10 Unusual Vacuum Tubes

What’s your favorite vacuum tube? What’s your favorite thing about vacuum tubes?

Above is the Arcturus Type 101A radio detector tube. Arcturus was one of the earliest American manufacturers of vacuum tubes, and they favored that lovely blue glass.

They made such cool looking tubes, I decided to include them twice in this list. Here’s a PZ/47 type tube from Arcturus. It’s a power pentode and was used as a power/output tube. I wonder what it might sound like in a guitar amp….

By the way, Arcturus is the name of the brightest star in the Northen Hemisphere, in the constellation Bootes.

The award for sexiest tube has to go to KR Audio’s sleek, but BIG, 1610 power tubes. They put out 22-50 watts, and a matched pair will set you back about $3500. (Hand, not mine, included for scale).

The gold colored base is a nice touch, don’t you think?

Yes, that’s a large tube, son. But, the tubes used to power radio transmitters are even bigger. This might not be the biggest one, but it’s big enough to be impressive, and it’s water cooled!

Ladies and gentlemen, meet the RCA UC-862! (Dude, also not mine, included for scale).

Now, size isn’t everything, you know. Raytheon developed the extremely cool subminiature tube types for military applications. They had to be sturdy enough to withstand the rigors of being in a submarine, or in a field radio, or in a fighter plane, and they had to be small. Here’s an example of the type 6112 dual triode. It’s two tubes in one, and is remarkably similar to the ubiquitous 12AX7 found in the preamps of many tube guitar amplifiers. These types are often called “pencil tubes.” Pencil (also not mine) shown for scale.

Next up in our cavalcade of thermionic emission is the weirdest looking tube. It’s the GE KR-9 kenotron! “Kenotron” is a fancy name for a high-power rectifier tube, such as might be used to power an X-ray tube.

Speaking of X-rays, did you know that any tube operating at several thousand volts can produce X-rays? Be careful next time you operate at tube at SEVERAL THOUSAND VOLTS. You might want to wear protective clothing. Below, you’ll see a Coolidge tube, which is one of the main types or X-ray tubes operated at several thousand volts. ZAP!

With over 3000 different tube types out there, it’s no doubt that some of them are bound to be a little strange looking. First up is the type 57 tube, used in radio and television signal detection. The little metal cap on top is how the anode, or plate, is provided with a separate voltage connection. These types of tubes are called “top cap tubes.” In some tubes, the top cap is used to provide grid, or bias, voltage.

Joining the type 57 in the freak show is the 316A triode, used as a transmitter/oscillator, but can also be used as a power amplifier. This versatile little guy is of a style known as a doorknob tube.

And finally, here’s a vacuum tube that I bet almost every reader has in their house right now.

It’s a cavity magnetron from a microwave oven! Yes, there is vacuum tube in there. They use magnetrons in radar systems as well. Some of us old-timers might remember when a microwave oven was called a “radar range.” Through an amazing bit of science involving resonance cavities, a wave guide, and curving electron paths by using a very strong magnetic field, microwave radiation is emitted to excite the water molecules in that bag of yummy popcorn!

I hope you enjoyed this little diversion. I had fun doing the “research,” and I learned a thing or two, as well. Tubes are cool.

4 thoughts on “10 Unusual Vacuum Tubes

  1. I have a genalex kt 88 that is a directly heated tetrode having filament on pin 7 & 8,no cathode pin.it is the same looks as any gec kt88,and it has 4.0 volt heater.


    1. Karl Wohlwend



  2. Donald

    Hi. There’s an error on the type 57 tube. The top cap is grid not plate. Some old drawings show the top cap as “pin 7) which doesn’t exist on a UX6 based tube. Other drawings clearly show a grid connection. The 57 (2.5 volt, UX6 base) became the 6C6 (6.3volt, UX6 base) then the 6J7, 6J7GT, 6J7G (6.3 volt, Octal base). All these tubes are electronically equivalent in circuit and all have the top cap as grid.


    1. Karl Wohlwend

      Thanks for the correction, Donald!


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