6 M Monoband Conversion For Heathkit SB-1000 Amplifier

Ron Berry, WB3LHD326 Sunset Drive, Bethel Park, PA 15102-1442: wb3lhd@arrl.net6 m Monoband Conversion forHeathkit SB-1000 AmplifierConvert this classic HF amplifier into a 6-m band work horse.After 38 years of chasing DX on the HFbands, I decided it was time to explore anew frontier and push the 50 MHz button onmy transceiver. I eagerly erected a 6 m twoelement quad antenna and started workingsome Maidenhead grid squares.1 I soonrealized that a bigger antenna and more than100 W of power would be most helpful. Icould afford a bigger antenna, but purchasinga 6 meter amplifier would be difficult. Thiswould be the perfect time to convert myold Heathkit SB‑1000 amplifier to 6 meteroperation. I had retired it 13 years ago fromHF duty because of a twice-blown bandswitch. It has been sleeping in my closetever since.Since I was not an expert on the subject ofamplifier conversions, and this was my firstattempt, I felt it best to contact other hamswho made the conversion successfully. I alsoconsulted the ARRL Handbook, RF amplifierbooks, articles in QST, and searched theInternet for additional information.2 Nowconfident, I began my HF to 6 metersamplifier conversion.For convenience, I assigned thecomponents with alpha-numeric designatorsdirectly from the original Heathkit SB‑1000amplifier schematic. You can download thecomplete manual from the Internet.3 Figure1 shows the schematic diagram of myamplifier.I needed to address the following detailsduring my conversion: strip the RF deck,change the plate and loading capacitors,modify the p-input circuit and p‑L outputcircuit, replace the parasitic choke, replacethe blocking capacitor, and replace the RFplate choke. I also replaced the internal RF1Notes appear on page 7output coax and safety choke.First, I removed the amplifier cover,knobs, vernier pointers, and lowered thefront panel. I saved all the hardware and partsin small plastic cups and labeled sandwichbags as I removed them. This saved me alot of trouble later on. I didn’t have to guesswhat screws or parts went where. Here arethe details.Stripping the RF DeckWith reference to the original schematicfrom the Heathkit SB-1000 manual, carefullyremove V1, the 3‑500Z tube, and set it aside.Remove the tank coils L7 and L8, inductorL9, plate RF choke RFC3, safety chokeRFC1, door-knob capacitors C26, C28 andC29, plastic standoffs, the band switch SW2,and the switch enclosure. When you removethe band switch enclosure do not cut off theinput matching section mini coax cables thatpass through the grommet. Unsolder thosecables to keep them as long as possible, theywill be used again. Mark the cables as inputand output so you don’t get them reversedlater. The input coax originates from pin 6of relay K1, and the output coax connectsto the 3‑500Z cathode capacitors C18 andC19. Remove the band switch with all thecapacitors, and the tunable slug inductorsfrom the enclosure, except L6, the original10 m inductor. L6 is the inductor with theleast number of turns on it, and is located atthe top next to the grommet hole. We willmodify and reuse inductor L6 as describedlater in the “p input circuit” section.Plate and Loading CapacitorsThe plate and loading variable capacitorsmust be replaced but you can swap theupper plate capacitor for the bottom loadingcapacitor. First remove the bottom loadingcapacitor C31 and set it aside for anotherproject. Now remove the top plate capacitorC27 and mount it in the bottom loadingcapacitor position. When doing this you mustreuse the original front mounting hole anddrill a new rear mounting hole. Before youmeasure and drill the new rear hole, be surethe capacitor shaft is fully set into the vernierdrive and is straight.A new upper plate capacitor will requireless capacitance than the