Determination of Typical Operating Conditions for

Transcription

1 1 1 ( A PUB.LICATION,OF THE RCA TUBE DIVISION Vol. 1, No. 3 Deceber, 195 Deterination of Typical Operating Conditions for RCA Tubes Used as Linear If Power Aplifiers During the past several years, there has been a treendous increase in the use of single-sideband, suppressed -carrier transission in aateur -radio radiotelephony. This type of transission offers several advantages over the widely -used aplitude odulation ethods. These advantages include reduced band -width and the eliination of heterodyne - interference probles. More useful power can be obtained with the sae tubes and power supplies or, conversely, saller tubes and power supplies can be used to deliver the sae useful power. ith high-level aplitude odulation, a carrier and two groups of sideband frequencies are generated. The total power in the two sidebands at 100 per cent odulation is equal to one half of the carrier power. Thus, for every 100 watts of total transitted power, 67 watts is in the carrier and 16.5 watts is in each sideband. Yet, one sideband contains all of the necessary intelligence for counication (provided certain receiver requireents are et). Half the Bandwidth Single-sideband, suppressed -carrier transission utilizes only one sideband. By the eliination of the other sideband, the bandwidth is cut in half. By suppression of the carrier, heterodyne interference is eliinated. Only 16.5 watts of power is required to convey the sae intelligence. Conversely, if the original 100 watts of power is transitted in a single side - band, six ties the forer useful power will be obtained. The literature contains considerable inforation on various ethods of generating By A. P. Sweet* single-sideband, suppressed -carrier signals. However, little inforation is available on the choice of tubes for aplifying these signals and the ethods of calculating typical operating conditions for these tubes.. Linear -RE Aplifiers Single-sideband signals ust be aplified by linear rf aplifiers. These aplifiers are identical to of power aplifiers except that resonant tank circuits are used in the grid and plate circuits instead of audio -frequency transforers. Consequently, the tube anufacturer's ratings for of power aplifier and odulator service for class A, ABI, AB2, and class B and typical operating conditions will apply, provided the tube is also capable of operating at the higher frequencies involved. The sae derating factors for plate voltage and input versus frequency shown by the anufacturer for class -C telegraphy ratings should be applied to single-sideband operation at the frequencies where they becoe applicable. Because the tank circuits act as energy - storage systes, it is not necessary (as in case of audio work) to use two tubes in push-pull in class -AB or class -B, linear, rf aplifiers. However, if only one tube is used, the rf haronics will be higher thereby aking the TVI probles ore severe. Although the anufacturer's ratings are based on 100 per cent odulation with sine - wave signals, noral voice odulation reaches this condition only on the peaks of odulation. The ICAS ratings shown by RCA have Power Tube Engineering, Lancaster, Pa.

