CQ Reviews:The HeathkitSB-200 LinearAmplifier

BY WILFRED M. SCHERER,. W2AEF

T HE Heathkit S8-200 Linear Amplifier hasbeen around for quite a few years; never­theless many requests a re still being receivedfor information on this piece of gear.

In view of this situation and inasmuch asCQ has never reviewed this table-top kilowattjob (ma inly due to the fact that its forerunner,the Modet HA-1 4 "KW Kompact" mobileversion had already been covered') . we shallat this time discuss the S8-200.

T his model is rated at 1200 watts p.e.p.input on s.s.b. and 1000 watts on c.w. whichis available with 100 watts r.f. drive such asthat obta ined from the Heath S8-400140 Itransmitter. the Heath S8 -1 00I 101 and HW­100 transceivers or other exciters in the samepower class. T his does not preclude its usewith lower- or higher-power drivers as wewill see later.

The S8-200 is a completely self-containedun it with a built-in power supply for opera­tion from 120/240 v.a.c., 50/60 c.p.s. It isquite well compacted in a 6%" X 14Y8" X13 'Ml " (H. W.O.) wrap-around cabinet withatt ractive styling to match the Heath S8­se ries of gear and thus makes a neat-lookingjob requir ing relatively little desk space. Itweighs 35 Ibs.

Full coverage is obtained on the 3.5-30 mcamateur bands using bandswitched input andou tput circuits . The input impedance isnom inally 52 ohms with broad-band pre­tuned-input circuits that need no tuning ad­justment during operation.

The output is dcsigned to work into 50/72­ohm impedances that present an s.w.r. within

*Technical Director, CQ.' C Q Reviews: The Heathkit HA-14 "KW Kom­pact" Linear Amplifier, CQ. February 1966,page 52.

62 • CO • June, 1969

2: I. This is realized with a Pi·network fur­nished with a variable loading control. Abuilt-in reflectometer provides s.w.r. readingsup to 3:1.

DetailsReferring to the schematic diagram at fi g.

I, the amplifier is an essentially grounded­grid affair and is opera ted in Class 8 using apair of 572-8 Or T-1 60-L graphite-anodetriodes connected in parallel.

R.f. drive is applied to the tube cathodes(fi laments) through individual matching net­works (pi or L types) tuned for each band.Use of such tuned circuits here ensures betterefficiency with less distortion than with apassive input. RFC, is a bifilar-wound chokethat maintains the tube filaments above r.i .ground and prevents the r.f-drive powerfrom being bypassed to ground through thefilament-suppl y windi ngs of t he powe rtransformer.

The Pi-network is made up using two in­ductors, L , and L,. L , is tapped and is usedalone for the 10- and IS-meter bands. L, isadded for the other bands for which it isappropriately tapped. C. and Co are switchedin only for 80-meter band operation.

The ret1ectometer is an in-line trough-linetype. The panel meter can be switched toread the relative forward power (for whichthere is a sensitivity control) or the reflectedpower for which the meter is calibrated interms of s.w.r, up to 3:1. The meter also canbe switched to read 0-100 rna grid-eurrent,0-1000 rna plate current or 1500-3000 v.plate potentia l.

A.L.e.The a.l.c. setup is a departure from the

usual in that its operation is based on an r.t,

See page 11 0 for New Reader Service•

L,L ,

--; ' 5 j-­..L1OO ...L..

C.>00

4.7 M;.02 .02

~ 4."L .02 .02

- T

C,

~C.[',.

+10 •.

.0 2

R.F. C. I

"02

"

.0 2

PC' c-<l--

.0 2

C,R,

"

2.

- 120 w.

........"!" -- - - - - - - - - - - - - - - - - ------------------ - - - - - - ---,,I '----,III,,

r---JII

i 28~"IIIt Rl,

Fig. I -Schematic for the 58·200 amplifier (powe r-supply and metering ci rcuits not shown). Thebondswitch alters L3, C7 and C8 to form impedance-ma tching Pi or l input networks fo r the va rious

bands. O the r details are in the text.

vo ltage. rather than on an a.j, voltage that isdeveloped during modulation when an amp­lifier starts to overload or draw grid current.

As shown at the schema tic. the tu be gridsarc not heavil y bypassed to ground (by C,­C" and some r.f. voltage thus appears at thegrids. C:I and C.. comprise a voltage divideracross the grid of V, from which a samplevoltage is derived and rectified by D,. Theresulting d. c, voltage provides the automaticbias for reducing the gain of the a.l.c-con­trolled stage in the exciter.

