Scanners & Radios

Scanners & Radios · Volume 7

Xiegu XPA125B

100-watt HF/6m amplifier with built-in antenna tuner

Contents

SectionTopic
1About this volume
2Hardware tour
3Operating modes
4Programming workflow
5Codeplug backups
6Field use
7Tips and tricks
8Resources

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 6) 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.

2. Hardware tour

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.

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 with Jeff (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 with Jeff 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).

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.

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 with Jeff which connector form factor his 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.

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.

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.

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 with Jeff the exact dimensions and whether his 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).

3. Operating modes

The amp is mode-agnostic: it amplifies whatever the drive radio sends. The interesting question is what each mode does to the amp’s thermal and protection circuitry.

3.1 SSB (USB/LSB voice)

The friendliest mode for any linear amp. Voice peaks hit 100 W only briefly; the average power is more like 20-30 W. The fan barely spins; thermal trip is essentially impossible during normal conversation. The amp will SSB indefinitely at full rated output, days at a time — the duty cycle is just too low to heat-stress anything.

ALC matters for SSB: without it, the rig’s audio compressor or speech processor can drive the amp past its linear region on voice peaks, producing splatter (intermodulation products spreading sideways into adjacent frequencies). Connect the ALC cable, set the ALC threshold so the green ALC indicator just barely flickers on voice peaks, and the amp stays clean.

3.2 CW (Morse code)

100 W key-down CW is a 100% duty cycle while the key is held. A long string of dahs (or a tune-up call) heats the heatsink rapidly. The amp handles QSO-rate CW (5-25 WPM, normal letter-spacing) without trouble — the gaps between elements give the heatsink time to shed heat. 30+ seconds of continuous key-down at full power will trigger thermal trip; the amp recovers in 1-2 minutes.

For CW tune-ups, drop the rig’s drive to 20-30 W output (i.e., 1-2 W of drive into the amp, ~20 W out) and tune at that level. There’s no reason to tune at 100 W — the tuner finds the same match at any power level, and you’re just heating the amp pointlessly.

The amp’s RF VOX has a hold time (the amount of time it stays in TX mode after RF disappears) that’s typically tunable in the menu. Set it long enough that a dit-pause-dit sequence doesn’t drop the amp back to RX between elements — typically 100-300 ms. Too short and the relays chatter; too long and the amp doesn’t release for the next station’s transmission. Hardware PTT bypasses this entirely (the rig controls TX/RX timing directly), which is the cleaner approach for CW.

3.3 AM

AM at “100 W” usually means 100 W carrier + 100 W peak sideband — i.e., 400 W PEP. The XPA125B is rated for 100 W PEP, not 100 W carrier; back off the drive so the carrier is ~25 W, which means PEP peaks (with 100% modulation) hit the rated 100 W. The amp will run AM at this level continuously.

Few hams operate AM voice these days, but the AM broadcast band, AM aero, and a handful of vintage-equipment 75 m AM nets remain. The mode works; just respect the carrier-vs-PEP math.

3.4 FM

FM is 100% duty cycle while the carrier is on. Treat FM like CW for thermal-management purposes: short transmissions (under 30 seconds) are fine at full output; long ones will thermal-trip. For 2 m / 70 cm FM you wouldn’t be using the XPA125B anyway (it’s 1.8-54 MHz only). For 10 m FM (29.6 MHz repeater inputs, mostly) and 6 m FM (50.0-50.1 MHz simplex, 52-54 MHz repeater inputs in some regions), the amp works, but be mindful of long PTT holds. Repeater operators in particular hold the key for the duration of a transmission, which can run a minute or more — drop to 50 W if you’re going to be talking that long.

3.5 Digital modes (FT8, FT4, RTTY, PSK31, JS8Call)

Digital modes are 100% duty cycle during a transmission. FT8 transmits for 12.6 seconds per 15-second slot; FT4 transmits for 4.5 seconds per 7.5-second slot. The duty ratio (TX time / total cycle time) is roughly 85% for FT8 and 60% for FT4 in continuous operation.

For FT8 at 100 W continuous, the amp will heat up steadily — the fan will run constantly, and after 15-30 minutes the heatsink will be hot enough that thermal trip becomes possible if the room is warm or air circulation is poor. The practical mitigation is to run digital modes at 50-70 W rather than 100 W: it costs you ~3-1.5 dB on the receiving end (often invisible to FT8’s marginal-signal detection), drops dissipation by half, and the amp runs at it indefinitely.

Most FT8 operators don’t actually need 100 W. The mode’s whole point is decoding signals 24 dB below the noise floor; 50 W getting through a contact is no different from 100 W getting through the same contact. Save the heat.

