and to keep the tube elements centered inside the tube envelope. There were other design attempts to try to reduce the microphonics in the tube. One of the more interesting visually are the mouse ear supports. These were two mica washers mounted on either side of the plates at the top of the tube to keep the tube elements stable and centered while minimizing the contact area with the tube glass. It didn't really help, but it does look interesting. Double and tripple stuff mount washers at the top and bottom of the tube were the most common method of trying to reduce microphonics. The way to reduce microphonics is to keep the tube elements from moving around. The more washers that are installed, the less likely the chance that the tube elements will have room to move around. One of the things that was discovered to be most important was to stabilize the grid. The grid has the greatest contribution to microphonics when it moves. A number of methods were used to try to stabilize the grids. One of the best ones devised was to use little "C" clips at the top of the tube welded to the grid support posts. These apply presure to the grid posts to keep them pushed against the mica washer holes thus minimizing any movement. It is a simple low cost solution that works well. It does however add a bit to the cost of the tube since the clips have to be welded to the posts as one of the last assembly steps. As a result, many tube manufacturers stopped using them to save on construction costs (especially for tubes used in TVs and low cost radios where saving pennies can make a big difference to the profits). So you will see the grid clips in some tubes and not in others even though the tube numbers are the same. Another area of tube construction that gets discussed and argued over a lot is about the type of getter and its placement. The getter is the device that produces the silvery patch inside the tube. The purpose of the getter is to remove the last remaining traces of air contaminants after the tube has had the air evacuated and has been sealed off. The getter is fired (usually with heat) which causes it to splatter the getter matterial on the tube wall where the getter is located. The material is highly reactive to the air contaminants (primarily oxygen) and quickly locks up the remaining molecules of air contaminants. The getter material remains active for the life of the tube and will pull in any stray air contaminants that might leak into the tube or be released from the cathode material or other elements in the tube. The getter is required because any stray air contaminants inside the tube will seriously degrade the performance of the tube, or prevent it from working at all by interferring with the flow of electrons between the cathode and plate (anode). The getter can be made from a number of different materials. A number of different elements have been used over the years for the getter material, including; Ba, Bi, Ge, Mg, Pb, Sb, Sn, Zn, Ta, Cb, Zr, Tb, Ti, Al and P. Sometimes the chemicals are used individually, but more often they are used in various mixes. Barium based getter material is the most common used for small tubes and results in the silvery mirror of the getter flashing. The resultant coloring of the flash material on the tube depends on the materials used in the getter and how it was fired once the tube was sealed up. The color can be gold, silver, gray or black. The shape and location of the getter has no effect on the operation of the tube. The getter has no direct contribution to the operation of the tube. It's only purpose is to trap contaminants. The location and shape is normally selected to be easy to manufacture. The location of the getter can be pretty much anywhere inside the tube (top bottom or side), although it does need to be visually accessable for the flash to be triggered. The shape of the getter holder is also unimportant. It can be a circle (halo), rectangular ("D"), a flat plate (normally with an indentation to hold the getter material), or a bar. The main value of the getter shape and location is as a clue of who made the tube and when it was made. It should be noted that the gettering process of a tube is not limited to just the getter flash on the side of the tube, all of the tube elements take a part in the process. Treatment of the all tube elements is an important aspect of the manufacture of tubes. As an example, the rough black appearence of the plates on tubes made by RCA is the result of iron being used in the coating material on the plate. As well as adding to the gettering process, the black coloration improves the ability of the plate to radiate the heat generated by the tube while in operation. --- Here is a quick summary of the various flavors of the 6SN7 tube This list includes the single triode and dual triode tube versions. 6CG7 - dual triode 9 pin mini 6V heater internal shield 6FQ7 - dual triode 9 pin mini 6V heater no shield 6F8 - dual triode octal with cap ST 6V heater 6J5 - single triode octal metal and GT 6V heater 6SN7 - dual triode octal GT 6V heater 7A4 - single triode loctal 6V heater 7N7 - dual triode loctal 6V heater 8CG7 - dual triode 9 pin mini 8V heater internal shield 8FQ7 - dual triode 9 pin mini 8V heater no shield 8SN7 - dual triode octal GT 8V heater 12FQ7 - dual triode 9 pin mini 12V heater no shield 12G4 - single triode 7 pin mini 12V heater 12H4 - single triode 7 pin mini 12V heater with center tap for 6V operation 12J5 - single triode octal metal and GT 12V heater 12SN7 - dual triode octal GT 12V heater 12SX7 - dual triode octal GT 12V heater selected for lower plate voltage designs 14A4 - single triode loctal 12V heater 14N7 - dual triode loctal 12V heater 5692 - dual triode octal GT 6V heater (special 6SN7) ------------------------------------------------- While the list is extensive of tubes that have similar characteristics to the 6SN7 tube, here is a short list of some of the more popular similar tubes. 6C4 - single triode 7 pin mini similar to 6J5 6C5 - single triode octal similar to 6J5 12U7 - dual triode 9 pin mini - very low power space charge tube for 12V only supplies 12AU7 - dual triode 9 pin mini - slightly different characteristics