D l is reverse-biased into non-conductionby an-a.l.c. th reshold potential of + 10 voltsapplied to its cathode from a tap on the plate­supply bleeder. When the r.f. voltage appliedto D ,'s anode exceeds I0 volts. the diode con­ducts and rectifies the r. f. to supply the neces­sa ry d.e. voltage fo r the a.l.c,

The threshold has been set for a.l.e. oper­ation at the instant the r.f. voltage at the tubegrids reaches the point just below that whic hoccurs when the tubes are overdriven.

The advantage of this arrangement, overthe conventional system. is that a.l .c. opera­tion is not dependent on some overdrive be­fore it takes hold. thus allowing it to beeffective prior to the occurrence of any flat­topping or overdrive.

Since the a.l.c. is not dependent on an a. I.component during modulation. the a.l.c.voltage can be produced even with steady-

state or c.w. drive.

Tra nsfe r Circu itsThe r. f. circui ts arc switched with a d.p.d.t,

antenna-cha ngeover re lay which is operatedby the v.o.x. relay in the exci ter. Operation isas follows: During receive. a bias of - 120volts is applied to the tube grids through therelay coil. The bias cuts olf the plate current.eliminating the possibility of the generationof hash within the tube that might he heardin the receiver.

Durin g this time the re lay is not energized,because the exciter- re lay contacts are notclosed, nor do the tube grids draw currentthrough the relay coil. Under this condition.the antenna is connected, through the normal­ly-closed relay contac ts, to the antenna ter­minal of the exciter if it is a transceiver ; o r tothe antenna-connector of a separa te t ransferrelay thaI would normally be used with areceiver-transmitter setup. This allows signalsto be received.

On transmit, the contacts on the exciterv.o.x. relay ground the top end of the amp­lifier-relay coil th rough R,. energizing therelay from the - 120-voll bias source. T henormally open contacts then close, connectingthe exciter-setup output to the r.f. input ofthe linear and transferring the antenna to thelinear's output.

At the sa me time, the resistance of the

See page 110 for New Reader Se rvice June, 1969 • CO • 63

l

•

Top view of the 58.200 . Enclosure for power­supply components is at upper left with circuit.breakers on its right wall. The amplifier enclosureis at the right. The input networks a re in thecente r foreground with the bifi la r fil a ment chokebelow the tube sockets at the right. A perforated

cover fits over the enclosures.

relay coil and R I form a voltage divider soproportioned th at the current drawn throughthe circuit produces a drop of 2 volts acrossR I. This appears as a negative voltage at thetube grids and thus provides operating biasfor the amplifier tuhes.

When the power switch on the 58-200 isturned off', there is no bias voltage to operatethe cha ngeove r relay , so the antenna remainsconnected directed to the exciter setup fornormal low-power operation witho ut theli near.

The amplifier tubes have instantaneous­heating filaments. so along with the use ofso lid-state rectifiers in the power supply whichalso are instantaneo usly-performing devices.linear-amplifi er operation may be had assoon as the power switch is turned o n. makingit possihle to instantaneo usly switch back andforth between low power from the exciteralone or high power from the amplifier.

Power SupplyOperating power is obtained from a single

transfo rmer that has three secondary wind­ings; o ne for 6.3-volt fil ament power. one fora - 120-volt bias so urce and o ne for a platesuppl y of 2500 volts. A full -wave voltagedoubler, consisting of eight silicon diodes ineach leg. is used for the latter along with 21

64 • C':l • June, 1969

mf of filter capacitance provided by six 125mfd, 450 v., electrolytic capacitors connectedin series. Voltage-equalizing resistors are in­sta lled across the capacitors. They also serveas a bleeder.

The power transformer has two primarywindings that arc connected in parallel forl20-volt opera tion. or in series for 240-voltoperation. An 8-ampere circuit breaker isinstalled in one leg of each winding.

Co nstructionThe 58 -200 is assembled on a chassis on

top of which are two shie lded compartme nts.One encloses the amplifier proper, the tubesfor which arc mounted horizontally in amanner that elimina tes the possibility of agrid-filament short due to filament saggingthat could occur after long usc.

A small motor-driven fan is located be­neat h the tuhes and provides cooling byblowing air upwards against the portion o feach tube envelope opposite the tube plate.Perforations on the chassis deck and in theenclosure cover and the amplifier cabinetprovide an air-flow path for cool-air intakeand warm-air exhaust. The fan goes on oroff when the power switch is operated.

