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Vacuum Tubes

2017-08-20 19:06  
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The voltage regulator requires a certain minimum current (about 5 mA) to function properly. If you are only drawing a few milliamperes from the supply, connect a 12k bleeder resistor across the output. Otherwise, the regulator will not adjust down to the lower voltages. Or, 220Ω and 10k fixed resistors, and a 15k pot, could be used at the voltage regulator, which would draw the necessary minimum current. The VR tube can be replaced by a high-voltage Zener diode.

 

 

A 25W isolation transformer is available at the date of writing from All Electronics (See theYour Laboratory page for a link) for $4.50. This transformer is surplus from the Power One firm, and is an excellent value. Solder a jumper between tabs 1 and 3, and another between tabs 2 and 4. The 120V input is connected between 1-3 and 2-4. The output tabs are marked B. This transformer would work well in the circuit above, or it could be put in a box and wired with line cord and output receptacle as a general isolation transformer. It should supply 200 mA without trouble, ample for our purposes.

An idea for an inexpensive B supply is shown at the right. The greatest expense is for the capacitors, which will cost about $15. It is based on a half-wave voltage doubler, and gives 300V for a 115V rms input. It cannot supply large currents, but is perfectly satisfactory for anything but power amplifiers. If supplied from a variable transformer, it becomes a variable supply for all voltages from 0 to 300. Note very carefully that one side of the supply is connected to the AC line, and this must be the grounded side, for your safety, and to avoid ground loops. You cannot ground the positive terminal of this supply to get a negative voltage supply (for use as a C supply, for instance). An isolation transformer, if you have one, would eliminate this hazard. If you don't have an isolation transformer, use a polarized plug to guarantee that the white wire is connected to the circuit ground. If you have a good ground, consider the old trick of connecting only one wire in the power cord, and using the ground to complete the circuit. It is best to observe the power ratings of the resistors and the voltage ratings of the capacitors. This circuit has been tested, except for the fuse. If the 0.5A slow-blow fuse fails, try a 1.0A. This fuse is to turn things off if a capacitor fails; nothing valuable is protected here, but it saves mess.

The RC ripple filter is worth the expense. Waveforms are shown at the left. The waveform at node "a" is the familiar one for a "tank" capacitor, and the ripple is fairly large. Since the impedance of a 100 μF capacitor is only 26Ω at 60 Hz, the ripple is reduced by a factor of almost 25. At 300V output and a load of 12 mA, the ripple is less than 0.1V, a very satisfactory result. Note that all that is left in the ripple is the 60 Hz component. The filter would work even better on a full-wave rectifier, but here it is very satisfactory, better and more economical than larger capacitors. Of course, a filter choke could be used for an even better result and less voltage drop, but this would double the cost of the supply.

You will also need a heater transformer, which can be quite small if supplying only one tube that requires 0.3A. The transformer can be put in a box with an on-off switch and convenient terminals. Ground the heater supply (at a center tap if one is provided) to the B ground, to avoid excess voltages between heater and cathode. If you have a 12.6V CT (center-tapped) secondary, you can supply both 6.3 and 12.6 V heaters. Many 12.6V miniature tubes can also be connected for 6.3V. Tubes whose designations begin with "1" have filaments that can be supplied from a single D cell. Obtain a holder for the cell so connections are easy. 6.3 V was chosen to be compatible with 6 V car batteries, but the supply is usually AC. Many rectifier diodes use a 5 V filament or heater supply, apparently for historical reasons.

A "C" supply, for the grid bias in measuring characteristics, can be any isolated low-voltage supply of say, 15V, and a potentiometer can be used to pick off a variable voltage, since little current is involved. A separate high-voltage supply for screen grids may also be convenient, though it is easy to pick off the necessary voltages with a Zener or a VR tube from the main B supply. This cannot be done, of course, if the B voltage is adjusted using a variable transformer.

