Spark Gap Tubes, Trigger Tubes & Passives

This section is a catch-all for simple vacuum and gas-filled devices used in relay and passive applications.


Bomac JAN-CBNQ-724B

The 724B is a TR cell, a device used to protect the receiver portion of a microwave transceiver which shares the same antenna for both transmit and receive functions. A TR cell functions like a triggered gap tube, and blocks the receiver hardware by shorting the receiver input during transmitter operation. Transmitter pulses cause the spark gap to break down, shorting the receiver input. The 724B is filled with water vapor, which produces a blue glow when ionized by a high voltage field.

The 724B was used in several WWII-era 10GHz mobile radar systems, including the AN/APS-1, ASD (AN/APS-3) ASH (AN/APS-4) and AN/MPG-1.


Bomac JAN-CBNQ-724B TR Cell Tube
Electronic Products 7616 / EP72M

The EP-72M/7616 is a small cylindrical Geiger tube capable of detecting alpha, beta, and gamma radiation. The front of this tube is sealed with a thin mica window, which allows weak alpha radiation to penetrate the tube and ionize the neon and halogen gas within. LND still manufactures a version of this tube, the LND-7616, which is a popular choice for electronic enthusiasts looking to upgrade the commonly available CDV-700 Geiger counter for increased sensitivity. The 7676 is ideal for this task, as it is small enough to fit inside the original probe that this meter was equipped with, though the builder would be required to cut a hole in the end of the probe if they intend to use the alpha particle detection feature of this tube.

LND-7616 Datasheet (PDF, 144kb)

Electronic Products 7616 / EP72M Geiger Tube
Western Electric 395A

The Western Electric 395A is a small tubular trigger tube designed for use in extreme environments. When properly mounted, this tube can withstand forces in excess of 1000Gs. The tube's three flying leads are intended to be soldered permanently into it's destination circuit. The conical structure at the top of the tube is the cathode; a mica washer keeps the cathode fixed in the center of the tube. The 395A is filled with neon, which lights brightly when the tube is in use. A small amount of radioactive krypton-85 has been added to the neon mix to allow the tube to trigger correctly in darkened enclosures.


Western Electric 395A Trigger Tube
Central Electric CVS-1

The Central Electric CVS-1 is a high voltage vacuum switch constructed into a large T-shaped envelope. This tube has three contacts; a fixed contact imbedded in glass, and two movable contacts that pass through sheet metal plates. The plates are flexible, allowing the movable contacts to opened and closed by an external force without breaking the vacuum. The tube's getter is located in one leg of the tube, attached to the base of the fixed contact.


Central Electric CVS-1 Vacuum Switch Tube
Sylvania 0A5

The 0A5 is a gas filled trigger tube designed as a high speed replacement for clunky mechanical relays. The 0A5 is a descendant of "Strobotron" neon filled flash tubes like the 1D21, but has an anode structure specifically designed for fast switching. In fact, one of the recommended uses of the 0A5 is as a trigger for gas filled flash tubes like the Strobotron. Sylvania departed from their normal print manufacturing techniques for this tube and put a lacquer coating over the logo and part number, similar to the coating used on some Russian tubes, to protect it from damage.


Sylvania 0A5 Trigger Tube
GA4 (Unknown Mfr.)

The GA4 is a large spark gap T/R cell tube of unknown specification, which was used in the SCR-270 'Pearl Harbor' radar system. The two electrode contacts on either side of the envelope are designed to be removed, and are held in place by a pair of Allen screws. The inner electrodes are made of tungsten, and had a tendency to become fouled with sputtered material after prolonged use. For this reason the GA-4 contains a small amount of powdered tungsten, and it was recommended that the tube be periodically shaken vigorously to scrub the inside free of fouling. This tube illuminates purple when subjected to an electrostatic field, which suggests it is filled with either hydrogen or water vapor.

Later versions of the SCR-270 radar system phased out the use of GA-4 TR cells in favor of the Westinghouse 1B32 spark gap tube.


