The Pine Board Project

The complete Pine Board Project. From left to right, the power supply, microphone preamplifier, and the transmitter. A return to yesteryear with a 5 W AM transmitter.

Bob Heil, K9EID great for learning because I could fol- and the like. I became fascinated by Among my most enjoyable and low the circuits visually and have a the signals Richard Beckett, WØBVT, meaningful pursuits as a new ham better understanding of how they put on the air with his “Pine Board” back in the mid-1950s was building worked. AM transmitters that run 3 or 4 W audio amplifiers, modulators, VHF using only vacuum tubes. Each I love to share this great hobby, and I transmitting and receiving convert- week, he would have a different tube was thinking of ways to get new ers, and VHF transmitters. Many of lineup, RF power level, and audio licensees attracted to the pleasures these early projects were assembled quality. of building. I wanted to come up with on pine boards. It was simple to do a project that would be a great It occurred to me that his Pine Board because the devices were laid out as teacher and something that would transmitters were exactly what I was shown in the schematic diagrams. not just be for static display, but a looking for. We made the projects As time went on, my projects usable transmitter. part of our weekly Ham Nation online advanced into scores of Heathkit show, and with Richard’s help, the Here in the Ozarks, the MOKAM products, along with aluminum Bud Pine Board transmitters were a hit. (Missouri, Kansas AM) group meets boxes that required learning about each weekday on 3885 kHz. Each Greenlee punches and how to work Gathering the Parts Monday, we converse with our low- with metal. Looking back, the proj- One of the first concerns was the power vintage AM transmitters — ects built on scraps of wood were availability of parts. Many old-timers Johnson, Harvey Wells, Heathkit, have junk boxes that are overflowing

Safety Concerns The design of the Pine Board Project is an homage to an earlier time when amateurs occasionally built homebrew projects with exposed circuitry. In those days, it was understood that working with high voltages entailed a certain level of risk. The need to be mindful of the dangers of electricity is just as impor- tant today, but especially so with the Pine Board Project. This project exposes the builder to potentially lethal voltages, and must be oper- ated with great care. Do not allow children, pets, or any unsupervised individuals other than yourself in proximity to this project. While the version of the Pine Board Project shown in this article uses an open construction architecture, for safety considerations we recommend packaging the entire transmitter, preamplifier, and power supply in an enclosure of wood, metal, or plastic to prevent contact with the circuitry. — QST Editorial Staff A close-up view of the power supply module.

www.arrl.org January 2018 31 with components, but that’s not so for Modular Design (The one we use most is stock num- most amateurs today. After a bit of The project is built in three separate ber 179408, the 5 × 7 inch rectangle research, I came up with several modules: the high-voltage supply, the plaques.) Give each board a light parts companies that could supply microphone preamplifier with two- stain, and you will be ready to build. crystals, transformers, chokes, sock- band equalization, and the two-band ets, resistors, capacitors, and hard- transmitter. Each of these modules The Power Supply ware needed to build an 80- and are designed with terminal strips that The Hammond 261G6 transformer 40-meter AM tube transmitter. Antique are wired to carry the B+, 6.3 V ac fil- has two secondary windings: 250 V Electronic Supply went so far as to ament voltages, and the ground con- at 130 mA and a 6.3 V winding for fil- put a Pine Board Project kit together nections. If you only want to work ament (see Figure 1). The P-QBR-34 with almost all the parts needed. The CW, you will not need the micro- bridge rectifier provides +350 V dc kit sells for $49.18 and is available at phone preamp module, just the from the full-wave rectifier circuit. This www.tubesandmore.com/products/ power supply and transmitter. is followed by two 20 µF capacitors pine-board-am-transmitter-kit-ham- along with a voltage divider com- nation-bob-heil. The pine boards are available, cut to prised of two 3.3 kΩ, 2 W resistors. size with finished edges, from Hobby One hundred and twenty volts feeds Lobby at www.hobbylobby.com. the mic preamp, while the +350 V feeds the transmitter. To complete the power supply, there is an ON/OFF T1 +350 V dc QS1801-Heil01 switch in 110 V primary, as well as a D1 110 V ac R1 pilot lamp connected to the 6 V fila- S1 B+ TX w/ground 33 kΩ F1 B+ ment leads. Preamp Hot 33 kΩ C2 6.3 V C1 R2 Neut. 20 μF ac Microphone Preamplifier 450 V Gnd and Equalization Gnd A single 12AX7, which has been the 6.3 V pilot light vacuum tube of choice for audio- philes for decades, works well in this circuit (see Figure 2). The first stage Figure 1 — The Pine Board Project power supply diagram. is a high-gain preamp that feeds the C1, C2 — 20 µF, 450 V electrolytic capaci- S1 — SPST switch tors T1 — Hammond 261G6 transformer second stage of the 12AX7, provid- D1 — Bridge rectifier (tubesandmore.com, (tubessandmore.com) Primary: ing sufficient output to drive the P-QBR-34) 120 V; Secondary: 250 V and 6.3 V. F1 — Fuse, 3A 6V6-based Heising modulator of the R1, R2 — 33 kΩ, 2 W resistors transmitter.

12AX7 B+ 0.001 C5 C10 4 9 5 R5 100 k Bright R10 6.3 V ac 10 μF 100 k J2 450 V C12 Tube filament wiring Output V1 S2 Flat V1 J1 1/2 12AX7 1 0.01 C6 1/2 12AX7 6 0.047 Mic in C4 Treble R4 R6 L1 0.0047 (low Z) 100 k 50 k 2 R8 Gain C11 2.5 mH 50 k 7 0.001 C3 C7 0.1 R3 3 8 0.001 1 M R7 C9 C8 0.1 10 k Bass R9 1 k 10 μF Decimal values of capacitance are in microfarads (µF); others are in 25 V picofarads (pF); Resistances are in ohms; k = 1,000, M = 1,000,000. QS1801-Heil02

Figure 2 — The preamplifier and audio equalizer diagram. C3, C5, C11 — 0.001 µF ceramic disc C10 — 10 µF, 450 V electrolytic capacitor R6, R8 — 50 kΩ potentiometers capacitor C12 — 0.047 µF ceramic disc capacitor R7 — 10 kΩ potentiometer C4 — 0.0047 µF ceramic disc capacitor J1, J2 — two conductor phone jacks R9 — 1 kΩ, ½ W resistor C6 — 0.01 µF ceramic disc capacitor L1 — 2.5 mH RF choke S2 — SPDT switch C7, C8 — 0.1 µF ceramic disc capacitor R3 — 1 MΩ, ½ W resistor V1 — 12AX7 vacuum tube (SND Tube C9 — 10 µF, 25 V electrolytic capacitor R4, R5, R10 — 100 kΩ, ½ W resistor Sales, www.vacuumtubes.com)

32 January 2018 www.arrl.org With the addition of only eight com- QS1801-Heil03 To antenna ponents, we add a two-band equal- C17 L3 izer with a treble boost that is a welcome touch for the low-power 500 pF transmitter. There are not many parts 6AG7 8 C16 365 pF to the microphone equalization pre- 0.005 μF V2 C13 1 amp, but the audio from this single- 6 6 stage 12AX7 is stunning. I borrowed L2 FT243 type 4 2.5 mH Y1 C14 the design from a guitar preamp I 3.885 MHz 5 100 pF built for many of our musician friends 7.290 MHz R12 1 during the 1970s. Two ⁄4-inch female 22 kΩ C15 1 W 0.001 μF microphone connectors are mounted R11 56 kΩ on the pine board: one for the micro- phone input, and the second to carry L4 the mic preamp output for the trans- 3 V3 4 H, 50 mA 6V6 mitter. R15 Audio 4 B+ 5 from (350 V dc) 22 kΩ preamp The Transmitter 8 1 W R13 10 μF The transmitter (see Figure 3) is C19 6AG7 6V6 designed to work on 80 and 40 100 kΩ C18 R14 450 V 47 μF 2 7 2 7 100 Ω 6.3 V ac meters. The plate coil is from the 50 V parts department of MFJ. Our circuit Gnd Tube filament wiring uses 41 turns of their 404-0811-1 inductor. The coil is mounted so that Figure 3 — Pine Board Project AM transmitter diagram. the last 14 turns can be shorted for C13 — 0.005 µF ceramic disc capacitor R11 — 56 kΩ, ½ W resistor resonance on 40 meters. This is our C14 — 100 pF silver mica capacitor R12 — 22 kΩ, ½ W resistor “band switch.” Change the FT243 C15 — 0.001 µF ceramic disc capacitor R13 — 100 kΩ, ½ W resistor C16 — 365 pF variable capacitor R14 — 100 Ω, ½ W resistor crystal to 7290 kHz, short the last 14 C17 — 500 pF silver mica capacitor R15 — 22 kΩ, 1 W resistor turns, and you are on 40 meters. It C18 — 47 µF, 450 V electrolytic capacitor V2 — 6AG7 vacuum tube (SND Tube Sales, C19 — 10 µF, 450 V electrolytic capacitor www.vacuumtubes.com) works great. L2 — 2.5 mH RF choke (Radio Daze, www. V3 — 6V6 vacuum tube (SND Tube Sales, radiodaze.com) www.vacuumtubes.com) The 6AG7 tube is configured as a L3 — Inductor, 1¾ inch diameter, 41 turns at Y1 — Type FT243 crystal. 3.885 or 10 turns per inch (MFJ 404-0811-1) 7.290 MHz would be best for AM. low-powered crystal-controlled elec- L4 — 4 H, 50 mA RF choke (Antique (www.af4k.com) tron-coupled oscillator. In this circuit, Electronics, www.tubesandmore.com) the screen grid serves as the “plate.” Thus, the cathode, control grid, and screen grid are connected as a tri- The transmitter is Heising modulated ode oscillator circuit. The only cou- with a 6V6 Class A modulator. When pling between the oscillator and the an audio sine wave, or a complex load is through the stream of the audio waveform, coupled to 6AG7. Due to the “electron-coupling,” Pin 5 of the 6V6 drives any changes in the load have very lit- the grid positive, cur- tle effect on the frequency-controlling rent through the choke circuits, so the oscillator frequency reduces the plate volt- remains very stable. age of the 6AG7. When the audio waveform The plate circuit is coupled to the L drives Pin 5 of the 6V6 network through a 500 pF coupling more negative, the plate capacitor. The purpose of the L net- of the 6AG7 goes more work is to match the high-impedance positive. The process results plate circuit of the 6AG7 to a 50 Ω in amplitude modulating the load. Maximum power is transmitted 6AG7 carrier. The results on when the impedance of the source the morning MOKAM nets have matches the impedance of the load. been excellent. The transmitter With the +350 V from the power produces clean audio with proper ­supply, the 6AG7 output is 5 W. equalization, and even with just The AM transmitter module.

www.arrl.org January 2018 33 nets have been excellent. The full-bod- ied audio with proper equalization, and the 5 W QRP AM output, is amazing. I A rear view of the microphone hope to be able to work you with your preamplifier and audio equalization module. own Pine Board Project!

My thanks to Ed Pfeiffer, WB4OJM, for his assistance with this project.

Bob Heil, K9EID, licensed in 1956 as KN9EID, began his career as an avid experimenter and builder of VHF equipment. Heil became involved in design- ing and building equipment, and in 1958, his was one of the first 1 kW 6- and 2-meter SSB stations. As a professional theatre pipe organist at the St. Louis Fox Theatre, he learned to voice and tune massive theatre 5 W, I work stations 150 to 200 miles amp/equalization module. This mod- organs, which taught him how to listen and mentally dissect what he heard. That ability, throughout the Ozarks. The Pine ule can also be used for other combined with his equipment-building skills Board Project is also a great exciter applications, such as a mic preamp learned from Amateur Radio, was the foun- dation for his career in building the first large to drive my Central Electronics 600L for other transmitter projects, or a arena sound systems for such artists as Joe linear amplifier to well over 200 W. microphone preamp for public Walsh, , the , and . His company, , address audio systems and the like. is the only manufacturer ever inducted into You will find detailed parts layout dia- the Rock and Roll Hall of Fame. grams and low-pass filter schematics “How To” Videos In 1980, Heil turned his focus to designing at www.arrl.org/qst-in-depth. more articulate microphones, headsets, and Watch the Ham Nation Pine Board audio gear for Amateur Radio, commercial Project video segments online for broadcast, and live concert stages. Heil con- For CW Operators tinues to bring new technologies to the more information. Each lasts 10 to 15 industry and is constantly designing new Should you want to build the Pine minutes. The video clips, along with and exciting audio products. You can contact Board Project for dedicated CW all the schematics and professional Bob at [email protected]. rather than AM use, the crystal-con- drawings by Gene Workman, For updates to this article, trolled transmitter would be perfect. W4IQN, have been assembled at see the QST Feedback page at Many of the Novice transmitters of https://heilsound.com/amateur- www.arrl.org/feedback. the ’50s and ’60s used this and simi- radio-post/the-pine-board-project. lar designs. The results using this Pine Board AM When you want to operate AM, you transmitter on the morning MOKAM simply connect the microphone pre-

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