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HF Section of Homebrew Radio

         photo: K5AM radio - HF section

The HF section includes mixers, amplifiers, and filters for
converting the main transceiver section range of 40-39
MHz to the HF amateur bands. A 200-watt tetrode
amplifier is included. Use of a tube at this point provides
low-distortion performance rarely achieved with solid-
state amplifiers at this power level. For protection against
out-of-band signals, a high-Q front-end preselector is
used. The front panel provides control of all essential

The main transceiver panel tunes the 39-40 MHz range in
reverse, enabling high-side injection in the front-end
sections. To cover the ham bands between 1 and 30 MHz,
LO injection is required on ten frequencies in the 41 to 69
MHz range. To avoid instability caused by a crystal switch,
ten separate crystal oscillators are used, with diode
switching of the oscillator outputs.

For transmitting, a mixer and a seven-stage solid-state
amplifier drive the PA. ALC is used to limit output power,
reverse power due to high SWR, screen current, and grid current.
External ALC input is also accepted from a kW amplifier.
Three of the driver stages are used primarily as buffers to
ensure smooth ALC functioning without distortion.

The PA uses a type 8072 tetrode; it is electrically identical to
the more familiar air-cooled 8122. The conduction-cooled
8072 has no cooling fins; the anode is clamped to a heat sink.

Complete details:
HF Circuits for a Homebrew Transceiver, QEX, Nov/Dec, 2001, 20-42.   pdf

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