Unusual Tubes Tom Duncan, KG4CUY March 8, 2019
Tubes On Hand GAS-FILLED HIGH-VACUUM Neon Lamp (NE-51) Cold-cathode Voltage Regulator (0B2) Hot-cathode Thyratron (884) Photomultiplier (931A) Magic Eye (1629) Low-voltage (12DY8) Space Charge (12K5) 2
Timeline of Related Events 1876, 1902 William Crookes Cathode Rays, Glow Discharge 1887 [1921] Hertz, Einstein Photoelectric Effect 1897 [1906] J. J. Thomson Electron identified 1920 Daniel Moore (GE) Voltage Regulator 1923 Joseph Slepian (Westinghouse) 1928 Albert Hull, Irving Langmuir (GE) Secondary Emission Thyratron [1928] Owen Richardson Thermionic Emission 1936 Vladimir Zworykin (RCA) Photomultiplier 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. Light-sensitive: photo-ionization 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 Volts across Device Chart details depend on element geometry and gas mixture. 80 70 60 50 40 30 20 10 Townsend Discharge Normal Glow Negative Resistance Region Abnormal Glow Glow (coronal) Conduction 10-20 10-15 10-10 10-5 1 Current through Device (A) Arc Conduction Destroys Lamp 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 General Electric Research Labs by Daniel Moore around 1920. Used as a voltage regulator with functionality similar to the zener diode. 6
Voltage Regulator Tube Internals 7
Thyratrons Heated filament releases electrons into a rarified noble gas mixture (mostly neon). The control grid, 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 transistor and silicon controlled rectifier Used as relaxation oscillators, switches, and triggers for high-power switching tubes like ignitrons. 9
Thyratron Control Characteristics As for neon lamps and voltage regulator tubes, chart details depend on element geometry and gas mixture. Non-Conducting Region Conducting Region 300 250 200 150 100 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 dynodes having gain, since more than one secondary electron is emitted for each incident primary electron, as described by Slepian. The anode current/photo current multiplication factor is about 10 6 (120 db) for the 931A tube, as compared with a vacuum photodiode. 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 electron multiplier section of the orthicon. Despite its high impedance, the photomultiplier tube has good immunity to thermal noise, so it is still used where high sensitivity (exceeding that of semiconductor photodiodes) 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-dynode tube has ~1000V across it. 14
Magic Eye Tubes William Crookes noticed that a suitablycoated surface could be made to fluoresce when bombarded by an electron beam. The funnel-shaped target anode of the magic eye tube is such a surface. A control electrode 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 triode amplifier 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 transistors 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 tetrode or pentode. 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! 23
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