40 m SSB Linear Amplifier Guidance (13 V, ~20–25 W) – Ready-to-Build

This document summarizes a recommended approach for building a 40 m HF SSB linear amplifier using 2SC1964 transistors, operating at 13 V, delivering 20–25 W SSB, and includes suggested LC and bias component values.

1. Project Goal

  • Amplify a 5 W SSB signal from a high-quality HF transceiver (~£600 radio) to 20–25 W output.

  • Supply voltage: 12–13 V DC.

  • Linear operation with minimal distortion.

  • Rig protection with optional driver stage.

2. Recommended Transistor Choice

Stage Transistor Quantity Notes
Driver (optional) 2SC1964 1 Provides isolation, smooths drive. Not required if rig output is clean 5 W.
Final stage 2SC1964 4 Two push-pull pairs. Each handles ~5–6 W input → 20–25 W combined output.

3. Amplifier Architecture

                                   │
┌───────────┴───────────┐
│                                                                          
Push-Pull Pair 1 Push-Pull Pair 2
2 × 2SC1964 2 × 2SC1964
│ │
└───────────┬───────────┘
│
LC Output Matching
│
50 Ω Load / Antenna
 

4. Biasing (Class AB1)

  • Final transistors idle current: 50–70 mA per 2SC1964

  • Driver transistor idle (optional): ~20 mA

  • Bias network (suggested):

– Two 1N4148 diodes in series across heatsink
– Rb = 22–33 Ω base resistor per transistor
– Rc = 0.5–1 Ω emitter resistor optional for current sharing
 

  • Thermal coupling of diodes or small transistor to heatsink ensures thermal stability.

  • Increase idle slightly (~70 mA) for ~23 W output.

5. Input Matching (50 Ω to ~10–15 Ω per transistor)

Suggested LC pi network (per push-pull pair input):

Component Value
L1 1.0–1.2 µH (air core)
C1 100–150 pF (series with input)
C2 100–150 pF (shunt to ground)

  • Fine-tune C1/C2 to maximize drive to each push-pull pair.

  • Keep leads short and symmetrical.

6. Output Matching (2–3 Ω combined → 50 Ω)

Suggested LC pi network (per push-pull pair output):

Component Value
L2 0.7–1.0 µH (air core)
C3 220–330 pF (series)
C4 100–150 pF (shunt to ground)

  • Adjust C3/C4 for maximum power transfer and low distortion.

  • Output network combines both push-pull pairs symmetrically.

7. Heatsinking & Thermal Management

  • Push-pull finals mounted on a large aluminum heatsink, preferably with fan.

  • Bias diodes thermally coupled to heatsink.

  • Even spacing of 2SC1964 transistors reduces hotspots.

8. Power Supply Requirements

  • Voltage: 12–13 V DC regulated

  • Current: 3–4 A peak for SSB peaks

  • Decoupling caps: 470 µF electrolytic + 0.1 µF ceramic near collectors

9. Expected Performance

Configuration Idle per transistor Output SSB Notes
4 × 2SC1964 push-pull 50 mA ~20 W Standard safe operation
4 × 2SC1964 push-pull 70–80 mA ~23–25 W Slightly more power, requires better heatsinking

10. Optional Protections

  • SWR protection: relay or clamp diode

  • Input limiter/attenuator: protects driver if overdriven

  • Mute/VOX control: reduces keying clicks

11. References & Similar Projects

  • W1FB’s 20 W HF Amplifier Designs – W1FB Linear Amp Guide

  • G0UPL 20 W 40 m Linear Amp – push-pull 2SC1964, 13 V operation

  • QRP-Labs Linear Amplifiers – QRP-Labs

  • ARRL Handbook – HF SSB amplifier biasing, LC matching

12. Summary Recommendations

  1. Use 4 × 2SC1964 in 2 push-pull pairs.

  2. Optional driver 2SC1964 for rig protection and smoother drive.

  3. Class AB1 bias, thermally stabilized (~50–70 mA idle per transistor).

  4. LC input/output matching: use suggested component ranges for ~20–25 W output.

  5. Heatsink/fan recommended.

  6. Operate at 12–13 V; expect 20–25 W SSB.

This setup maximizes linearity, reliability, and safety, and is achievable with your available 2SC1964 transistors.