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.