Xiegu XPA125B — Vol 1: Introduction & Hardware

1.1 About this volume

The Xiegu XPA125B is a 100-watt linear amplifier covering 1.8-54 MHz, intended as the matched output stage for the Xiegu X6100 (Vol 9) and other QRP-class transceivers in the 5-10 W drive range — the Xiegu G90, the Xiegu X5105, the Elecraft KX2/KX3, the Yaesu FT-817/818, the Icom IC-705. It earns the bench slot as the HF amplifier in the X6100 family: stack the X6100 on top of the XPA125B, run two cables (RF and an ALC/CAT control lead), and the combination behaves as a single integrated 100-watt HF station that runs from one 13.8 V supply.

The amp also carries its own LDG-style internal autotuner, which collapses the “amp plus separate tuner” stack that this class of station normally requires into a single box.

The decision graph for “do I want this or something else” sorts cleanly along three axes:

• Power class. The XPA125B sits firmly in the 100-watt category — the legal HF maximum in most countries’ general-class privileges and the natural pairing for a QRP rig you don’t want to leave at 5 watts. If the goal is serious DX where 500-1500 W matters, look at an Elecraft KPA500/KPA1500, an Acom 700S/1010/1200S, a Yaesu VL-1000 — different bench slot, different price tier ($1500-7000+ USD mid-2026), different power-supply class (separate 50 V supply rather than 13.8 V).
• Integration. The X6100 and the XPA125B share a vendor — the ALC line, the band-data line, and the CAT/PTT cable are designed to drop straight in. Mixing brands (X6100 driving a Tokyo Hy-Power, or an FT-817 driving the XPA125B) works fine electrically but you’ll spend an evening on a custom ALC cable and band-data interface. If the rig is already a Xiegu, take the integrated path.
• Tuner integration. Pairing the amp with an external autotuner (LDG AT-200ProII, MFJ-993B, Palstar AT2KD) adds another box, another cable, and ~$250-500 USD (mid-2026) to the bill — but gives a wider impedance range and survives mismatches the XPA125B’s internal tuner can’t. For a portable/home-shack 100-watt station with reasonably-matched antennas (anything between roughly 16-150 Ω at the feedpoint), the integrated tuner is good enough; for stealth wire antennas, random-length end-feds, or extreme mismatches, plan on an external tuner downstream.

Where the XPA125B falls down: it’s not a Tier-1 amp. The build is competent but not Elecraft-grade. There’s no remote-control protocol, no SO2R port, no narrow-band notch filter, and the protection circuitry — while functional — relies more on foldback and thermal trip than on the kind of fast over-current limiting you’d find in a $4000 amp. For its target user (the X6100 owner who wants 100 W out of their portable HF rig without spending five figures), it does the job. For a contest station running multiple radios and stacked Yagis, look elsewhere.

This volume covers the hardware tour, the operating-mode envelope, the front-panel “programming” workflow (no codeplug — front-panel menus + DIP switches inside the case), and field use including antenna pairing, power supply, and the duty-cycle math that matters at 100 watts.

1.2 Hardware tour

1.2.1 RF performance envelope

• Frequency coverage: 1.8-54 MHz continuous (160m, 80m, 60m, 40m, 30m, 20m, 17m, 15m, 12m, 10m, 6m amateur bands plus the WARC and intermediate gaps). The amp will pass through any frequency in this range when properly band-selected.
• Output power: ~100 W PEP on HF (1.8-30 MHz); ~70-80 W PEP on 6 m (50-54 MHz) — the LDMOS final and output network are optimized for HF and roll off slightly on 6 m. This is the typical pattern for LDMOS PAs of this class.
• Input drive: ~3-7 W typical (5 W nominal for full output). The published gain is ~13 dB, which means 5 W in → ~100 W out. The amp tolerates up to ~10 W of drive briefly without immediate damage, but ALC will kick in well below that.
• Modes: SSB, CW, AM, FM, all common digital modes (FT8/FT4, RTTY, PSK31, JS8Call). The amp is mode-agnostic — whatever the drive radio sends through, gets amplified linearly. Class AB1 push-pull LDMOS finals, no class-C nonsense that would munge SSB.
• Harmonic suppression: better than -45 dBc on the standard amateur HF bands, meeting FCC Part 97.307(d) (spurious emissions ≥ -43 dBc for sub-30 MHz, ≥ -60 dBc above 30 MHz — TBD on the 6 m number; verify against the FCC spec sheet before relying on it for the 50 MHz band).
• Intermodulation distortion (IMD): 3rd-order ~-30 dBc at full output (typical for LDMOS amps in this class). Adequate for SSB voice; not contest-grade for crowded bands.

1.2.2 Drive interface

• RF input: BNC connector on the rear panel. The X6100’s RF output is also BNC, so the drive cable is a short ~30 cm BNC-to-BNC patch — RG-58 or LMR-240, terminated with quality BNC plugs. Loss across this short jumper is negligible (~0.05 dB at 30 MHz).
• RF output: SO-239 (UHF female) on the rear panel. Connect to the feedline going to the antenna. UHF is fine at HF; if the station ever moves up to 2 m or higher, swap to N (the XPA125B’s 1.8-54 MHz range stays below the PL-259/SO-239 SWR-discontinuity frequency around 300 MHz, so UHF connectors are not a limiting factor here). See Antennas Vol 5 (Feedlines) for the connector deep dive.
• ALC output (control): a control line back to the drive radio that tells it to reduce drive when the amp is approaching its safe operating limits. The X6100’s ALC input port mates directly with the XPA125B’s ALC output via a vendor-supplied (or quickly hand-built) 3.5 mm TRS or DIN cable — TBD: verify exact pinout against the X6100 service manual (the X6100 service manual documents this, and the XPA125B manual specifies its ALC output voltage range, usually 0 to -5 V negative-going). Without ALC, the amp protects itself via internal foldback, but you risk over-driving from the rig side, which compresses SSB peaks and degrades on-air audio.
• PTT/keying line: the amp uses RF VOX (carrier sensing) by default — it transitions to TX mode within ~1-2 ms of RF appearing at the input — but also exposes a hardware PTT input via a 3.5 mm jack for radios that have a PTT-out line (the X6100 does). Hardware PTT is the cleaner approach for CW and digital modes where RF VOX can chop the first dit of a CW character or the first symbol of an FT8 transmission.
• Band data (CAT): the amp exposes a serial CAT port that reads band data from the X6100’s CAT line. With CAT connected, the amp auto-selects the band filter every time the X6100 changes bands — no manual band-switching required. Without CAT, the amp falls back to RF-sensing band detection (which works but is slightly slower) or to manual front-panel band selection. TBD: verify which CAT cable form factor the XPA125B exposes (3.5 mm stereo TRS, DIN-6, or DB-9 are the candidates; the Xiegu ecosystem typically uses 3.5 mm TRS for inter-Xiegu communication).

1.2.3 Internal antenna tuner

• Topology: relay-switched L-network (the LDG-style “relay tuner” architecture — a bank of switched inductors and capacitors that gets reconfigured to find a low-SWR match). Not a continuously-variable tuner (no roller inductor, no variable cap); the matching is discrete-step.
• Impedance range: roughly 16-150 Ω at the antenna feedpoint, equivalent to SWR up to ~3:1 against a 50 Ω reference. Beyond 3:1 the tuner gives up and the amp’s foldback protection kicks in.
• Tune time: ~1-3 seconds for a first-time tune on a new band/frequency. Subsequent tunes on the same band are near-instantaneous because the tuner remembers the last good match per band (the memory is volatile — power-cycling clears it; TBD: verify whether the XPA125B preserves tuner memory across power cycles).
• Bypass mode: a front-panel toggle disables the tuner entirely. Useful when feeding a known-resonant antenna (a dipole cut for the operating band) where tuner insertion loss (typically 0.2-0.5 dB on a clean match, more on a hard mismatch) is gratuitous.
• Tune procedure: press the front-panel TUNE button. The amp keys up at ~5 W (low power) and the tuner cycles through L/C combinations until SWR drops below ~1.5:1. If the tuner can’t find a match within ~3 seconds, it gives up and displays an error — typically meaning the antenna is presenting >3:1 SWR, in which case you need an external tuner (see Antennas Vol 17 (Antenna tuners)) or a different antenna.