RTTY at 100 W is the worst case — it’s a continuous AFSK tone, 100% duty cycle, for as long as the transmission lasts (multi-minute contests aren’t unusual). Don’t run RTTY contests at 100 W on this amp: drop to 50 W or expect thermal trips. RTTY is dying out anyway, but if you operate it, plan around the thermal budget.

4. Programming workflow

The XPA125B has no codeplug in the traditional sense — no software, no USB-loaded channel database, no “CPS” tool. Configuration is split between front-panel menus (the runtime configuration that you’ll change between operating sessions) and DIP switches on the internal main board (the install-time configuration that you set once and forget).

4.1 Front-panel menu items

Enter the menu via the FUNCTION button. The exact menu structure varies by firmware version; the canonical items are:

  • ALC threshold: 0-100% (or equivalent voltage scale). Sets how aggressively the ALC output line clamps drive power. Start at ~80% and adjust empirically — too low and the amp can’t hit full output; too high and you risk over-driving and splatter.
  • Fan mode: auto / always-on / always-off. Auto is the default and the right answer for nearly every use case. Always-on is useful for hot-shack contesting where you want maximum cooling from the start. Always-off is a footgun — don’t.
  • Band detect mode: auto-sense / CAT-driven / manual. Auto-sense reads the input frequency and picks the band filter. CAT-driven reads the X6100’s CAT data. Manual locks to the band you select on the front panel. For an X6100-paired station with CAT connected, use CAT-driven; for a portable setup with no CAT, use auto-sense.
  • RF-sense sensitivity: 0-100%. How much input RF triggers the TX changeover. Too low and the amp keys on stray pickup; too high and it misses real drive. Default ~50% is typical.
  • VOX hold time: 50-500 ms. How long the amp stays in TX after RF drops. Tune for the mode — short for digital, longer for CW.
  • Display brightness: 0-100%.
  • Standby/operate: same as the front-panel switch but exposed in the menu for completeness.
  • Reset to factory defaults: a one-button factory reset. Useful when you’ve fiddled too much and want to start over.

4.2 Internal DIP switches

A small DIP block on the main PCB (visible after removing the top cover) controls install-time options:

  • Default ALC polarity (positive-going vs negative-going voltage to the rig). Different rigs expect different polarities — the X6100 expects negative ALC (the most common convention). TBD: verify with Jeff the DIP-switch position for X6100-compatible ALC.
  • Band-data input format (open-collector vs 0-8 V analog vs serial CAT). Older Xiegu rigs used an analog band-voltage convention; newer ones use serial CAT.
  • Fan thermostat threshold: low/medium/high. Default medium.
  • Tuner enable/bypass at power-on: choose whether the tuner starts in bypass mode or active mode after a power cycle.

Once these are set for your station, leave them alone — they’re not session-level adjustments.

4.3 Firmware updates

The XPA125B exposes a USB serial port for firmware updates. Xiegu publishes occasional updates on their support site; the update tool is a Windows utility that uploads new firmware over USB. The process is similar to the X6100 firmware update procedure (Vol 6 §4) — connect USB, run the updater, wait for the progress bar.

Firmware updates are rare and not strictly necessary. The amp’s RF behavior is set by hardware (LDMOS finals, output network, tuner relays); firmware controls only the display, menus, and protection logic. Check Xiegu’s site once a year; otherwise don’t touch it.

For the broader programming-software view of the Xiegu ecosystem (X6100 + XPA125B + G90), see Vol 21 (Programming software).

5. Codeplug backups

Not applicable. The XPA125B holds no per-channel data, no per-frequency memories, no contact lists. The only state worth documenting is the DIP switch configuration, the menu-set ALC threshold and fan/VOX preferences, and the physical cabling setup between the amp and the X6100.

Document these in Scanners_and_Radios/programs/xiegu-xpa125b/notes.md (file pending Jeff’s first bench setup session — placeholder for now). Items to capture:

  • DIP switch settings (each DIP position, on/off, with the function it controls)
  • Front-panel menu settings (ALC threshold value, fan mode, band-detect mode, RF-sense sensitivity, VOX hold)
  • ALC cable wiring: pin-out from XPA125B ALC OUT to X6100 ALC IN — the specific 3.5 mm TRS or DIN pinout, including which conductor carries the signal voltage and which is ground
  • CAT cable wiring: pin-out from XPA125B CAT to X6100 CAT — likely a 3.5 mm TRS or stereo cable
  • PTT cable wiring: pin-out from XPA125B PTT IN to X6100 PTT OUT
  • Power-supply cabling: which Powerpole or binding-post conductor is positive, which is negative, and the fuse rating (typically 25 A inline)
  • Heatsink temperature observed at idle, after a 10-minute SSB rag-chew, and after a 30-second key-down at 100 W (baseline for spotting future cooling degradation)

These aren’t a codeplug, but they are the operational state that a “rebuild the station from scratch” recovery would need.

