WHY HAM RADIO - WHY NOT CB?
YOUR FIRST RIG
HF AMPLIFIER BASICS
GROUNDED GRID BASICS
POWER SUPPLY BASICS
WHAT NOT TO DO AROUND HV!
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The amplifier circuit, is quite simple. Tricky bells and whistles have been omitted in favor of a basic circuit capable of many hours of maintenance free operation. The tetrodes in the circuit are in parallel, requiring approximately 85 watts to drive. There is a degree of mismatch in the input circuit on all bands. This does not present a problem if the rig used to drive the amp uses tube finals in combination with a pi or pi-l network. If the finals are solid state, on the other hand, a matching network will be needed between the driver and amplifier input in order to drive the tubes to full power.
The filaments are wired in parallell and use a common bi-filar choke to isolate the filament transformer secondary from RF (see parts is parts). In addition, generous use of bypass capacitors aid in avoiding feedback and self oscillation in the input circuit.
Switch S1 controls AC to the filament transformer and prevents applying B+ to the tubes before the filament has been lit.
S2, when open, disables the driving transceiver from switching K2 in and out. Relay, K2, externally switched by a spare set of contacts in the driving transceiver, controls antenna change over and the plate current limiting resistor. When K2 is open, the antenna bypasses the amplifier, and places the plate current limiting resistor between the center tap of T1 and the B- supply. This limits the idling plate current to 75-80 ma. When K2 is closed, the transceiver is connected to the amplifier input, the antenna is connected to the amplifier output, and the plate current limiting resistor is shorted.
The meter circuit at M1, permits selective monitoring of either high voltage, or plate current. Meter, M2, monitors grid current and relative RF out. Grid current is monitored by metering a shunt between B- and ground. The relative power indication is obtained through the use of a sampling circuit on the antenna side of L2.
The pi-network functions extremely well in matching a limited load variation. However, harmonic attenuation suffers somewhat above 40 meters. To avoid problems, care must be taken to ensure that the amplifier is not over-driven. Over driving the amplifier will increase the IMD products and spurious emissions. Of course, an ALC circuit would be helpful. However, careful adjustment will accomplish the same goal without many of the problems that accompany ALC circuity.
SELECTING A TUBE
RF INPUT CIRCUIT
RF TANK CIRCUIT
TRANSFORMER POWER CAPABILITIES
PARTS IS PARTS
ROLLING YOUR OWN - TRANSFORMERS AND CHOKES
POWER SUPPLY PROJECTS
LEGAL LIMIT AMP PROJECT