2 2 RCA HAM TIPS Deceber, 195 taken this factor into account. Consequently, no attept should be ade to operate above these axiu ratings. Such operation will result in shorter tube life and the possibility of early tube daage during transitter adjustent or unexpected overloads such as icrophone "howl." Since only rf power aplifiers are being considered, class A operation will not be discussed further. Of the reaining classes, AB1 operation with tetrodes or pentodes is the siplest since only the plate- and screen -voltage supplies require good regulation. Table I includes the axiu ratings and typical operating conditions for several RCA tubes used as linear rf power aplifiers. If it is desired to operate at conditions other than those given, typical conditions can be calculated by eans of the following procedure: 1. Make sure Eb is within tube ratings. 2. Refer to the published curves. On the average plate characteristics curves, select a point on the zero grid -voltage curve near the "knee," and record i'b,* and ebin; fro the average screen -grid characteristics curves, deterine i'c2 for this point. (E02 equals the value shown for the curves used.) 3. Calculate Ibe: Ibe = i'b/3.. Calculate PD: * PD = 5. Calculate SI: SI = E02i'c2/. 6. Calculate PI: PI = EbIbe. Eb ebin E02 Eel ec E'g Ib Ibo Fly 1,2 l cl 1 el PD PI PO DP SI Dc plate voltage. Miniu plate voltage for the required peak current (fro the characteristics curves). Dc screen voltage. Dc control grid voltage. Maxiu grid -voltage drive to obtain the required peak plate current at a given iniu plate voltage. Peak value of grid -voltage swing. Maxiu -signal, do plate current. Zero -signal, do plate current. Instantaneous peak plate current. Maxiu -signal, do screen current. Instantaneous peak screen current. Instantaneous peak grid current. Plate dissipation at axiu signal. Plate power input at axiu signal. Power output at axiu signal. Driving power at axiu signal. Screen input at axiu signal. 7. Check the values found in steps, 5, and 6 to deterine whether they are within tube ratings. Norally, they will be within ratings for AB1 operation. If they are not, a lower value of i'b (either in the negative -grid region or at a lower screen voltage) ust be selected and steps 2 through 7 repeated. 8. Calculate PO: PO = PI - PD. 9. Calculate Ibo: Ibo = Ibe / EC1 can now be found on the plate characteristics curves as the grid voltage where the plate voltage is Eb and the plate cur- rent is Ibo ec, E', = EC1 This value of Eg is the absolute value of E01 (the brackets ean ignore the sign) plus the algebraic value of ec (include the sign). If the original point in step 2 was selected on the zero grid -voltage curve, then ec is equal to zero and E'g = [E01]. 12. Calculate IC2: I C2 = 1'02/. E' i' e2 13. Calculate DP: DP = g2 (for AB1 operation, i'c1 = O so DP is zero). Class -AB, Tetrode or Class -B Triode Operation Class-AB2 tetrode and class -B triode operation provide ore power than class-ab1 operation, but have the disadvantage of placing stiffer requireents on the driver and grid - bias supply regulation. Calculation of typical operating conditions other than those given in the tube data sheets is slightly ore coplicated for class-ab2 and class -B operation than for class AB1, but is still relatively siple with the procedure outlined below: * 1. Make sure Eb is within tube ratings. 2. Assue a value of Ibe. A good starting point is at 3 (rated PD), Ibe Eb Check this value to see whether it is within ratings. If it is not, use the axiu rated value of Ibe. 3. Calculate i'b: i'b = 31bs. Fro the plate characteristics curves, select a value of ebin near the "knee" of the curves at which i'b can be obtained. Also record E02, ec, i'e1 and i'c2 for this point. 5. Calculate PD: * Calculation for fetrodes Is discussed; the triode case Is the soe except for the oission of the calculation of screen -Input power.

3 3 RCA HAM TIPS Deceber, 195 pd = I ss (Eb + 3ebin)' Ec2ic2 6. Calculate SI: SI - 7. Calculate PI: PI = EbIbs Check the values found in steps 5, 6, and 7 to deterine whether they are within the axiu ratings for the tube type. If the calculated values exceed the axiu ratings, choose a lower value of Ibs and repeat steps 3 through 7. If the plate dissipation and input are below the axiu ratings but the screen input is high, it ay be possible to choose a higher value of ebin in step (and repeat steps 5, 6, and 7) to get all values within ratings. The reverse case can also be applied. If all the values are well below axiu ratings, a higher value of Ibs can be chosen in step 2, and steps 3 through 7 repeated to see whether the operation is still within ratings. If so, this latter set of operating conditions will provide slightly ore power output. hen values that are slightly below the axiu ratings are obtained for plate dissipation, screen input, and plate input the corresponding value of Ibs represents the axiu value which can be used at the original plate voltage selected. Lower values of Ibs, which give ore conservative operation but less power output, can also be used. Once the value of Ibs is selected, the reainder of the calculation follows steps 8 through 13 shown for class AB1 operation. The driving power (DP) calculated does not include the rf tube and circuit losses. Consequently, for adequate perforance, at least ten ties this value of power should be available fro the driver. The following exaple illustrates the calculation of "typical operation" conditions for the class-ab2, CCS operation of the type 807 with an Eb of 600 volts: 1. The axiu plate voltage rating is 600 v. 2. Deterine Ibs book or tube bulletin) ; therefore, the axiu rated value of 120 a will be used as a first approxiation. 3. i'b = 3ibs = 3 (120) = 360 a.. Fro the 300-v EC2 curves, Fig. 1, select ebin = 90 v, and read ec (= + 12 v). Fro Figures 2 and 3, read i'c1 = 12 a, and i'c2 = 35 a, respectively. 5. PD = I ss 3 (rated PD) 3 (25) bs =.125 ap. Eb 600P These values copare reasonably well with This value is above the axiu -signal, the published values. dc plate -current rating (fro tube hand- (Continued on Page 5) [Eb + 3(ebin)] 10 [ (90)] = 26 w. Ec2ic2 300(.035) 6. SI - = w. 7. PI = EbIbs = 600 (.120) = 72 w. PD and PI are both above ratings, and a lower value of ebin at the required current cannot be found on the curves. Therefore, a lower value of Ibs ust be chosen; try a value of 100 a, and repeat steps 3 through 7: 3. i'b = 3 (100) = 300 a.. Fro the 300-v E,2 curves: ebin = 70 v, ec = + 7 v, i'c1 = 8 a, i',2 = 35 a. 5. PD =' 0 [ (70)] = 20.3 w. 6. SI - 300(.035) 2.6 w. 7. PI = 600 (.100) = 60 w. These values are within ratings; therefore, the reainder of the calculations can be copleted: 8. PO = PI - PD = = 39.7 w. 9. Ibo = Ibs = 20 a. 10. E,,1 (fro Fig. 1) = -35 v. 11. E'g = [Ect] + ec = 35 + (+7) = 2 v. i'c = Ic2 = = 8.7 a. 13. DP = E/gl '`2 2(.008) 2 2 =.17 w.

