Note
QST, Jun 1959, p. 131.
QST, Nov 1960, p. 115.
QST, Jan 1961, p. 119. (Available in kit form)
Allied Radio catalog 1961 (copyright 1960), p. 394.
The B&W Model LPA-1 is a 1000 watt linear power amplifier designed for desk-top operation. It features a self-contained filament and bias supply for its power amplifier tubes. It is designed to be paired with the companion B&W Model LPS-1 Power Supply, which provides the necessary high voltage, power controls, and protection circuits.
Tube Complement: (2) fan cooled 813 beam power tubes.
Output Impedance: Nominal 52 or 75 ohm unbalanced output circuit designed to feed standard antenna systems.
⚠️ Safety Warning: The LPA-1 chassis must be securely bonded to the power supply chassis to ensure a proper ground return path, and the negative power supply lead must be insulated from the chassis ground for the metering circuit to function correctly.
Output Power
CW & SSB: Full legal input power. Normal SSB operation is configured for a resting plate current of 40 mA.
Linear AM: Supported but limited to within the plate dissipation ratings of the 813 tubes (typically driven by an AM transmitter reduced to 25–30 watts output).
Design Evolution (Serial No. 251 and above)
Later production units (starting at Serial No. 251) introduced several circuit updates and safety features:
– Simplified Bias Switching: The original continuously variable bias control was replaced with a robust, adjustable 500-ohm, 25-watt wirewound resistor paired with a 1-pole, 2-position rotary switch. This allows operators to easily switch between preset CW (0 mA static plate current) and SSB (40 mA static plate current) bias levels.
– Cabinet Safety Interlock: A positive-acting interlock switch was added. It automatically removes primary AC voltage from the LPS-1 plate transformer if the amplifier cabinet lid is opened.
– B+ Grounding Assembly: A safety grounding bar was integrated to physically ground the B+ line when the lid is lifted, protecting the operator from residual high-voltage charges.
Requires the LPS-1 (or equivalent) power supply.
