Pentodes and Tetrodes Operating As Triodes

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♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ PENTODES & TETRODES OPERATING AS TRIODES

by C. C. McCallum

screen current 15 one-fifth of the INeE commercial radio receiver H.T. + S designers have nece!'s.lI·ily to anode current. Thi!'< relation�hip can and public address equipment be assumed to remain suhstantially provide a maximum audio power constant throughout the ranges of output for a mllumum D.C. power potc'ntial in which we ale intere!>tt'd input (to reduce cost of H.T. supply and (�nable� the "triode" anode equipment) and .1 minimum audio currt'nt to be derived by addition of input swing (to reduce cost of early INPUT screcn and anode current. 'Vhere stages), the pentode and its successor the manufacturer gi\·cs a family of ,he beam tetrode has been almost anode currt'nt-control grid vult age universally chosen for the output OOTPUT lOAD curves for identical anode and �crt'en stage. voltages, th� procedure of conversion .T.- to anode current-anode voltage curves The result of this concentration on H is simplified. When' the anode power efficiency has very naturally currcnt-control grid voltage cuncs· cau&ed the valve designers to neglect are given for varying anode voltages triodes and to devote their skill to and fixed screen voltages, interpola the still further improvement of the HI+ tion methods will have to be adopted. pentodes and tetrodes. This can be In the ca�e of a true pentode, as dis· very readily seen by looking through tinct from a beam tetrode, the screen, current catalogues and finding that supprcssor grid and anode must the old original PX4, PP3/:ZSo, TETROOE all linked together in making an PPS/400, etc., are still the be PX:zS, artificial triode. " ultimate" in triodes much as they were over ten yea'rs ago. INPUT The adjoining chart ha;; been pre pared to indicate' the power OlltPUt to Now among a large number of be expected from a representati\'e quality-conscious enthusiasts triodes range of British and American are still used extensively because, OUTPUT lo\O . triodes, pentodes and beam tetrode�, \\ give the chart greater advantages if etnciency is not of value the input swing nece!'�ary to paramount importance. This large favour because of the damping im achieve the power output stated can body of discriminating people are, posed on the speaker. The impor tant thing about this circuit (from also be lead off from a scale. The teo, realising that the triode has not the point of view of this article) is figure!' are in general given with due progressed to any marked extent for that, whether one employs a triode regard tll the maker's maximum dis years and they have had to resort to beam tetrode� or pentode, all function sipation requirements, restriction of methods not altogether orthodox to effectivelv as triodes as will be seen secolld harmonic distortion to a produn� lor tht'mseh'es more efficient· " by the diagram. Thus it becomes maximum of 5 per cent. of the funda triod:·s. Thi:- they have done by important to know the triodf' nlt'ntal, and utilisatiflll fit optimum linking electrodes in the pentode and characteristics of tetrodes and load re�i�tance. tctrode class and thus evolving for pt�ntodes if one i!' to asses!' the themselves "new" valves having The table may ut value to tl\ do not give, c'xcept in rare will give approximately the !lame cases, the details wanted and the user powcr output. How far thl!" succeed will be shown will have to "work it out" for later, but bet-ore doing .so, another n addition, the -ensitivit\" of a 0\' himself. 1 ' point will be touched upon that is individual ,·al \"t. Clppearing on th� causing the growing interest in the This can be done fairly easily by chart may b«- simply obtained by U!iC of pentodes and tetrodes as reference to the catalogues. Taking didding tht' ptlWl'r output hy the triodes. for example the Mazda A.C.4/Pen input vohage swinJ.:", thu" deri\·ing an valve, the details given in the expression milliwatts per \'oh The cc cathode-follower ., as applied of maker's list are:- to the output stage, where the lot:d input., British pentodes and beam tetrode!' connected as triodes will speaker transformer appears in the Anode Current (,.$ m/a at V. z50' be found to cons;derablv ahead of aO\· cathode circuit, is beginning to find ' Screen " 13 m/a at F.250. be ----_ . . --_.- other types from thE' p�int of "iew of

• R8Cl1o Dhision. The i::4i1Oll Swan Electric Co. From this it mav be seen that the sensitivity. r-- 100 90 80 70 DO.26. 60

50 �o _

40 .45

.2,043 30 6r60 .,J.042 .00.24 I I I I I' I Ipps/«JO - .AC.Pl1!Jl20 PP3/250 -- •• p��

?A� 15 £�2e 4 �15 Kf6�• . !'''' 6i .KT66 � 606. AC� " .AC.P4 L.: 6K6. .AC.P .. g 10 • 6 I N. Cl) - rC .... 8 � • � • AC6,P€4 7 .g P. 'N.45 IEL6 . +-' • AC,4,liN 6 _CLJ � .1'.41 I 5 .El

• AC2P£N ..

• TRIOD£S • PENTODES & T£TROO[s I ,- 'C;: 2 �,S

1-5 Audio Outp.ut watts -

' 15 ·2 '3 ... ·5 '6 '7 -I -8 I' 1'5 2 2-5 .. 5 6 7 8 S f o I I I I I I I 1 1 I I I "{ I I I I I I J

Chart of typical British and American trlodes, tetrodes, and pentodes d.Alsifled accord In, to power output and Input volta,. swinl_