original, and ahigher voltage rating. I purchased a Cardwell#154‑11‑1 (9 – 38 pF) 4 kV air variablecapacitor. There are several sources for theseand they vary greatly in price. To mountit, you must reuse the rear mounting holeand drill a new front mounting hole. Becareful when drilling the hole, as the highvoltage power supply is directly behind thepanel. Again, make sure that you mountthe capacitor straight. The newly mountedCardwell capacitor has the same shaft heightas the original capacitor, but the shaft will notreach the vernier drive. To fix that, I purchaseda piece of 0.25 inch O.D. aluminum shaft. Icut off a piece approximately 2.5 inches long,and connected the two shafts together using acoupler that I had in my junk box. Before youcut the aluminum shaft, measure it carefullyto be sure it’s long enough to fit fully into thecoupler and into the vernier drive.The p Input CircuitThe original Heathkit input matchingcircuit is a p‑type configuration that uses avariable inductor and two fixed capacitors.Some hams have opted to modify their6 m conversions to a T‑type network thathas a variable capacitor and two fixedinductors. I wanted to retain the p circuitfor its superior harmonic suppression andQEX May/June 20163

improved efficiency. I also decided to makeit broadband enough to cover the entire 6 mband. This way no adjustments would benecessary with changes in frequency, and Icould keep the original enclosure without theInput PI Matching Sectionband switch. I started by modeling the circuitusing an online Input Impedance MatchingL6Designer.40.138 μHInput Coax toC18In Designer, enter 50 W for SourceRelay K1, Pin 60.01 μFResistance, 110 W for Load Resistance,52 MHz for Frequency, and Desired Qof 2. Source Resistance is the exciterC17C16output impedance, Load Resistance is the75 pF62 pF3‑500Z input impedance, and Q determinesC19bandwidth of the circuit. After modeling0.01 μFthe circuit, I built and tested it using a HPnetwork analyzer. I added a 10 pF capacitorin parallel with the 110 W resistor at thePC1output of the circuit to better simulate the1 Turn3‑500Z input impedance. The final circuithas the following values: C16 is 75 pF, L6Filament To T1is 0.138 mH, C17 is 62 pF. I measured less47 Ωthan 0.2 dB insertion loss, and return loss2Wgreater than 30 dB, which is equivalent to aVSWR of less than 1.1:1 across the entire 50– 54 MHz span of the 6 m band.Output PI-L Matching SectionModify the 10 m band inductor L6 to theneeded inductance as follows. Unsolder theC24Output Coax toL1L2top end of the coil from the lug, undo one0.002 μFRelay K1, Pin 20.200 μH330 nH10 kVturn, trim it to length, tin and re-solder it backon the lug. You should now have 2.75 turns(Figure 2). Apply some Q‑Dope if necessaryC27C31RFC1RFC39-18 pF20-250 pFto hold the coil winding in place.Z-5042 Turns #245 kV1 kVSolder capacitors C16 and C17, and the7 μH1" Dia.mini-coax cables between the L6 coil lugsPlateLoadand ground, see Figures 1 and 2. The inputcoax and the 75 pF capacitor C16 are onC230.01 μFthe left and below inductor L6. The outputcoax and the 62 pF capacitor C17 are on theright. I fastened the ground lug to one of theB unused screw holes, and covered over the 3000 V DCQX1605-Berry01remaining holes with copper tape to helpkeep the enclosure RF tight. If you don’thave copper tape, some aluminum duct tapeFigure 1 — Heathkit SB-1000 6 m band conversion schematic. See Table 1 for the projectbill of materials.will work as well. [Copper tape used for RFhas copper filings embedded in the adhesive,so there is dc contact between the tape and itsTable 1.Bill of Materials: Digi-Key, www.digikey.com; RF Parts, https://www.rfparts.com/; target. Aluminum duct tape does not make dccontact with its target through the adhesiveeBay, www.ebay.com; Amidon, www.amidoncorp.com/,— Ed.].Mouser, -408L1RFC1RFC3V1Misc.Misc.Misc.Misc.Part75 