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Home , Control grid, Hot cathode, Magic eye tube, Photomultiplier tube, Space charge, Tetrode

Unusual Tubes

Tom Duncan, KG4CUY March 8, 2019 Tubes On Hand

GAS-FILLED HIGH-

• (NE-51) • Photomultiplier • Cold- Voltage (931A) Regulator (0B2) • Magic Eye (1629) • Hot-cathode • Low-voltage (12DY8) (884) • (12K5)

2 Timeline of Related Events

1876, 1902 William Crookes Cathode Rays, Glow Discharge 1887 [1921] Hertz, Einstein 1897 [1906] J. J. Thomson identified 1920 Daniel Moore (GE) 1923 Joseph Slepian (Westinghouse) 1928 Albert Hull, Irving Thyratron Langmuir (GE) [1928] Owen Richardson 1936 Vladimir Zworykin Photomultiplier (RCA) 1937 Allen DuMont Magic Eye 3 Neon Bulbs

• Based on glow-discharge (coronal discharge) effect noticed by William Crookes around 1902. • Exhibit a negative incremental resistance over part of the operating range. • -sensitive: photo- causes the ionization voltage to decrease with illumination (not generally a desirable characteristic). • Used as indicators , voltage regulators, relaxation oscillators , and the larger ones for illumination .

4 Neon Lamp/VR Tube Curves

80 Normal Abnormal Glow Glow 70 60 Townsend Discharge 50 40 Region 30

Volts across Device across 20

10 Conduction Destroys Lamp Destroys Arc Conduction Arc Chart details (coronal) Glow depend on -5 element 10 -20 10 -15 10 -10 10 1 geometry and Current through Device (A) gas mixture. 5 Cold-Cathode Voltage Regulator Tubes • Very similar to neon bulbs: attention paid to increasing current-carrying capability and ensuring a constant forward voltage. • Gas sometimes includes radio-isotopes to reduce sensitivity to photo-ionization. • Developed at Research Labs by Daniel Moore around 1920. • Used as a voltage regulator with functionality similar to the zener .

6 Voltage Regulator Tube Internals

7 • Heated filament releases into a rarified noble gas mixture (mostly neon). • The , when sufficiently positive, ionizes enough gas to cause the tube to “avalanche” into glow-discharge conduction, at which point the grid no longer exerts any control. • The grid can draw considerable current once the tube conducts.

8 Thyratrons (2) • Conduction stops when the plate voltage is sufficiently reduced, and after sufficient time has elapsed for gas molecules to de-ionize. This limits the switching speed to 10s of kHz. • Invented at GE Research Labs by Albert Hull in 1928. • Functionality similar to the unijunction and controlled • Used as relaxation oscillators , , and triggers for high-power switching tubes like . 9 Thyratron Control Characteristics

300 As for neon lamps and voltage 250 regulator tubes, chart details 200 depend on Conducting Region element geometry 150 and gas mixture. Non-Conducting 100 Region

50

0 -30 -20 -15 -10 -5 0 Grid Volts 10 Thyratron Internals

11 Photomultiplier

• Based on the photoelectric effect described by Einstein (1921 Nobel Physics prize) • …aided by secondary emission of electrons from having gain, since more than one secondary electron is emitted for each incident primary electron, as described by Slepian. • The current/photo current multiplication factor is about 10 6 (120 dB) for the 931A tube, as compared with a

vacuum . 12 Photomultiplier (2) • The tube was a by-product of efforts by Vladimir Zworykin of RCA to develop a TV camera tube (1936), and developed into the section of the orthicon . • Despite its high impedance, the has good immunity to thermal , so it is still used where high sensitivity (exceeding that of ) is required. • Formerly used as “electric eyes”, now mostly for precision analog illumination measurements .

13 Photomultiplier Tube Internals

Typically voltage between anode and cathode is distributed evenly across the dynodes, 75~100V between adjacent dynodes. So a 9- tube has ~1000V across it.

14 Magic Eye Tubes • William Crookes noticed that a suitably- coated surface could be made to fluoresce when bombarded by an electron beam. The funnel-shaped target anode of the is such a surface. • A control near the funnel, if at a lower voltage than the funnel, shields the funnel from the electron beam, so a part of the target doesn’t fluoresce.

15 Magic Eye Tube Internals

16 Magic Eye Tubes (2)

• Because a large voltage change on the control electrode is required to change the eye size, some tubes like the 1629 have a built-in stage. • Allen DuMont (DuMont labs, DuMont TV network) invented the eye tube as a radio tuning indicator (an alternative to more expensive meter movements) and eventually sold it to RCA, around 1937.

17 “Usual” Tubes at Low Voltages

• The synchronous rectifier “vibrator” power supply was the bane of car radios for years: there was a real need to operate with 12V on the plates. • Many receiving tubes that usually operate with 100 to 250 volts on the plate will function at 12V, but it is difficult to deliver significant power at these voltages. This is because in order to draw enough plate current to deliver power, the control grid must be biased positive, and the input impedance then becomes uselessly low. 18 A Conventional Car Radio Tube

19 Tubes at Low Voltages (2)

• How can we ever drive a speaker with 10 mW tubes? With a hybrid design using tubes in the RF/IF/detector/audio preamp stages, and for the power amp. • But even then, in the age of not-so-reliable germanium transistors, several stages would be required to boost 10 mW to a couple of watts necessary to make the radio audible over the road noise, with windows down in the days of no air conditioning. 20 Space Charge Tubes

• What if we bias the first grid positive to increase plate current, but applied the signal to the second grid? This is the notion behind space charge tubes . The second grid is spaced much closer to the first grid than in the usual or . The voltage gain isn’t much, but there is much greater power gain than with conventional tubes, and the input impedance is still acceptable.

21 A Space Charge Car Radio Tube

22 Conclusion

• Many other unusual tubes have come and gone – shown on the next slide. • A few remain – traveling-wave tubes, magnetrons, photomultipliers . • My hat is off to the imagination and inventiveness of those who brought us these hollow-state devices. • Thanks for coming!

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