Radios

Xiegu XPA125B · Volume 4

Xiegu XPA125B — Vol 4: Reference

100W HF/6m amp with built-in antenna tuner

4.1 Tips and tricks

4.1.1 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 against the unit and the X6100 service manual.)

4.1.2 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.

4.1.3 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.

4.1.4 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.

4.1.5 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.

4.1.6 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%.

4.1.7 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.

4.1.8 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.

4.2 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: