What is an octal socket?

The octal socket is a standard connector for plug-in devices such as vacuum tubes and relays. It is an old design with roots back to the second generation of vacuum tubes circa 1935. The octal (8-pin) socket provides a robust mechanical connection. The socket fostered a technological revolution where the greater number of pins allowed increased vacuum tube diversity. The direct outcome was tubes such as the 12SN7 pictured in Figure 1 along with classic audio and guitar output tubes such as the 6V6, 6L6, EL34, KT-66, and 6550. The nearly 100-year old socket remains popular in industrial control systems today with products such as the featured Finder and matching 90.02 socket.

Figure 1: A modern Finder brand octal relay and socket along with a pair of vintage Sylvania 12SN7 dual triodes. The vacuum tubes and relay share the same octal socket.

Features of the octal socket

Figure 2 presents a close-up image of the vacuum tube and relay connection. The eight pins with their symmetrical placement are clearly visible. In the center of the socket, the round plastic keying post is visible. Close inspection shows that the key pin located at the 12 o’clock position for each socket.

This keying post serves no electrical function. However, it is an elegant solution for quickly installing the device into the socket. To use this feature:

  1. place the device’s keying post into the socket hole

  2. spin the device until the locator key drops into the socket

  3. push down to fully seat the device

This “spin indexing” is a highly desirable way to install a device. This is especially true when the cumbersome alternative is to first observe the pin orientation and then carefully locate the corresponding socket position. Experienced guitar players appreciate this fact as a 6L6 octal base vacuum tube is much easier to install than a 12AX7 with its compact 9 pin noval base (root from the Latin word novem for nine). The 6L6 can be replaced in the dark, while a steady hand and a flashlight are required for the 12AX7.

Figure 2: Close up image of the tube and relay pins. The keying post is located in the center of the pins. This allows spin-indexing for easy and fast replacement of the device.

Tech Tip: The device may still be used if the keying post is damaged. However, great care must be taken to prevent improper installation. Observe that the socket is symmetrical in all directions. Don’t guess, as the odds are not in your favor. There is one valid position and seven incorrect.

Pin numbering for the octal socket

Pin numbering is a challenge for all electrical devices. Like many devices, the pins of the octal socket are numbered consecutively. Pin #1 is immediately to the right of the index key as shown in Figure 3. The pin numbers increases as we travel in a clockwise direction. This assignment is simplified as the pin assignments can be read directly from Figure 3.

The logic of this numbering scheme depends on which side of the device we are on. If we are working in an old radio chassis (underneath the socket) this makes sense as the locator pin is easy to find, and we move in a clockwise direction. However, when viewed from above, the perceived situation is reversed. The pin assignments seem counterintuitive as we travel in a counterclockwise direction.

This numbering scheme is shared by many devices included integrated circuits such as the 741 op amp and the 555 timer. When viewed from above, the numbers increase in a counterclockwise direction. When viewed from the solder side, the pin assignments increase in a clockwise direction.

Figure 3: When viewed from below, the pins are consecutively numbered in a clockwise direction starting with pin 1 immediately to the right of the central locator pin.

Derivatives of the octal socket for relays

The pin count of the octal socket is limited. A typical relay such as the Finder provides Double Pole Double Through (DPDT) operation. This includes 2 pins for the coil and 3 pins for each of the two switch sections.
The 11-pin socketable relay is common variant. Relays such as the Finder are available. The corresponding sockets as shown in Figure 4 include the 90.02 (8-pin) and 90.03 (11-pin).

Figure 4: Datasheet description of the Finder 90.02 (8-pin) and 90.03 (11-pin) sockets. The 8-pin socket supports a DPDT relay while the 11-pin socket supports a 3PDT relay. The pin assignments are viewed from the top of the socket.

Tech Tip: Relays such as the ones featured in this article are designed as a family with interchangeable components. This keeps the cost low as the same socket may be used for relays with different coil voltages. One important family option is the snubber to control the inductive kick when the relays coil is turned off. A diode such as the Finder is used for 24 VDC systems while a 120 VAC system would use a varistor module such as the Finder

Derivatives of the octal socket for vacuum tubes

Vacuum tubes were considered the weakest link in older electronics. Consequently, nearly all were socketable. We could fill a small book describing the history of tube development with section for receiving, transmitting, and cathode ray tubes (picture tube). Some of this variety is displayed in Figure 5. This represents a small sample of the total variety developed over the last century.

Figure 5: A small collection of vacuum tubes displaying a variety of plug-in connections.

Tech Tip: Have you ever encountered an electronics device with the warning, “CAUTION: No user serviceable parts”? Back in the 1960s all the way to the 1990s you would see this warning. It served as a reminder that the new solid-state electronics could not be repaired by the consumer. This is very different from the older vacuum tube equipment where the consumer was expected to test and replace the tubes. In fact, if your radio wasn’t working, you could remove all the tubes and then bring them to the drug store to perform the test.

Parting thoughts

The octal socket has been with us for nearly a century. It is commonly used for both industrial relays and for vacuum tubes. The form factor provides a robust connection in a convenient package. Personally, I find the spin indexing to be the most useful feature.

The real question is how long do you think it will last? If I had to guess, the 6L6 tube will be with us for another century. It was designed in the 1930s and is still going strong with multiple factories across the world. As long as Jazz, Rock, Blues, Metal and Country music exist, we can expect guitar players will cherish their vacuum tube amplifiers.

As for industrial controls, the equipment has a lifetime measured in decades. If we were to ban all new implementation of the octal socket, I suspect you could still purchase replacements a hundred year from now.

What are your predictions?

Best wishes,


About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

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