4 i Deceber, 195 RCA HAM TIPS 60 q 2 j 0.. if' 20 I I I TYPE 807 Ef=6.3 - VOLTS GRID -=2 VOLTS=300. E = OAPP Ib(=360) -- ec =1z) +10 -_ +5 all1í -GRID 11,1_ V_ E PLATE VOLTS 92CM Fig. 1. Average plate characteristics for the type 807 tube (grid -No. 2 voltage = 300). Ef'=8.3 VOLTS GRID -Nº2 VOLTS=300 I II 11 Ef = 8.3 VOLTS GRID -Nº2 VOLT S= í 10 CO 1.1 K o. 50 J 5 0 á I 0 30 K EC =+25 VOLTS +20 yi 120 a 100 J 5 80 N e O 80 fó I = CI( 12) ebin (=90) PLATE VOLTS PLATE VOLTS Fig. 2. Average control -grid characteristics for Fig. 3. Average screen -grid characteristics for the tyoe 807 tube grid -No. 2 voltage = 300). the type 807 tube grid -No. 2 voltage = 300).

5 Deceber, 195 RCA HAM TIPS 5 (Continued fro Page 3) Table I shows the axiu ratings and typical operating conditions for several popular RCA tubes in linear rf aplifier service for single-sideband, suppressed -carrier transission. It should be reebered that the typical operating conditions shown by the anufacturer (or calculated by the preceding ethods) are approxiate only. Minor adjustents are usually ade in actual operation by varying the grid bias or screen voltage slightly. In linear rf aplifier circuits for single-sideband, suppressed -carrier transission, it is particularly iportant to check the actual operating conditions when the transitter is first set up to assure that linear operation within the axiu tube ratings is being obtained. - ``" -`s Zsog Illn %122,--1-:,,,,--,:-,-- :.. o... sr. ` ;.<... ^ :;,7e a o dere M '......s33g.. ^' d.<212,e3 ; o a C2R2 R22a 1< RZ RC2RI , RR3o8 P... '...,7.R~3SS133333` e<22a5e ;ZPIISS 1~11I1`1áTRé~ oo I'~R~SIá31qIT91º $b «2 e2~1aimrrirr 111QIIMIIIII 8II "282M11R b 88eI 8558IM8IIM8MS8MRg11Si 1R5tYA31138Sio11883S8R81SdooI8II8 r 3 1.., d e KC o N R n N NM ",,:l M , 1. d ' - "-'1'" o«y,s S, N V2 R R / g 11, R I e 8 I Si II 38,. I 3 S , 1n r `_$ irt < 1 `a 1 < a < < < < a < < < 1 < á" 86

6 ., TMK Fro your local RCA distributor, headquarters for RCA receiving and power tubes. RCA HAM TIPS is published, bythe RCA Tube Dh ision, Harrison, N: J. It is available free of charge,fro RCA Distributors Joseph Pastor, Jr., 2KCN Editor Copyright 195 Rodio Corporation of Aerico FORM 357 REQUESTED Devices and arrangeents shown or described herein ay use patents of RCA or others. Inforation contained herein is furnished without responsibility by RCA for its use and without prejudice to RCA's patent rights. r ;1` \ 1 or r 9 KK y r,.--, 71 Iti T écr, : / u d Pgg& 2 ` 3 T e. -_á ó1á16r 0 t 11 1 `` 11 v1s> I'l IS,Q < y1.; 11 ' É r a i.' ` tó< ág='filgl s5c á{l $VQ r i3búó2ú NA -.t=1-- cu ác:ó Cr,!,í` a,«ó ye ó9óo:t 7.,211,-'. S Eá óóázrg6oú "u-,:. w Q.= _e3es 'á : st = ` ó :231-.'.1:'3.E129. a 1ao - I : st= Úú s e.l1s3vis1_ái U DúóCCx ij Í II YI El. I Ell