An all-in-one economical supply for vacuum-tube measurements is shown below. It uses an inexpensive isolation transformer from All Electronics, and can be made for about $30.00. The most expensive single part is the aluminum chassis. The grid potentiometer could be a precision 10-turn pot, but this would be expensive, and an ordinary carbon or plastic potentiometer (1/2 W or better) will be satisfactory. The maximum plate voltage of 120V and maximum plate current of 35 mA is adequate for many measurements. If you use a three-wire line cord, ground the chassis to the green wire. If you use only a two-wire line cord, it is probably better not to ground the chassis.

Vacuum tubes come with metal or glass envelopes, and in latter days with either the familiar octal arrangment of 8 pins, or as miniature glass tubes with 7 or 9 pins. There were earlier bases with four, five or six pins. Later, 12-pin miniature "compactron" or "duodecal" tubes were used in TV sets. Miniature tubes were not miniature, simply tubes with a button seal and all-glass envelope closely fitting a normal-sized cage. Subminiature tubes were actually miniature. Sometimes connections to grid, plate or (rarely) cathode were made to caps at the top of the tubes. In small tubes, these caps have a diameter of 1/4". The pins are numbered consectively clockwise, starting from the left of the index key for the octal, or to the left of the wider space, for the miniature, always looking at the bottom of the tube. This is shown for the octal base at the left. Pin numbers are given in the circuit schematics here. Most sockets have pin numbers marked. You will need to get sockets for the tubes you study, one for each type of socket. Solder wires to the tab at each pin that can be inserted in the solderless breadboard. I use the resistor color code for the pin numbers. A convenient octal socket fixture is available that comes with screw terminals for making connections. It was intended for relays, but is very useful for tube experiments. Heater connections for octal tubes are typically (not always!) to pins 2 and 7, and often to pins 3-4 on 7-pin, or 4-5 on 9-pin, miniature tubes. Sometimes halves of the heater can be connected in series or parallel, for two different voltages. Sockets were originally mounted in holes punched in aluminum chassis, secured by locking rings or by screws and nuts with a mounting plate. The chassis was, not surprisingly, the ground or common.

The "Loktal" tube was an excellent idea that was never universally adopted, mainly because miniature tubes took over in the 1950's. Since loktal was a trade name, RCA used "lock-in" instead, and you sometimes see "loctal." The loktal tube has an 8-pin button-seal (like the seal on miniature and octal GTB tubes). A natural metal base (of some aluminum alloy, apparently) shields the base of the tube and has a central pin with a circumferential locking groove. The pins project only 6 mm, and are 1.4 mm in diameter, much smaller than octal pins, so the locking action guarantees that the tube will stay in the socket in spite of the small pins. The tubes are roughly the same size as an octal GT tube. Most are one size, but a few power tubes have a slightly longer envelope. There are no grid caps on any Loktal tube, and the heater connections are always to pins 1 and 8. Among the thoughtful features of loktal design, the type number appears in a hexagon on the top of the tube where it is visible from above, not on the side as on octal tubes. There is a dimple on the base corresponding to the key of the central pin, making the tube easy to orient for insertion. It seems that a lot of getter was used, so the tops of the envelope appear heavily silvered. The available types are only those used in AM and FM receivers. There are, nevertheless, enough types for a broad variety of experiments, and the prices are not excessive, so you may want to standardize on Loktals. Type numbers beginning with 7 have 6.3 V heaters, while type numbers beginning with 14 have 12.6 V heaters. There are some 7xx and 14xx tubes that are not Loktal, and some tubes that actually take a 7 V heater supply. One loktal rectifier, the 5AZ4 (a 5Y3 equivalent), has a 5 V filament. Loktal tubes designed specifically for battery-powered equipment had 1.4V filaments. The type numbers began with "1L." There were also rectifier and beam power loktals with 35, 50 and 70-volt heaters for AC/DC sets with series heater connections.

A tube designated simply 6N7 will be a metal-envelope octal tube with a 6.3V heater. A 6N7GT will have a cylindrical glass envelope. A 6N7G would have a shouldered glass envelope of the graceful shape designated ST. The electrical characteristics of such tubes were the same, whatever the envelope shape.