The variable capacito rs for the pi-outputcircuit arc ceramic-insu lated. The one forloading is a two-gang job somewhat huskierthan the usual t.r.I. receiving type. The plate­tuning capacitor is a wide-spaced high-volt­age type. L, is a se lf-supporting inductorwound with # 10 wire, L:! is wound with # 14wire on a fiberglass form.

The tuned circuits for the cathode inputare installed on the rear of the amplifier com­partment on which the tube sockets also arclocated . Slug-tuned inductors arc used alongwith mica fixed capacitors.

The bifilar fila ment choke is wound with# 14 wire on a ferrite core, the use of whichcuts down the number of turns o therwise re­qu ired. raising the Q and holding down thewire resistance fo r a minimum fi lament volt­age drop.

The bandswitch employs ceramic-insu latedsect ions. Those for the output tank have dualcontacts for adequately handling the r.f. cur­rent at the high power levels involved.

Except for the transfo rmer, the power­supply componenls are located in a secondenclosure and are assembled on a pr inted­circuit board. The circuit breakers aremounted on one wall of this enclosure in away that allows them to be handily reset

See page 110 for New Reader Service

Bottom view of the 58· 200. The printed-circuitboard for power-supply pa rts it a t the lowerleft. The reflectometer trough is at the ce nte r ofthe rea r apron. The blower motor is neor rig ht

cen ter.

It also may be found that the tuning ofthe excite r may be slightly off when the amp­lifier is turned on. because the S.W .f. (or re­flected impedance) seen by the exc iter willva ry btween I : I and 3: I due to the fact thatthere is a varia tion in the nominal inputimpedance of the amplifie r, depending on theband, and the portion of the band used. Thereflected impedance also will change withvarious line lengths between the two un its.In some cases, sligh tly retouching the exci te rtuning may be needed .

PerformanceWith a monito red 100 walls of steady­

state drive and with a power-line potential of120 or 240 volts, measured under load, theplate-power input of the 58 -200 at fu ll tune­up amounted to 1150 watts with an r.f', out­put of 775 watts. T his was essentially the sa meon all bands. With 100 watts of s.s.b. p.e.p.drive. the power input and r.f. output were10% higher, thus adeq uately fulfilling therat ing of 1200 watts p.e.p. input.

Under these conditions an oscilloscopetrapezoid display indicated excellent linearity;while observations on a spectrum analyzershowed the 3rd-order distortion products tobe within Heath 's specification of - 30 dbrelative to 1000 watts p.e.p. input, Actually,this was a limitation imposed by the excite rused at the time, T he 5th-order product was- 45 db.

•

•

,

d ' h

•

- --,....-,~~

- ----

•••

-----

th rough access holes when the hinged coverof the cabinet is raised.

T he reflectometcr is located beneath thechassis and on the inside of the rear apron. Itis built in a U-shapcd metal trough with thetransmission line conductor and the pick-upwires held in place by nylon spacers. Oneside of the trough is open whic h, together withthe fact that the antenna re lay also is exposedunder the chassis (as is a long unshieldedlead below the chassis between the band­switch and the loading capacitor), might allowsufficient r.f. to be induced into power leadsto cause TVI , even through the extemalleadsare bypassed. The job otherwise is pretty wellboxed in with shieldi ng to minimize the pos­sibility of TVI difficulties.

AssemblyNo problems were encountered with the

assembly work, thanks to Heath's standard­ized easy-to-follow instruc tions and proced­ures. Upon its completion, the amplifier func­tioned properly in all respects "right off thebat." A total of 12 hours was required to dothe job; however, for a less ex periencedbuilde r, 15 hours or so may be needed.

OperationTuneup is a simple procedure during which

time you don't have to hurry the adjustmentsor wo rry about burning up the tube grids assome times is the case with high-power rigs.Furthermore, the range of the tuning con­trols is so proport ioned that the adjustme ntsare rather broad, th us making them non­cri tical.

With the linear turned off, the excite r istuned up for maximum output in the norm almanner. With such drive applied, you thenclose the amplifier's power switch and adjustits plate and loading controls for maximumr.f. output as indicated by the meter which isfirst set for relative-power readings. You'renow ready to go with high- or low-powertransmissions available simply by flipping the58 -200 power switch on or off (this switchis a rocker type).

During initial tuneup with the exciteralone, it is best to check the s.w.r, of the loadbefore turning on the linear. to make sure theload presented to the amplifier will fall withinthe 2: I matching range of the linear amplifier.This is especially important fo r proper load­ing as described later. If needed, the s.w.r.,as seen by the amplifier, often can be broughtdown by altering the length of the trans­mission line accordingly.

See page 110 for New Reader Service June, 1969 • CO • 65

With 200 walls drive, the amplifier couldbe pushed to 1500 walls input with 1000walls output. With s.s.b. peak power at theselevels, fl attopping was not evidenced, exceptfor on ly a slight tendency to rou nd off thepeak of a trapezoid display. At the sametime, a spectrum analysis showed the 5th­order product to rise to ncar the 3rd-orderlevel which itself did not change significantly.

At these higher-power levels, no evidenceof excessive tube heat ing or deterioration wasfou nd after periods of on-the-air operationwith s.s.b. or during rigid bench tests; never­theless, it must be kept in mind that suchoperation exceeds Heath's specifications andrecommended power levels. as well as thetube ra tings. In this respect. Heath's specifi­cations ca ll for a maximum p.e.p. input of1200 walls (with 100 walls drive) using con­tinuous voice modulation on s.s.b.: while forc.w, the input is ra ted at 1000 walls with a50-pe rcent duty cyc le (key-down time not toexceed 5 minutes) ,

With low-drive levels of 45-50 walls, suchas obtai nable from the Heath HW-1 6 c.w.transceiver. the amplifier will put out 450­500 walls. From the power levels given here­in, it may be noted that the apparent ga in inpower output decreases with drive levels(from 10 db with low drive to 7 dh with highdr ive). This is primarily du e to feedth roughof the r.f-dri ve power which bears a sma llerra tio to the power produced by the amplifieritself at high drive levels than with lowerdrive.

In order to maintain good li neari ty, par­ticularly with high-power drive levels, duringtuncup the amplifier must be loaded to be­yond the maximum-output point; that is.past th is spot to where the output starts todrop off. This also will be indicated by a verybroad or hardly noticeable plate-current dipat resonance. The plate current under theseconditions may be higher than recommendedin the manual; however. the tubes will take it.at least during s.s.b. operation.

Insuffi cient loading can result in flattoppingor other de teriora ting effects with high peaklevels, especially on the 40-, 20- and 10-meterbands where the tank impedance for highpeak levels apparently is not quite as favor­able as on 80 and 15. As a matter of fact, itwas found that with proper loading as justdescribed. usc of the a.l.c, was not required.since no fl attopping or other bad effects showup, especially at the recommended powerlevels.

66 • CO • June, 1969

On the other hand, where the exciter de­livers more than 150-200 walls, the use ofa. l.e. would be desirable to limit the drive.Such exci ters usually will require a highera.l.e . voltage than that normally provided bythe 5B-200. T his may be realized by reduc­ing the a. l.c, threshold which can be done byconnec ting the ca thode of D l to ground, in­stead of to the lO-voll tap on the power­supply bleeder.

Adjustments both for optimum loadingand a .l.c. operation arc best made while ob­serving a trapezoid display on an oscilloscope(such as the Heath HO·I O or 5B-6 1O Mon­itor-scopes) by sampling both the d . inputand ou tput of the amplifier. Either a two-tonetest signal may be used or speaking into themicrophone will suffice.

Observat ions thereby ca n be much moreaccura tely and easily made than with an r.f. ­envelope display. Furthermore, with thetrapezoid d isplay obtai ned as described,you're looking at on ly the amplifier perter­mance towards its proper adjustment, thuseliminating possible confusion with the ex­citer preformance as could otherwise be thecase with an r.f.-cnvelope displ ay.

One fin al admonition-with a I20-volt sup­ply line the amplifie r draws up to 16 a .; whilewith a 240-volt source the current is 8 a.Therefore, for best regulation and overallperform ance, a 240-volt source should beused wherever possible. Also make sure thisis a 230·240 line and not a 208-volt line.

As may have already been judged, theHeath 5B-200 Linear Amplifier can reall y" take it" as well as "dish it out" while stillmaintaining a good-quali ty signal. It is a buyhard to beat at the kit price of $220. The pro­ducer is Heath Company, Benton H arbor,Michigan 49022. - W1A EF

" Actua lly you'll be the la,t QSO fram thisaTH .. ."

See page 110 for New Reader Service