Vacuum Tube 80-40M Transmitter Circuit

The transmitter utilizes a 6BW6 vacuum tube to achieve an output power of approximately 5 watts. The circuit includes a component CI that is calibrated to produce the cleanest continuous wave (CW) note. The tuning capacitors C8 and C9 have values of 365 pF and dual-365 pF (connected in parallel). The inductor LI consists of 35 turns of #24 enamelled wire wound on a 1-inch plastic tube. FT-243 crystals are employed for operation at frequencies of either 3.5 MHz or 7 MHz. It is important to note that this circuit is not designed to generate a 7-MHz output from a 3.5-MHz crystal, as it does not support frequency doubling.

The transmitter circuit is designed around the 6BW6 vacuum tube, which is known for its efficiency and reliability in RF applications. The output stage, powered by the 6BW6, is capable of delivering a stable output of around 5 watts, making it suitable for low-power transmission applications. The component CI, which is likely a variable capacitor or an inductor, plays a crucial role in tuning the circuit to achieve the clearest possible CW signal.

The tuning capacitors C8 and C9 are critical for frequency selection and stability. The use of a dual-365 pF capacitor in parallel with a single 365 pF capacitor allows for fine-tuning and provides a greater range of capacitance, which is essential for achieving precise frequency adjustments. The inductor LI, constructed from 35 turns of #24 enamelled wire on a 1-inch plastic tube, forms a resonant circuit with the tuning capacitors, enabling effective filtering and signal generation at the desired operating frequency.

The choice of FT-243 crystals for 3.5 MHz or 7 MHz operation provides a reliable frequency reference for the transmitter. However, it is imperative to adhere to the design specifications, as the circuit is not intended for frequency multiplication. Attempting to use a 3.5 MHz crystal to generate a 7 MHz output could lead to undesirable performance and potentially damage the circuit components. Proper selection and installation of the crystals are essential for optimal operation and signal integrity. Using a 6BW6 vacuum tube, the above transmitter delivers about 5 W output. CI is adjusted for cleanest CW note. C8 and C9 are 365 pF and dual-365 pF (paralleled) tuning capacitors. LI is 35 turns of #24 enamelled wire on a 1 plastic tube. FT-243 crystals for 3.5 or 7 MHz are used. Do not use this circuit to produce a 7-MHz output from a 3.5-MHz crystalit is not intended to double over crystal frequencies.