A very important part of vacuum-tube technology was bringing the metal leads through the glass envelope. Coefficients of expansion must be exactly matched, and the seal must be strong. Originally, tubes had bases (usually Bakelite) to support the contact pins mechanically, taking the strain off the pressed-glass seal, which was made of lead glass. Around 1935, the metal envelope was developed, but there was still a base. The all-glass "miniature" tube was made possible by the "button seal" that supported the contact pins mechanically as well as bringing them through the glass, allowing the base to be eliminated and tube size to be reduced. The insides, or "cage," was the same size as in previous tubes, however. It is supported on its leads, which are welded to the contact pins before the envelope is fused in place and evacuated. The button seal is also used, in a larger form, on tubes designated by GB at the end of the type designation, and by Loktals. The final step in manufacture was "flashing" the getter, usually barium or magnesium, to perfect the vacuum by adsorption of any remaining gases, leaving a shiny coating. This was generally done by heating a loop inductively by RF from outside.

Diodes

Thermionic diodes, like semiconductor diodes, are divided intosignaldiodes that handle small currents at low voltages, andrectifierdiodes that handle large currents, often with large inverse voltages. A diode has an electron-emitting cathode and an electron-receiving anode or plate. The arrangments of cathodes and plates in commercial tubes, and what they are called, are shown in the figure. Signal diodes are also often added to a triode or pentode, sharing the same cathode and with one or two plates. Current flows only from plate to cathode, and this unidirectional conduction is the purpose of a diode. Diodes cannot amplify.

Signal diodes always have indirectly-heated cathodes, so they are easy to use. It is only necessary to make sure that the heater-cathode voltage does not exceed specified limits, usually a few hundred volts. Rectifier diodes often have filamentary oxide-coated cathodes, since these cathodes are more efficient when large currents are needed, requiring less power. We are considering only vacuum diodes,kenotrons, in this section. Thermionic gas diodes, orphanotrons, will be treated below, since they have rather different properties.

Thermionic diodes have now been completely superseded by semiconductor diodes, largely for economic reasons, physical size and the need for a filament supply. A silicon diode capable of carrying 1 A is available for $0.04 or so, and takes up very little room. However, diodes can teach us a lot about thermionic emission and other interesting things. They do work rather well, and it is good to make their acquaintance.

The forward voltage (in the direction of current flow) of a diode is always relatively low, less than 15 V or so. The plate current is roughly proportional to the 3/2 power of the anode-cathode voltage (Langmuir-Child law), and the proportionality factor is called theperveance. The perveance depends on the geometry of the tube, increasing with larger area and closer spacing. It's remarkable that most diodes agree with Langmuir-Childs so well, in spite of different geometries. Since the voltages are low, contact potentials may affect your measurements. Contact potentials are discussed below in the section on low-voltage tubes. The easiest way to find the perveance is to plot I2/3against V, and to draw the best straight line. The intercept gives the value of the "true" zero plate voltage, and the slope, raised to the 3/2 power, is the perveance. Perveances range from 0.02 to 2.4 mA/V3/2for a representative assortment of 12 diodes of all types. There is no turn-on voltage drop for a thermionic diode, as there is for a silicon diode. Conduction begins immediately when the plate is positive with respect to the cathode, and stops immediately when the plate goes negative. It is easy to measure the V-I characteristic of a diode with a low-voltage DC supply, a voltmeter and an ammeter. I use a 100Ω resistor in series to make adjustment easier and safer. Thermionic diodes are not as easy to destroy as semiconductor diodes, and will take a good deal of abuse.

The 6AL5 dual diode, whose basing is shown at the right (7-pin miniature socket), is a typical signal diode. IS is an internal shield between the diodes. The two diodes and the shield are easily seen through the glass envelope, and you should notice how close the plates are to the cathodes. The close spacing means a large perveance, so only small plate voltages are required. Don't connect this tube directly across high voltages! A peak inverse voltage of 330 V can be resisted, and the DC plate current should not exceed 9 mA. Peak currents can go up to 45 mA if necessary, however. I measured the perveance as 2.42 mA/V1.5, for one plate, a large value. The 6AL5 gives 9 mA with a plate voltage of only about 2.5 V! The heater, connected to H-H, pins 3 and 4, takes 0.3 A at 6.3 V.

