Special Tubes by Sylvania
January 1945 Radio-Craft

January 1945 Radio-Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

With most solid state devices, be they discrete elements or integrated circuits, discerning their functions by visual inspection is nearly always impossible. A trained observer can use a microscope on a bare die to get a good idea of what the various parts do. Memory banks, transistors, resistors, capacitors, etc., are usually pretty obvious even to someone relatively new at the job. Opaque overmolded packages and/or extremely small features complicate the task. In the vacuum tube days, anyone accustomed to working with them could fairly easily look at the the arrangement of heaters, cathodes, grids, and plates - sometimes singular but often multiple - and figure out the function. Experts could even make a fair guess at the key electrical specifications. Of course that only applied to those with clear glass envelopes. Tubes with special shapes might have been a bit more difficult to guess the application. To wit, the long, skinny R-1110 "Pirani Tube" shown here is an example. It is designed to measure ambient pressure using an ingenious principle that measures a change in the resistance element as a function of pressure even though it resides inside the sealed tube. Sylvania made many such special purpose vacuum tubes, many of which can be found using a Google search.

Special Tubes by Sylvania

The role of the electronic-tube in industry is ever more and more varied. To meet special needs, many types of tubes have been and are being devised. Some are not widely known - others are at present restricted to war uses only. These and others will play an increasing part in the development of modern electronics when civilian needs again assume priority. A brief description of a number of interesting specialized Sylvania tubes is given below:

Strobotron

Type 631-P1 and SN-4. These two tubes are similar except that the former is designed for viewing moving objects. the latter for electronic relay work. They are cold cathode tubes. The 631-P1 emits sharp, brilliant flashes of red light at low frequency. By viewing rotating objects under its light, an illusion of "stopped" or "slow" motion is created. thus allowing study, adjustment and alignment of rapidly moving machinery. This tube, for instance, is used in the General Radio 631-B Strobotac, a device directly calibrated to produce from 600-14,400 flashes per minute, (of 5-10 microseconds in length). The SN-4 is an electronic relay with no moving parts operating at a maximum of 60 pulses per second.

A basic circuit for either tube is given in Fig. 1. Switch closure places the voltage on C₂ between cathode and grid #1 to initiate discharge. The condenser assumes the full DC voltage while the switch is open. Other tube elements may be utilized as shown in the chart.

Pirani Tube

R-1110. This tube may be used to measure a vacuum between the limits of 10^-1 to 10^-5 millimeters pressure. The Pirani principle is that of measuring the resistance of a wire placed in an atmosphere of low gas pressure. Since heat conduction changes with the pressure, a current meter determines the degree of vacuum. The tube tip (which is open) is sealed into a vacuum chamber which it is to measure. The filament resistance is about 6.6 ohms when cold and rises to about 16 ohms with 100 M.A. flowing (in a vacuum).

Fig. 1 - Strobotron. light is emitted directly from the tube.

Thermocouple Tube

R-1100. This tube may be used for similar applications to the above and has the same range of pressure measurement. It also works on the principle of varying thermal conductivity with gas pressure. In this tube, however, the center point of the filament is the hot junction of a thermocouple, the output of which is measured to indicate degree of vacuum. Its filament resistance is 1.5 ohms, thermocouple resistance 3.5 ohms. Like the previous tube, its open tip is sealed to form part of the vacuum chamber under measurement.

Regulator Tubes

R-1159. This tube finds application in low current drain circuits for voltage regulation, The voltage across it varies with the current flowing through it, thus maintaining the voltage across the load.

OB3/VR90 OC3/VR105 OD3/VR150. These three tubes may be used in circuits requiring higher operating currents. The load in this case is placed in parallel with the tube, which maintains a constant voltage with widely varying current flow. The first code letters in the tube names determine operating current range, the last number being the operating voltage.

Facsimile Tube

R-1130. This is a cold cathode tube designed to emit a pinpoint of photographic light in the range of 3500 6500 Angstrom units. It may be used in facsimile receivers where a modulated EMF is applied to the tube. The emitted light is focused onto a sensitized photographic paper attached to a revolving cylinder.

Near Ultra-Violet Lamps

Blacklight Lamp. Sources of near-ultra violet light, these lamps resemble ordinary fluorescent tubes, with a red-purple glass envelope, which acts as a filter to absorb visible light. They can be operated with regular fluorescent lamp equipment and fixtures, though the smaller sizes have a special base. As they cause many naturally fluorescent materials to glow, they are useful for many inspection purposes, as well as their ordinary uses in lighting fluorescent dials, etc. As an example, the lamp is used in sorting acetate and cellulose yarns, which glow differently under the light though they may resemble each other very closely under ordinary light.

RP-12. This fluorescent lamp represents a compact source of visible and near ultra-violet emission in the range of about 3600 Angstrom Units. Operating from a 24-volt D.C. source it may be used to illuminate fluorescent painted dials such as an airplane instrument panels, for inspection lights, etc.

Blacklight lamps resemble ordinary fluorescents.

Posted March 1, 2021