GA4 (Unknown Mfr.) Spark Gap Tube (GA-4)
Unidentifed TR Cell

We do not know much about this mysterious device or it's intended purpose, other than that it appears to be a TR cell or possibly a Geiger tube based on it's outward appearance. The tube is divided into two chambers and has only two electrodes, a pin electrode in the smaller chamber and a tubular one that sits to one side. The longer chamber is capped with an evacuation nipple and contains no other structures. The tube appears to be filled with neon and other unidentified gasses, and lights brightly under the influence of an ion source.

If you can identify this device, please contact us.

Unidentifed TR Cell
Westinghouse 1B32

The 1B32 is a medium-sized spark gap T/R cell tube produced by Westinghouse for use in the AN/TPS-2 radar system as well as later SCR-270 radar installations. The 1B32 has a breakdown voltage of 1000 volts, and is filled with an unidentified low pressure gas. The heatsink is likely an aftermarket addition, and is attached to a threaded stud on one of the tube's electrodes. The other contact contains the tube's evacuation nipple. This tube was produced in versions with both black and red print, the black print variant is shown here.


Westinghouse 1B32 Spark Gap Tube (532A)
Sylvania 1B40

The Sylvania 1B40 is a small cylindrical TR cell which was likely released sometime during the 1940s. The 1B40 is filled with argon, which produces a purple discharge when the cylindrical keep-alive electrode is energized. This tube is designed to be mounted in an external cavity, which would mate with the grounding ring on the bottom edge of the tube. We do not know the release date of the 1B40, but by 1947 the tube was out of production and no longer readily available.


Sylvania 1B40 TR Switch Tube
Sperti 473384-2

This device, which functions as a vacuum relay, consists of a spark gap bridged by a pivoting metal contact mounted on a glass bead. The entire mechanism is sealed in a complex T-shaped vacuum envelope. The underside of the tube is sealed with a flexible metal plate, allowing the contacts to be mechanically opened and closed by pressing the terminal exiting the bottom of the tube.


Sperti 473384-2 Vacuum Relay Tube
Sylvania 6068A

The 6068A is a relatively undocumented TR cell with a complex internal construction. The cavity of this tube appears to be wholly separate from the actual waveguide interface, which is the large copper block in the center of the tube. The center of the copper block contains a tunable spark gap, which can be adjusted by turning a screw in the large steel cylinder at the base of the tube. Note that this tube has two separate sealing pips, and possibly contains separate cavities.

In outward physical appearance, this tube appears almost identical to the 1B24, a tunable TR switch tube with a rating of 8.5 to 9.6GHz. The 1B24 was developed in 1943 by Westinghouse


Sylvania 6068A TR Cell Tube  (1B24)
RCA 991

The RCA 991 is primarily used as a voltage regulator, but has also been known to be used as a indicator in some situations. Packaged in the same envelope as many T-4-1/2 bulbs, the 991 has a three connector bayonet base and is filled with neon gas. In operation, the bulb's two electrodes will both illuminate simultaneously when used with an AC power source.


RCA 991 Neon Lamp (Glow Regulator Tube)
Sylvania 5823

The 5823 is an argon-filled trigger tube in a small 7 pin envelope. This tube is triggered by a 70 volt signal, and can switch a load of up to 200 volts. The 5823 is a cold cathode device; unlike some gas filled switching tubes, no filament voltage is required. This tube produces a purple discharge when subjected to a high voltage field.


Sylvania 5823 Trigger Tube
Sylvania BW382

We don't know much about this TR cell, but it is almost surely one of the smallest of such devices ever put into production. Little more than a glass window, the BW382 is quite small; dimensions are 24mm by 12mm and the device is a mere 1mm thick. The BW382 does not have any mounting holes, and is likely designed to be held in place by friction.


Sylvania BW382 TR Cell Tube
G.L.A. 20750

The 20750 is a small spark gap tube in a cylindrical glass envelope. We can find no information on this device, but we suspect it was most likely used as an overvoltage protection system in some high power transmitter. The tube does not appear to be gas filled, and has no discharge when subjected to an RF field.


G.L.A. 20750 Spark Gap Tube

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