Try the 6AL5 in the circuit shown at the left, which is a basic signal rectifier with a 4.7k load resistor. Feed it with the signal generator, and compare the output and input with the oscilloscope. Try input peak-to-peak voltages of only 2 V or so. You will notice that there is no "diode drop" with the 6AL5--it acts like a perfect diode, rectifying down to small voltages. We know how to do this with a semiconductor diode and an op-amp, but here it's done quite simply. The 6AL5 has an incremental resistance of only about 237 Ω, and is nearly linear. It is easy to run a plate voltage versus plate current curve with a low-voltage power supply. Keep the load resistor, and subtract the voltages at plate and cathode to find the plate voltage.

The 6H6 is an octal dual signal diode like the 6AL5, in a unique small metal envelope. The heater is connected to pins 2-7, the cathodes to 4 and 8, the plates to 3 and 5. 3 and 4 are one diode, 8 and 5 the other, and completely independent. It can be used for any reasonable service, such as AM detection, as a full-wave rectifier, or as a voltage doubler, so long as the current per plate is 8 mA or lower, and inverse voltages do not exceed 420 V. The voltage between heater and cathodes should not exceed 330 V. Measure the plate current as a function of the plate voltage up to 10 mA (the plate voltage will be about 7 V), and plot the current against the 3/2 power of the voltage. I obtained a rather straight line, showing agreement with Langmuir-Child, with a permeance of 0.5 mA/V1.5. At 8 mA, the incremental resistance was 590Ω, and V/I = 785Ω. The 12H6 and 7H6 are similar tubes with different heater ratings and basing.

The 7Y4 is a typical small full-wave rectifier with an indirectly-heated cathode, like the more common 6X4 (miniature) and 6X5 (octal). This "Loktal" tube is inexpensive. Many of the common rectifier diodes are rather costly, for the curious reasons associated with the current tube market. The heater, taking 6.3V at 0.5A, is connected to pins 1-8 (as with all Loktal tubes). The cathode is pin 7, and the plates are pins 3 and 6. The peak inverse voltage is 1250 V, the peak current 180 mA, and the average dc current 70 mA. The heater-cathode voltage should not exceed 450 V. Measure the plate voltage for currents up to, say, 50 mA, and plot the results as for the 6H6. Again, we find a straight line and a perveance of 0.58 mA/V1.5. Note that the plate voltage varies considerably as the current changes, from 4 V at 7 mA, to 16 V at 40 mA. Compare these voltages with those for a mercury-vaporphanotronas discussed in the next section. The 7Z4 is a somewhat larger full-wave rectifier (with perveance 0.40), the Loktal equivalent to the types 80 or 5Y3 that are now much more expensive.

An excellent diode for observing the Langmuir-Child law is the 2X2A. This tube has a 4-pin base like the 82 phanotron discussed below, and the large, bell-like anode is brought out to a cap at the top of the ST envelope. The oxide-coated cathode thimble is easily seen. The heater takes 2.5V at 1.75A, so it can use the same transformer as the type 82. The rated DC current is 7.5 mA, and the maximum voltage is 4500V. A plate voltage of about 60V is needed to reach 7.5 mA plate current, so measurements can be made over a wide range of voltages. Plot your results as I2/3vs. V. A straight line will be found, that intercepts the V axis at -1.2V. The perveance of the 2X2 is found to be 0.0165 mA/V3/2. The unusually low value is due to the large cathode-anode spacing.

