US2198317A - Multistage vacuum tube amplifier - Google Patents

Description

April 23, 1940. w. VAN B. ROBERTS MULTISTAGE VACUUM TUBE AMPLIFIER Filed May 27, 1938 GLASS ENI/FL OPE i m-MI L7 INVENTOR WAL rm VAN B. ROBERTS BY ATTORNEY Patented Apr. 23, 1940 PATENT OFFICE H l MULTISTAGE VACUUM TUBE AIVIPLIFIER Walter-van B. Roberts, Princeton, N. J assignor to Radio Corporation of America, a corporation of Delaware App ication May 27, issaserial No. 210,384

2 Claims. (or. 250-415) My present invention relates to multi-stage vacuum tube amplifiers generally, and especially to portable battery-operated amplifiers.

Conventional amplifiers as used in the past for portable purposes have suffered the drawback that/relatively heavy and expensive batteries have been required. Furthermore, two distinct batteries have customarily been employed, one for the filaments of the tubes and another battery of relatively high voltage for the plate circuits.

Usually one of these batteries runs down more rapidly than the other and the user may not know which battery requires replacement. Furthermore, it is necessary for him to buy two difierent styles of battery and the high voltage battery is not available at all stores and is rather expensive.

It is, therefore, the object of the present invention to provide a. multi-stage vacuum tube ampli- 2U fier capable of operation on a single battery composed of individual cells which are widely distributed at a very low price for flashlight purposes. A further object is to provide such an amplifier capable of operation on a relatively small number of such cells, for example or of them.

If it is attempted to utilize the filamentary cathode type of tubes on a single battery with uniform drain of current throughout the bat- 30 tery by connecting the filaments in series across the whole battery, the difiiculty arises that the potential difference available between the positive terminal of the battery and the cathodes of the various tubes is different for the different tubes. Furthermore, if a number of separate tubes-are used, there is inevitably a considerable heat loss at the ends of the filaments of each tube. In accordance with the invention, therefore, a plurality of tubes is constructed having a w common cathode of the heater type and a reflect ing enclosure about the entire structure for reducing radiation heat losses. With this arrangement cathode heating may be obtained from a battery which is also used for the plate circuit energization of the various stages, and end losses are reduced to a minimum by the use of the single cathode, while radiation losses are also reduced by the reflecting enclosure so that the power drain for cathode-heating purposes is 0 aminimum.

The accompanying drawing represents a typical amplifier system constructed in accordance j with the invention. A two stage amplifier arrangement is shown for simplicity, but it will 5 be understood that more stages may be employed if desired. In the drawing, reference character 5 denotes a source of signals to be amplified, such as a dynamic type microphone. The ground symbol in. the drawing represents a common point rather than an actual ground, inasmuch as the 5 amplifier is particularly intended for portable use such as, for example, in deai-aid sets. A grid electrode 2 is wound around a cathode 3 which is heated by a heater wire 3 passing through an insulating tube 5. The tube structure indicated in the drawing is a sectional view taken through the axis of the tube. One end of heater wire 4 is brought out through a glass envelope 6 to the positive terminal of battery 1 while the other end of heater wire 4 is connected to cathode 3 and thence, by Way of Wire 8 and resistance 9, to the negative terminal of the battery.

Resistance 9 is shunted by condenser, Ill and provides a positive bias for the cathode with respect to the ground in a well known manner. Signals impressed upon grid 2 causes variation of space current between cathode 3 and plate l l the latter plate being connected through the primary winding of transformer [2 to the positive terminal of battery 7. Voltage developed across the secondary of transformer 12 is impressed upon the grid 2 of a second grid-plate assembly similar to that previously described, and the output plate I! of the second tube section is utilized in telephones l 3. It is not believed necessary to describe the operation of the second tube section in further detail, as its operation is quite conventional.

In order to reduce radiation losses from cathode 3 to a minimum a metallic, reflecting enclosure or shell l4 surrounds the entire electronic system. Enclosure It contains a metallic apertured disk it which acts as an electrostatic shield between adjacent tube sections. Further disks l t of mica act as spacers for centering the cathode 3 within the grid and plate structures which are supported by glass beads H where the leads from said structures pass through the reflecting enclosure Hi. A second reflecting metallic enclosure or shell It may surround the first mentioned structure M if desired, both of the structures l4 and it being attached to an insulating disk It through which insulating tube 5 projects. Cathode connection wire 8 may be conductively connected to both of the reflecting enclosures l4 and it, but the grid and plate leads must, of course, pass through these enclosures without conductive connection thereto. The grid and plate leads and the cathode and heater leads are brought out through glass envelope 6 by way of the usual seals.

Cathode 3 is coated on its outer surface with electron emissive material capable of producing electron emission at a much lower temperature than electron emission occurs from the grid wires, so that grid emission will be negligible even if the grid temperature should arise to thermal equilibrium with the radiation temperature in the space between cathode 3 and reflector l4. While reflector M reduces the radiation losses, nevertheless the actual temperature of M will rise to a rather high value, so that it would cause some radiation losses from its outer surface. Such losses are further reduced, however, by the second enclosure I8, and, in order to prevent thermal conduction between enclosures l4 and I8, they are not connected metallically together at any point other than possibly wire 8 whose thermal conductance is small, due to its small cross section.

The grid and plate elements are preferably spaced as close as possible to cathode 3, in order to permit satisfactory operation at relatively low plate voltage, but, if desired, such operation may be obtained by interposing a space charge grid in well known fashion between the cathode and the control grids. It is also a matter of individual choice whether screen grids be provided between the control grids and plate of any tube sections.

While I have indicated and described a system for carrying my invention into efiect, it will be apparent toone skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is: y

1. A space discharge tube including a hot electron emission element, an electrode mounted to receive electrons therefrom, a reflection shell surrounding the emission element and said electrode, a second reflection shell surrounding the first shell and spaced therefrom, and a conductor connecting the shells and emission element solely at single points thereof.

2. An electron discharge device comprising an envelope, a cathode, a grid and an anode assembled in a unitary electrode assembly, a heat reflecting shell enclosing said electrode assembly, a spacer of good heat insulating material, said electrode assembly being mounted on said spacer and said shell being supported on said spacer, said envelope enclosing said shell and thermally insul'ated therefrom.

WALTER VAN B. ROBERTS.

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