Power Amplifier Behaviour

The simulated power amplifier (PA) heats up under load, reduces power to protect itself and can suffer permanent damage when abused. Its state is visible on the TEMP meter of the Radio Deck and in the deck header.

Heating

Transmitting dissipates heat in the amplifier finals. The temperature shown on the TEMP meter starts at room temperature (about 24 °C) and rises with output power, duty cycle and drive level. Constant-envelope modes (FM, CW key-down, steady tones) heat the amplifier considerably faster than speech on SSB, which has a low average power. When transmission stops the amplifier cools back down over time.

Thermal foldback

When the finals get hot the amplifier automatically reduces output power:

• Foldback begins at roughly 78 °C and the deck header shows a thermal foldback notice.
• It becomes aggressive above roughly 92 °C; forward power visibly sags.
• The header shows PA OVERHEATED while the amplifier is in thermal distress.

Foldback is protective and reversible: power returns as the amplifier cools.

Permanent damage

Sustained operation above roughly 110 °C starts to damage the finals. Damage:

• accumulates faster the hotter the amplifier runs (minutes just above the threshold, seconds at extreme temperatures),
• is permanent for the session; cooked finals do not heal on cooldown,
• permanently reduces the achievable output power and adds distortion and slow drift to the signal.

Operating guidance

• Watch the TEMP meter during long transmissions, especially FM, AM and digital modes at full power.
• If the temperature approaches 78 °C, reduce power, shorten transmissions or pause between overs.
• High final drive and low PA headroom (see Advanced Transmitter Settings and Radio Presets) increase dissipation at the same output power.
• A badly matched antenna also stresses the transmitter; the protection circuit reduces power at high SWR, see SWR and Transmitter Protection.