2x GU-81M HF Linear Power Amplifier – 2000W (FJ-2000)

Welcome to another high-power project in my amateur radio portfolio. This showcase features a heavy-duty, custom-built linear power amplifier designed around a pair of legendary GU-81 (GU-81M / ГУ-81М) glass pentode tubes. Engineered for maximum durability and sustained high output, this amplifier is a true powerhouse for demanding DXing and contest operations.

Technical Specifications

• Bands: 1,8-28 MHz
• Active Components: 2 x GU-81M (ГУ-81М) Vacuum Tubes
• Input Power (Drive): 30 - 40 W
• Output Power: 1700 - 2000 W
• RF Switching: High-speed, highly reliable V1V and V2V (В1В / В2В) vacuum relays.
• Power Supply: Built around a massive, military-grade anode transformer sourced from the renowned R-140 (Р-140) radio station.

Design & Construction Highlights

As detailed in the internal chassis photos, this amplifier is built without compromise, utilizing massive components capable of handling extreme RF voltages and currents.

The RF deck features a beautifully crafted Pi-network with heavy-gauge copper tubing for the tank circuit, ensuring high efficiency and low losses on the HF bands. To manage the immense heat generated by producing up to 2000 watts of RF power, the chassis employs a robust forced-air cooling system with multiple high-throughput exhaust fans located on both the top cover and the rear panel.

The power supply section is equally impressive. Relying on the heavy-duty R-140 anode transformer, it provides a stiff, stable high voltage under heavy load. The use of V1V and V2V vacuum relays guarantees quiet, fast, and arc-free T/R (Transmit/Receive) switching, which is crucial for modern digital modes and fast CW break-in.

Operational Layout

The front panel is designed for straightforward operation and monitoring. It includes dual analog meters for precise tracking of tuning parameters and operating currents. Dedicated band switching and tuning/loading controls allow the operator to achieve optimal impedance matching and maximum power transfer across the HF spectrum.

*This page serves as a technical showcase for the amateur radio community to share build concepts, classic tube implementations, and high-power RF engineering.