6. Field use

6.1 Antenna pairing

At 100 W out, antenna quality matters more than it does at 5 W. The forgiveness budget shrinks: a stealth random-wire that worked passably at QRP can present 8:1 SWR at the amp, which the integrated tuner cannot match, and the amp will foldback-protect rather than transmit.

For specific per-radio antenna recommendations including the X6100 + XPA125B pairing, see Antennas Vol 29 (Use-case Matrix). High-traffic-pairings:

  • Resonant HF dipole (single-band or trapped multi-band) — the easiest pairing. Cut for the band, fed with 50-100 ft of LMR-400 or RG-213, presents ~50-75 Ω at the feedpoint. SWR <1.5:1 across the band; the internal tuner barely has to work. See Antennas Vol 6 (Single-band dipoles).
  • End-fed half-wave (EFHW) — the popular portable choice. The 49:1 UNUN at the wire’s high-impedance end (Antennas Vol 16 (BALUNs and UNUNs)) drops 2450 Ω to 50 Ω. Pair with a 1:1 current choke at the UNUN to suppress common-mode currents on the feedline — at 100 W, common-mode currents can light up household electronics and give you RFI complaints from the neighbors. See Antennas Vol 10 (Random wire & end-fed).
  • Multi-band trap dipole or fan dipole — the home-station alternative to swapping single-band wires. Slightly lossier than dedicated dipoles, but the amp’s internal tuner makes up the difference. See Antennas Vol 7 (Multi-band & specialty dipoles).
  • Vertical (Hustler 4/5/6-BTV, GAP, R-7000) — fine at 100 W; the tuned-trap verticals expect ~50 Ω at resonance; the amp’s tuner trims out any installation-deviation mismatch. Requires a good ground/radial system (see §6.5 below and Antennas Vol 20 (Grounding)).
  • Random wire (any-length, fed via 9:1 UNUN) — the stealth choice. Works at 100 W but the tuner gets a workout, and SWR varies dramatically across bands. Pair with an external tuner (Antennas Vol 17) for the bands where the internal tuner can’t find a match.
  • Magnetic loop — careful. Most amateur mag loops are rated for ~100 W intermittent but with arc-over risk at the high-Q tuning cap. Check the loop’s rated power; some are 50 W or less.

6.2 Feedline

For a 100 W station, feedline loss is real money. A 100-foot run of RG-58 at 28 MHz loses 2 dB (40% of your power, dissipated as heat in the coax). The same run in LMR-400 loses 0.5 dB (~10%). At 50 MHz the gap widens to 2.5 dB vs 0.8 dB. Use LMR-400 or better for any feedline run over 30 ft. See Antennas Vol 5 (Feedlines) for the full coax-loss table.

For portable use where weight matters, LMR-240 is a reasonable compromise — half the weight of LMR-400, ~30% more loss. RG-8X is acceptable for short (under 15 ft) jumpers but the cumulative loss across a long deployment is brutal.

6.3 Power supply

The amp draws 20-22 A at peak; the supply needs headroom. A 25 A bench supply (Astron RS-25M) is the minimum; 30-35 A (RS-35M, MFJ-4230MV, Powerwerx SS-30DV) gives margin. Sub-25 A supplies will sag under load and the amp will throttle back, costing you output power on voice peaks.

For portable battery use, the math is roughly:

  • 100 W SSB voice, average drain ~6 A. A 30 Ah LiFePO4 battery lasts ~5 hours of active transmit (or 12+ hours of mixed RX/TX at typical operating ratios).
  • 100 W FT8 continuous, average drain ~18 A. The same 30 Ah battery lasts ~1.5 hours.
  • 100 W CW QSO, average drain ~10-12 A. ~2.5-3 hours.

The 1 A idle drain matters for battery ops — power-down the amp during RX-heavy stretches.

Voltage cabling: use 12 AWG or heavier for any run over 6 ft. Voltage drop across thin cabling at 22 A will hold the amp at ~12.0 V instead of 13.8 V, and the amp will protest with reduced output. Anderson Powerpoles are the de-facto standard amateur connector; crimp them properly with the right tool (not pliers).

6.4 SWR and protection

The amp foldback-protects when:

  • SWR exceeds ~3:1 (variable by band)
  • Heatsink temp exceeds ~70-75 °C
  • Supply voltage drops below ~11.5 V or rises above ~15.5 V
  • Forward power exceeds ~120 W (drive too hot)
  • Reflected power exceeds ~25-30 W (mismatch too severe)

Foldback means the amp drops to a low-power “safe” state and lights the fault indicator. It does not immediately damage the amp — the protection is designed to be re-armable by power-cycling. Repeated foldback events do cumulative damage to the LDMOS finals over time, though, so treat foldback as “fix the problem before transmitting again” rather than “ignore and continue”.