pF 500 V mica capacitor62 pF 500 V mica capacitor0.002 mF 10 kV capacitor9‑38 pF CapacitorElectrolytic Capacitors0.25” OD Copper Tubing7.0 mH safety choke15’ of #24 Thermaleze wireTaylor 3‑500ZG TubeQ-Dope, GC Electronics0.25” OD Aluminum ShaftHyper Plate CapSilver Plating SolutionSourceDigi-KeyDigi-KeyRF PartsCardwellRF PartseBayMouserAmidonRF PartseBayACE Hardware73CNCAmazonPart 211T450R4N/A588-Z50EAWG#24 HAPT3-500ZG-TAYN/AN/ACap for 3-500ZMedallion BrandMy total cost4QEX May/June 2016Cost 1.82 1.82 2.95 50.00 159.60 10.00 5.61 3.00 169.95 10.00 5.00 39.99 16.99 476.73The p‑L Output CircuitThe original Heathkit output tank circuitwas a p configuration. I was concernedthat the original p circuit would not haveenough second harmonic suppression tomeet the FCC Part 97 requirements of60 dB suppression, so I decided to use a p‑Lconfiguration. A p‑L network has a broadertuning range, higher Q, and greater harmonicsuppression compared with the standard pnetwork. I started by downloading the PI-ELoutput network designer.5Make L2 from #12 AWG copper wire

stripped from a piece of 12‑2 Romex . Windfour turns, 0.75 inches long around a one inchO.D. form. The inductance of L2 should beabout 330 nH. One end connects to the loadcapacitor and the other end to the RF outputcoax center lead and the safety choke. I hada ceramic standoff insulator in my junk boxto secure and insulate these connections, seeFigure 3.I made L1 from 0.25 inch O.D. thin wallcopper tubing. Thin wall tubing is mucheasier to form than solid conductor. Windfive turns, five inches long, around a 1.50inch O.D. form, see Figure 4. A video I sawon YouTube suggested to fill the tubing withtable salt and crimp the ends tightly. Thesalt will prevent the coil from flattening andkinking as you wind it. Be sure to knockthe salt out when finished. I cut the ends tolength and mashed them flat with a hammer.I then drilled holes in the flats and fastenedthe ends between the variable capacitors. Theinductance of the finished coil will be around0.20 mH. I silver-plated the coil with silverplating solution.6 This not only looks nice,but will help make it as efficient as possible.The Parasitic ChokeThe original parasitic choke PC1 was4.5 turns of wire wound around a 100 W2 W resistor and was designed for an HFamplifier. I tried replacing it with fourturns on a 47 W 2 W resistor. I measureda reactance of 16 W at 54 MHz. I felt thechoke was far too aggressive so I wound amore 6 m friendly version. It has only oneturn of #16 AWG buss wire wound aroundthe 47 W 2 W resistor. This presented 1 Wof reactance at 54 MHz but still has enoughroll-off above the 6 m band to be an effectiveparasitic choke.Figure 2 — Modified inductor L6 and routing of the input coax (left) and output coax (right).Capacitors C16 (left) and C17 are below L6.The Blocking CapacitorI wanted a higher voltage rating on theblocking capacitor to provide a higher marginof safety, so I replaced the original twoparallel 0.001 mF 6 kV blocking capacitorsC24 and C25 with a single 0.002 mF 10 kV,capacitor C24 (Figure 1).RF Plate ChokeThe original RF plate choke RFC3measured 120 mH inductance, which hasgood reactance for the HF bands, but waymore than necessary for 6 m, and it mighthave a self-resonance somewhere withinthe 6 m band. I removed all the original coilwires but reused the original 1 inch diameterceramic form. I close-wound 42 turns of#24 AWG heavy armored polythermalezetop-coated (HAPT) magnet wire about0.25 inches down from the top, and securedthe coil turns to the ceramic form withFigure 3 — Inductor L2 mounts on a ceramic stand-off and connects to the RF output coax(below and left) and to choke RFC1. The other end of RFC1, and the coax shield connect tothe ground lug at the bottom.QEX May/June 20165