The 6V3-A is a strange miniature tube with a cap on top that is the cathode connection. Its heater, connected to pins 4 and 5 of the 9-pin miniature base, takes 1.75 A at 6.3 V. The plate is connected to pins 2, 7 and 9. This tube is designed for the rugged service of a televisiondamper diode. During horizontal retrace, the damper diode conducts, charging the boost capacitor while absorbing the large inductive kick. The peak inverse voltage is 6000 V, the peak current 800 mA, and the average current 135 mA. The large-diameter cathode tube and long plate imply a large perveance, which, in fact, is about 2.3 mA/V1.5. This tube happens to be very cheap, but would serve as an excellent half-wave rectifier for practically any purpose. There are other damper diodes, such as the 6W4 and the 6AX4GT (perveance 1.42), that would have similar characteristics.

As an example of the small signal diodes that are often combined with a triode or pentode in the same envelope, and share the same cathode, the 6AV6 or 6AT6 furnish good examples. The 6AV6 has its heater at pins 3-4, cathode at pin 2, and the signal diode plates at pins 5 and 6. The maximum current for each diode is 1 mA. I connected the two plates together for measurement, and took the current up to 3 mA, for which a plate voltage of 6.4 V was required. The curve of I against V1.5sagged a little at low currents, but the upper part was quite linear, showing a perveance of 0.085 mA/V1.5for one plate. The incremental resistance was 4.55kΩ, and V/I was 5.05kΩ at 1 mA. The current for one plate obeyed the formula I = 0.15 0.085V1.5mA. In this tube (and similar ones) the plates are flat, one on each side of the cathode.

The 1A3 seems to be the smallest signal diode of all. It was designed for portable measuring apparatus. The heater takes 0.15A at 1.4V (a D cell), connected to pins 1 and 7 of the 7-pin miniature envelope. The cathode is at pin 3, the anode at pins 2 and 6. The peak inverse voltage is 330V max., the maximum plate current 5 mA, and the average plate current 0.5 mA DC. Maximum heater-cathode potential is 140V. The anode is only a few millimeters high; most of the envelope contains only vacuum. The measured perveance was 0.075 mA/V1.5.

The Noise Diode

A special kind of diode should be mentioned here, because experiments with it are quite interesting. It is thenoise diode, intended for the specific purpose of producing wide-band RF noise through theshot effect. Shot effect noise is fluctuations in the anode current due to the random collection of electrons. We have already mentioned that the anode current is controlled by the space charge around the filament. It was discovered, to some surprise, that this correlated successive electrons so that they were emitted regularly to maintain a constant current, and therefore the shot effect was nearly completely eliminated. That is, a normal diode has no shot effect noise in its plate current.

The noise diode is designed so that at reasonable plate voltages, all electrons emitted by the filament are immediately drawn to the plate without forming much of a space charge. Since the electrons are emitted randomly, the anode current will show the full shot effect noise. This is done by purposely making the filament to have low emission. To do this, a tungsten filament is used. Noise diodes give us the opportunity to observe a tungsten filament, as well as temperature saturation.

An available noise diode is the 5722, whose basing is shown at the right. The 7-pin miniature tube was made as late as 1977, and now costs about $14, which is probably not much more than when it was new. The maximum plate voltage is given as 200 V, and the maximum plate current as 35 mA, so apparently the plate can dissipate 7 W. The plate has wings that make a good dissipation probable.

A circuit for testing the 5722 is shown at the left. Note that an RF choke is put in the plate lead to act as a load for the current fluctuations. This choke should be rated for the plate current employed. I connected a variable DC supply to the filament as shown, to pins 3 and 4, leaving the center tap alone. This supply should be rated at 2 A or more. Increase the filament voltage gradually, looking for the glow. There will be no plate current until the filament current reaches about 1.3 A, but it increases very rapidly beyond this point. The filament glows brilliantly, like an incandescent lamp, since its operating temperature is about 2400K, not the 900K of an oxide-coated filament. The filament current should not be allowed to exceed 1.6 A. If the power supply has current limiting, it can be useful here. By setting the plate voltage at near 200 V, you can see the saturation current as a function of filament current.

