Those of you who are using auto-tuners with the 8000 may have noticed that it can present some problems, with the LCD microcontroller throwing high VSWR or overdrive faults, the tuner refusing to tune, and high VSWR indications in PowerSDR and on other metering.
The reason why this is so is because the 8000 amplifier uses no voltage feedback to lower its output impedance combined with the fact that LDMOS devices are structurally a cascode. This combination causes the 8000 amp to behave like a pure current source that, when presented with a high impedance load, will produce over 100V at the output with drive that only produces a few watts into 50 ohms. This will often trip the overdrive fault. Further combine this behavior with the normal auto-tuner safety feature that has the tuner refuse to tune with high drive power to keep relay contacts from arcing, when the tuner sees the high voltage as an indicator of excessive drive it will refuse to tune.
The natural reaction is to simply lower the tuning power level. However this is also not successful because the current source characteristics of the amplifier, as discussed above, delivers very low power into low impedances, and so when a tuner offers low reactances to the amplifier the amp doesn't produce enough power for the tuner to function and tuning again fails to occur.
So what do you do?
What I've found to work is as follows:
1. Make certain that the radio is running the latest 1.08 LCD microcontroller firmware. This version of the firmware substantially relaxes the fault limits. The LDMOS devices used in the 8000 amp are quite tough, it's unlikely they would be damaged even if there were no limits at all, so this is not a problem. With the more relaxed limits, VSWR and overdrive faults are much less likely.
2. Make certain that in Setup > General > Ant/Filters > Antenna, SWR "Enable Protection" is turned ON and that "Disable on Tun Pwr < 35W" is turned OFF. This will cause the radio to cutback power when the tuner is stalled due to an apparent excessive power input condition, thereby breaking the stalled condition in those cases. This is exactly the opposite approach that many auto-tuner owners take with the 100 and 200 series radios, as on those radios the output power cutback associated with that feature frequently causes tuning to stall because of too low of an input power level.
3. Be prepared to monitor output power during tuner and to manually ride the RF drive level to keep it low enough or high enough to prevent a tuning from stalling when it gets to an impedance condition that causes the 8000 amp to generate erroneously high or low output power.
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