Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

'Filament Lighting from the A. C. MainsA Discussion of a Practicable Method for Batteryless ReceiverDesign The Advantages of Series Connection of FilamentsBy ROLAND F.BEERSA NATURAL query when discussingwhy such an arrangement iscrease the values of the constants untilFig 2, may in- thecircuit shown in I- 1C.. 2subject of series filaments is to inquire we obtain the same degree ofrequiredfiltering for 23either necessaryamperes at 6 volts. Our structure will then lookor desirable. This inquiry ably illustrates like Fig The i. weight of such a device would bethat growth of tradition which radio is rapidly approximately 300 pounds, and its cost, on theaccumulating. With a few exceptions, modern same basis as previously assumed, would averageradio receivers up to the present time have $i 500.00! For the reason of its weight^and cost,employed the parallel filament connection. The it is therefore not feasible to effect A-batteryfilaments of the radio tubes have been operated elimination at 2\ amperes.from a constant 6-volt source, and variations of There are two main reasons why the seriesindividual tube filaments have been made by filament connection is so much more desirablechanging the applied voltage.from the power standpoint. The first reason isIt is not difficult to trace the reason for the that the total current to be filtered is very muchprevalence of the parallel filaments scheme. In less than with the parallel system. With anythe beginning, audion filaments often required number of tubes in series, the maximum currentone or two amperes for each tube. A multi-tube required is only that taken by one filament, andset would therefore require 3 to 6 amperes. as the size and cost of filter chokes increasesWhat better source of power was there, then, rapidly with the increase in the amount of currentthan the familiar 6-volt storage battery a lowvoltage,they are required to pass, we realize the economyhigh current capacity affair? Its availa-possible by a reduction of current.bility through the regular channels of distributionWe are somewhat assisted by the fact that thewas assured, and everyone was acquaintedwith its operation and maintenance. As the firstuses of the vacuum tube were determined bythe amateurs, so were its tendencies bound tofollow along their pioneer activities. It did nottake long, therefore, for the 6-volt parallel filamentscheme to become firmly entrenched in theminds of those who were to become radio setdesigners and builders of later periods.In answer to the query concerning the systemwherein radio tube filaments are arranged consecutivelyC 2000 to 8000 mfd. for 6 Volts, 2.5 Amps.in series, it is first necessary to showwhy A batteries have not yet been completelyFIG. Ieliminated by the use of the parallel filamentscheme. Let us first consider the general principlesfiltering efficiency of a choke coil increases withof power supply, illustrated by the modern the current through it; therefore, we should beB-power unit. Here we have the customary able to design a more efficient filter system for Atransformer, rectifier, and filter circuit, as shown supply than for B supply, as far as the chokes arein Fig. 2. The current and voltage capacity of concerned. The amount of energy that can besuch a device is of the order of 85 mA. and 200 stored in a given choke coil isequal to one halfvolts d. c. The degree of filtering of high-grade the inductance times the square of the current.units of this type In spite of this fact, however, there are otheris such that a variation orripple of but o. i per cent, is attained in the currentconsiderations which work to our disadvantageoutput.in the design of a high-current choke coil. ForIf high-quality reproduction is desired, with example, on account of the direct-current satura-freedom from hum and "motor-boating," it is tion of the steel core it is necessary to includeabsolutely necessary that the variation in current large air gaps in high-current chokes. These airsupply be of this order. In order to achieve this gaps reduce the effective permeability of the coredegree of smoothing in plate supply devices, to low values, makingitnecessary to increasea filter structure of such design as is shown in the amount of copper and iron to large quantities,Fig. 2 is used. The retail cost of a structure of in order to attain sufficient inductance at highthis type is approximately $20.00.direct-current saturations. We are also limitedIf we are to adhere to the same standard of in this consideration by the amount of copperquality with regard to the A powersupplysource, it is also necessarythat the current ripple shall not exceedo.i percent. This statement has _ Tbeen theoretically demonstrated andexperimentally verified, using anaverage radio receiver with no po- ^tentiometers which may be used tojbalance out any hum. With such areceiver the storage battery mightbe replaced with this theoretical Afjpower-supply unit. Basing our judgmentupon the design ofwethe filterthat can be used in a choke coil, on account ofthe d. c. resistance. If the choke coils havehigh d. c. resistance, the voltage output of apower unit intended for parallel filament supplywould be much too large on a j-tube set, forexample, if the unit were designed to supplysufficient voltage for a 6- or 8-tube set. A powerunit having sufficiently low regulation for parallelfilament operation would therefore requiremonstrous inductances.A second reason why the series filament connectionis desirable is that higher voltages areavailable for filtering. For example, a j-tube receiverwith 100 tubes in series requires 15 voltsfor the filament supply, and an additional 15volts may be advantageously employed for gridbias. Therefore, the total voltage required is 30volts instead of 3 volts (grid bias is obtainedfrom external batteries) for the parallel filamentconnection. Additional voltageis also availableat the filter circuit by virtue of the fact thatthe A current is obtained through a series rheostat,the voltage drop through which may rangefrom 100 to 150 volts (Regulation in this circuithas not the same importance as in the parallelfilament scheme, because it is a constant-currentsystem and not a constant-voltage system). Thetotal voltage applied to the filter circuit isequalto the sum of these values or approximately200 volts.Now it is a matter of common knowledge thatcondenser efficiency in a filter circuit is muchgreater at high voltages than at low voltages.The amount of energy stored in a condenser isequal to one-half the capacity times the squareof the voltage applied. Therefore, from thisstandpoint alone, a great saving is gained. Forexample, if a total capacity of 12 mfd. is requiredfor a given degree of smoothing at 200 volts, thecapacity required for the same degree of smoothingat 3 volts would be in excess of 50,000 mfd.The saving in inductance and capacity affectedby the use of the series filament scheme is thereforeenormous.Fig 3 shows a typical filter circuit for use inconnection with IQQ type tubes in series. Thisarrangement actually gives the same degree offiltering found in IFigs. and 2. Its cost is withinreasonable limits, in comparison with othermethods of power supply, while its size is notbeyond the scope of average radio cabinets.One obvious way in which to attack the problemof series filament connections is to place thevacuum tubes consecutively in sequence, in thesame order in which they normallyoccur. That is, we may begin withthe radio-frequency stages and runthrough the detector and audiostages. While this method may appearto be the most straightforward,it isopen to several criticisms.In the first place, it has been demonstratedthat less difficulty with humwill be experienced if the detector isplaced nearest the B-minus or groundconnection. Several different theorieshave been advanced to explain thissituation, but the important fact to