For two or more reasonable values of the saturation current, say 5 mA, 12 mA and 20 mA, record the current as a function of plate voltage and plot your results. For If= 1.5 A, the plate current saturated for about 50 V on the plate, approaching a value of about 12 mA. It is easy to find out what plate voltage to use to ensure saturation when making shot noise in this way. It is very difficult to make noise measurements in the usual breadboarding environment. I thought it just possible to have seen some on my 100 MHz scope with a plate current of 20 mA, without amplification. See the page on Noise for more discussion of noise measurements.

 

The Phanotron

General Electric and Westinghouse liked to coin names for their products that drew on Greek. Aphanotron(fanos, "bright") was a gas-filled thermionic diode, while akenotron(kenos, "empty") was its vacuum cousin, which we have just been studying. All these tubes, once so common and useful, have been totally replaced by the much cheaper and smaller semiconductor diode. There is still, however, quite a lot of interesting physics and electronic involved with gas tubes, which makes their study profitable.

The most convenient phanotron to study is the type 82. Its kenotron cousin is the very familiar type 80, later available as the 5Y3, which is still, remarkably, in production. In the curious contemporary tube market, these are rather expensive, and the 82 was not cheap. Both are full-wave rectifiers with two plates and filamentary cathodes. The 80 and 82 have a 4-pin base, once rather common,and the graceful ST shouldered glass envelope. When you pick up an 82, the droplets of mercury on the inside of the envelope will be evident. There are two cylindrical plates, with an oxide-coated filament ribbon in an upside-down V inside each.

The filaments will glow orange when you apply the 2.5V at 3A they require across the larger pins 1 and 4. The plates are connected to pins 2 and 3. A low dc voltage can be applied between plate and cathode, using perhaps a 100Ω resistor in series to soak up extra voltage. Some current will flow even at low voltages as the plate attracts electrons from the cathode space charge. When you raise the voltage, it will stabilize at about 12 V and a bright blue glow will fill the plates. This is probably the stimulus for the name "phanotron." As you increase the current, the voltage across the tube will increase a little. I found about 14 V at a current of 100 mA. The rated average current for the tube is 115 mA.

The glow can be examined by a spectroscope, such as the Edmund 30823-05, the Project STAR spectroscope, available for about $30. This is a low price for an instrument that can show Fraunhofer lines in the solar spectrum and resolve the sodium doublet, even though it is somewhat hard to use. The 82 is not designed as a lamp, but the glow is sufficiently bright to give a good spectrum. The violet line at 405 nm, the cyan line at 436 nm, the green line at 546 nm, and the yellow doublet at 577 and 579 nm can be seen. The lines are sharp, much better than with a fluorescent lamp.

The reason the tube was designed was to offer a voltage drop that was more constant with changes in current than was the drop across a vacuum diode. The 12-14 V drop is not particularly low, especially for low currents, but there is some advantage at high currents. This did not seem to appeal greatly to designers, and the tube was rather little used, and eventually was discontinued without the appearance of a later version. The 866, a half-wave phanotron larger than the 82, remained popular for amateur transmitter power supplies. It could handle 250 mA with a peak inverse voltage of 10,000 V, and was generally used in full-wave pairs.

When the tube reaches its operating temperature, the upper part of the bulb, which at first condenses a mist, will clear of mercury, which will still collect in the cooler lower regions. At 20°C the vapor pressure of Hg is about .001 mmHg, and at 60°C, about .025 mm Hg. These are roughly the limits of the mercury pressure in the tube. The 82 does not contain argon to start the discharge, since no self-sustaining discharge is initiated. Distinguish carefully between the operation of a phanotron and that of a glow tube, such as the voltage regulators mentioned below. All the current in a phanotron comes from thermionic emission, as aided by the ionic and field effects at the cathode. The maximum current is about 1.8 times the saturation thermionic emission in a vacuum. One should be careful to heat the cathode before applying plate voltage, so that the tube drop does not exceed about 25 V. If it is higher than this, positive-ion bombardment soon destroys the cathode.