1.2.4 Power supply and current draw

• Supply voltage: 12.0-15.0 V DC; nominal 13.8 V. Below 12 V the amp throttles back; above 15 V you risk damaging the LDMOS finals.
• Peak current: ~22 A at 100 W output on a long key-down (CW or FM full-duty). PEP SSB averages much lower (typically 5-8 A in normal voice).
• Idle current: ~1 A in RX mode (the amp’s relays, fans, and front-panel electronics consume this whether you’re transmitting or not). For battery-powered portable use, turn the amp off when you’re not actively transmitting — 1 A for 8 hours is 8 Ah of battery you didn’t need to lose.
• Power connector: rear-panel binding posts (red/black) or a 4-pin Anderson Powerpole connector — TBD: verify which connector form factor the unit has; both have shipped in different production runs. Powerpoles are the de-facto amateur standard and the easier connection for a portable battery.
• Recommended supply: an Astron RS-25M (25 A continuous, 28 A peak) or RS-35M (28 A continuous, 35 A peak) linear bench supply is the bench-classic pairing. For portable use, a Bioenno BLF-1230A or 1240A LiFePO4 battery (12 V nominal, 30/40 Ah) handles a full POTA activation. Switching supplies (MFJ-4225, Powerwerx SS-30DV) work too but are noisier on receive — listen for hash before committing.

1.2.5 Front panel and controls

• Display: backlit LCD showing band, forward power, reflected power (or SWR), ALC level, temperature, and any active fault codes.
• Power/standby switch: front-panel toggle. Standby disables the finals (the amp passes drive straight through to the antenna with negligible loss, essentially turning into a high-grade BNC-to-SO-239 cable) — useful when you want the antenna and tuner connected but don’t need the amplification.
• Band selector: rotary or push-button selector for manual band selection. Used when CAT is disconnected or when you want to override auto-detect.
• TUNE button: starts an autotune cycle.
• Function/menu button: enters the setup menu (ALC threshold, fan speed, RF-sense sensitivity, display brightness, etc.).
• Status LEDs: TX/RX indicator, fault indicator, and (on some units) a separate ALC-active indicator.

1.2.6 Cooling

• Heatsink: large finned aluminum heatsink covers the rear half of the chassis. The LDMOS finals are clamped directly to the heatsink with thermal paste.
• Fan: variable-speed thermostatically-controlled fan. Quiet at idle and low-duty operation; ramps up audibly on long CW or FM key-downs. The fan continues to run for ~30-60 seconds after you stop transmitting — this is normal and lets the heatsink shed residual heat. Don’t power-cycle the amp until the fan has spun down on its own.
• Thermal trip: the amp shuts down the finals (drops to bypass) when heatsink temp reaches ~70-75 °C, protecting against thermal runaway. Recovery is automatic once temp drops back below ~60 °C — typically 1-2 minutes if you’ve actually been hammering it.

1.2.7 Mechanical

• Form factor: ~210 × 145 × 90 mm (W × D × H), ~1.5 kg. Sits naturally as a desktop box under or beside the X6100. TBD: verify the exact dimensions and whether the unit has a rack-mount option — older XPA125B production runs were desktop-only.
• Case: anodized aluminum extrusion top + bottom with bolt-on end caps. Heat-sink fins are part of the rear extrusion.
• Internal access: 4-6 case screws on the top cover; opens to expose the LDMOS finals, the tuner relay bank, and the DIP switch block (used for some configuration that isn’t exposed on the front panel).