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

44 RADIO BROADCAST MAY. 1927The filter is one of the most important partsof a B socket power device, and it is essentialthat it be carefully designed if it is to give bestoperation. The best filter will, of course, be thatone which gives the least hum in the output consistentwith reasonable cost. It is preferable toaccomplish the filtration with a filter as smallas is feasible because, in this way, it is possibleto keep the resistance of the filter circuit at alow value. In " F" the filter issupplied withenergy at the rate of 120 cycles per second. Ittherefore seems logical to design the first sectionof the filter to eliminate this ISO-cycle rippleand to then use one additional filter stage toeliminate any other harmonic frequencies orany residual ripple that gets through the firstsection. A selective filter of this type has beenpatented by Kendall dough and is used in theSilver-Marshall 331 Unichoke and also is incorporatedin this unit because, with such acircuit,excellent filtering action can be obtainedand, at the same time, it is possible to keep thefilter resistance down to a fairly low value.At the output of the filter we obtain thethe device. Amaximum voltage delivered byreceiver, however, requires several differentpotentials for its operation. In most cases a highvoltagetap, say about 180 for a 171 power tube,a go-volt tap for the first audio amplifier and ther. f. stages, and a 45-volt tap for the detector, areresistance of the rectifier, the poorer the voltage the B power unit will be called upon to supply aregulation. In the case of batteries in good varying load. If its regulation curve is not flat,condition, the internal resistance being very low, any variation in the milliampere load will causethe voltage remains substantially constant under a change in the voltage supplied to the receivernormal current loads and a regulation curve for and this will produce another surge in the receiveragain affecting the average plate currenta battery is substantially a straight line. In itself,a falling characteristic may not, at first, appear and repeating the process. "Motor-boating" dueto be a disadvantage so long as the curve indicatesthat the device is capable of supplying curve of the power unit be made substantiallysufficient voltage at the particular load at which flat, and this has been accomplished in theto this cause will be preventedif the regulationit is to be used, but, a poor regulation curve, particular unit illustrated on page 43 by connectinginto the output circuit a glow tube, asindicating a high internal resistance, is doubtlesslyan important contributory cause of shown in Fig. 2. The problem of eliminating"motor-boating" in a receiver."motorboating" is complicated and for this reason"Motor-boating" can also be produced by RADIO BROADCAST Laboratory expects to makeoverloading, which causes a change in the averageplate current drawn by the receiver. If the the exact causes of the trouble. These dataa series of tests in an endeavor to determineaverage current drawn by the receiver varies. will be published in a later article.required.To obtain the various voltages it is necessaryto equip the device with an output potentiometer,which merely consists of a resistance, orseveral resistances, connected in series acrossthe output, as will be seen in drawing "G," Fig.I. This circuit diagram is similar to many powerunits in use to-day. It has one important disadvantage,which is that the voltage obtainedfrom the various terminals depends upon theload. If one tube issupplied from the tap markedgo-volts it might really receive 1 10 volts, twotubes 90 volts, and three tubes only 70 volts,and so on. This is one important respect in whicha B power unit differs from a dry battery. Froma battery we obtain the same voltage at allreasonable loads, but from an ordinary powerunit we obtain voltages that depend on the milliamperedrain."MOTOR BOATING" CAUSESTHAT the output of most B power unitsvaries with load isimportant because it isvery likely that one of the causes of "motorboating"is to be found in this fact."Motor-boating" is seldom experienced (althoughit is not an impossibility) when anamplifier is operated from good new B batteries.Ordinary B batteries must be discarded as thevoltage runs down, the actual reason for thisbeing that the internal resistance of a batterygoes up as the voltage goes down, with the resultthat the high resistance, being common to thecircuits of all the tubes in a receiver, may causeoscillation, and most certainly distortion.Considering a B power supply from this angle,we find a comparatively high internal resistanceunavoidable with standard rectifying devices,and we further find a comparatively high resistanceis necessary for the mechanical constructionof satisfactory filter chokes. The net resultispoor voltage regulation; that is, as the currentdrawn from the B supply increases, the voltagedoes not remain constant as with a battery but,instead, falls off at a fairly rapid rate. Curve A,Fig. 3, indicates the regulation that may be expectedfrom a standard B supply with an extremelylow-resistance filter. The higher thefilter choke resistance, the poorer the voltageregulation; and, likewise, the higher the internalZero LineFIG.IZero Line