Mercury has an ionization potential of 10.43 V. When electrons have been accelerated to this energy in the cathode-plate field, they can knock electrons off the neutral atoms and produce positive ions. These positive ions neutralize the space charge, producing a plasma that is very conductive. This is the effect of the gas; no glow discharge with its characteristic cathode and anode phenomena is initiated. The anode-cathode voltage must only remain high enough to replenish the stock of ions. Electrons of lower energy can excite mercury atoms to upper levels. It takes only 4.9 eV to excite the atom so that it emits its strong ultraviolet line at 253.7 nm. Most of the glow is produced by such excitation by inelastic electron collisions, as well as by recombination of the ions. With a hand spectroscope, you should see the familiar lines 454 nm (blue), 546 nm (green) and 578 nm (yellow) of the mercury spectrum in the glow.

If the voltage across the tube should rise above 22 V, thedisintegration voltage, the positive ions acquire such energy that they sputter and destroy the oxide cathode. This can happen if the current is raised too high, or if anode voltage is applied without sufficient gas pressure. These tubes work with an efficient oxide cathode only because the discharge is maintained in mercury vapor at a low enough voltage. For large phanotrons, the filaments should be energized, and the tube brought to operating temperature before anode voltage is supplied

A curiosity is the 0Z4 gas rectifier. This tube, which is indeed a phanotron, has two plates and one cathode, really two diodes in the same envelope, as was typical for rectifier diodes intended for full-wave rectification with a center-tapped transformer secondary. It contains, I believe, argon gas at low pressure. The positive ions heat the cathode, as well as neutralize the space charge. The 0Z4 was used with vibrator power supplies for automobile radios, and had the advantage of not requiring a filament supply. A vibrator was a mechanical chopper that turned the DC from the car battery into AC that could be transformed to a higher voltage and rectified for the B supply. Solid-state replacements may now be obtained. The 0Z4 is guaranteed to break down below 300 V, and requires a current of at most 30 mA to keep the cathode hot. The circuit at the right can be used to test the properties of an 0Z4. The 5.5k resistance has to be 15 W; I used two 11k power resistors that I happened to have on hand. My 0Z4 broke down at 268 V, and had an operating voltage drop of 20-22 V, which seemed to fluctuate. When the voltage was reduced, the tube did not fall out until about 60 V, probably from too low a current to keep the cathode hot. These tubes produce a large amount of RF noise, and so are shielded to reduce it. My metal 0Z4 was silvery in color. The 0Z4 can be used with any power transformer from 250-0-250 to 300-0-300 volts. It requires at least 300V for breakdown, and the peak inverse voltage is 800 V. The current should be between 30 mA (minimum) and 90 mA (maximum).

The 0Z4-G is well worth obtaining, even if it does cost more than the more common metal tube. It has a small tubular glass envelope that displays everything inside. The cathode is an 11 mm long spiral, apparently coated with oxide to increase emission, 4 mm in front of the two post anodes. These are circular rods inside metal cylinders. As you look at the tube from the side not silvered by the getter, the pin 3 anode is to the left, the pin 5 anode is to the right. The tube can be tested with a variable-voltage DC supply and a series load resistor of 3.3k and 25W dissipation. The resistor will get hot. When the tube breaks down, a bright cathode spot forms surrounded by a bluish glow. These are the cathode glow and the negative glow of a DC discharge. There is a greenish glow at the anode, which is probably the positive column of the discharge. Between the two glows is a dark space, probably the Faraday dark space. This is the only place I have found where the glow discharge can be seen with all these details in a commercial device. There is considerable flickering, both at the cathode, where it is most persistent, and at the anode. This flickering occurs in the DC discharge.

The 0Z4 was found to generate and radiate a large amount of RF noise in operation, so it is well-shielded when in operation. This curious phenomenon is said to result from the turning on and off of the current, which creates waves in the plasma in the tube. I have not studied this, but it might be an interesting diversion. The discharge was observed to flicker even in a DC discharge.

TheTungarlow-voltage rectifier tubes had a tungsten filament and graphite anode close together in rather high-pressure (5 cm Hg) argon. They were used for battery charging and similar duties involving only low voltages. Selenium rectifiers replaced them even before the appearance of silicon diodes. Their filaments glowed brightly, because oxide-coated filaments could not be used.


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