The single most important habit: do an autotune on every new band before transmitting at full power. It takes 3 seconds, prevents 90% of foldback events, and protects the finals. Build the muscle memory.

6.5 Grounding

At 100 W, RF on the wrong surfaces is a real problem. Ground the amp chassis to the station ground via a heavy (8-10 AWG) bonding strap to the same single-point ground as the rig and the antenna system. Don’t trust the negative power lead to provide ground — it’s a 12 V DC return, not an RF ground.

The full grounding deep dive is Antennas Vol 20 (Grounding). The minimum: a single-point ground bus near the operating position, with the amp, rig, computer, and any external tuner all bonded to it; lightning suppressors (Polyphaser IS-50UX or equivalent) at the feedline entry point; ground-rod field outside the shack tied to the bonding system.

6.6 Posture — portable vs home base

The XPA125B is heavy enough (~1.5 kg) and power-hungry enough (~20 A peak) to be a marginal portable companion. Most users keep it as a home-base amp paired with a wall-powered station, and run the X6100 standalone (5-10 W) for true portable POTA/SOTA activations. A few users do haul the amp out to field-day setups with a Bioenno 40 Ah LiFePO4 — workable but heavy.

The sweet spot is: X6100 standalone for portable, X6100 + XPA125B for home/base. Same rig, two operating modes depending on where you are.

7. Tips and tricks

7.1 ALC cable is non-negotiable for SSB

Without ALC, the rig’s audio compressor will routinely overshoot the amp’s linear region on voice peaks, generating IMD that splatters into adjacent frequencies. The fix is a $5 cable. Build the ALC cable first, before you even attempt SSB at full power. The cable’s pinout is in the XPA125B manual and the X6100 manual; verify both ends before powering up the first time. (Pin-out specifics — TBD: verify with Jeff against his unit and his X6100 service manual.)

7.2 Use CAT for band-data, not RF auto-sense

CAT-driven band switching is instant and accurate; RF auto-sense takes 100-300 ms of carrier presence to detect the band, during which time the amp passes drive through the wrong band filter — which can momentarily distort and (in pathological cases) damage the filter. Wire up the CAT line; the few minutes of cabling pay off forever.

7.3 Set fan mode to AUTO, not ALWAYS-OFF

Sounds obvious, but the menu lets you do it. Some users disable the fan thinking they’ll get quieter operation — and then thermal-trip on a 30-second CW transmission and wonder what happened. Don’t.

7.4 Tune at low power, transmit at full

Drop drive to ~10-20% before pressing TUNE. The tuner finds the same match at any power level, and tuning at 100 W is gratuitous heating. After the match is found, bump the drive back up and transmit normally.

7.5 Watch the temperature on long digital sessions

A small thermometer probe (or just a finger after the fan stops) on the heatsink during a long FT8 run will tell you whether you’re approaching thermal trip. If the heatsink is hot enough that you can’t keep a finger on it for 5 seconds (~50-55 °C), back off to 50-70 W and let it recover. Better yet, drop to 50 W at the start of the session and don’t worry about it.

7.6 The built-in tuner has limits — keep an external tuner in the kit

For random-wire antennas, EFHW configurations near band edges, or any setup where SWR routinely exceeds 3:1, an LDG AT-200ProII or MFJ-993B downstream of the amp will tune what the internal tuner can’t. See Antennas Vol 17 (Antenna tuners) for the full external-tuner deep dive. The internal tuner is great for the 80% case; the external tuner is the backup for the other 20%.

7.7 Turn the amp OFF, not standby, when leaving the bench

Standby still draws ~0.5-1 A (relays plus front panel). Power-off draws zero. For overnight, weekend, or any extended off-time, hit the switch. The amp boots in ~2 seconds; there’s no reason to leave it powered.

7.8 Don’t power-cycle until the fan stops

After a long transmission, the fan runs for 30-60 seconds to shed residual heat. Wait for it to stop before flipping the power switch. Cutting power while the heatsink is still 60+ °C and trapping the heat with no airflow can stress the LDMOS junction.

8. Resources

Manuals:

  • Xiegu XPA125B operator’s manual — ../manuals/xiegu-xpa125b/ (PDF; covers front-panel operation, menu reference, DIP switches, ALC pinout, troubleshooting). The manual ships with the unit; if missing, downloadable from Xiegu’s official site.

Vendor and distributor:

Community and reviews:

  • eHam reviews: https://www.eham.net/reviews/view-product?id=14017
  • Reddit r/amateurradio + r/amateursatellites threads on XPA125B (search “XPA125B” + “ALC cable” or ”+ X6100” for the high-signal threads)
  • QRZ.com forums — Xiegu user group thread
  • YouTube — Frank K4FMH, Walt KZ1X, and a handful of POTA-focused channels have multi-part XPA125B reviews and setup walk-throughs (especially the X6100 + XPA125B integration cable build)

Related volumes: