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Rich Evan thanks for the pointers. I’ll have to research the past archive from my desk later

I was trying to see the safe limit if the output power the other day by increasing the powe and supply voltage. Around 12V supply I got 10W output. A bit above the finals blew and I was doing the postmortem analysis. The final transistors should be able to push more power easily and I was thinking of a possibility of what went wrong. I suspect the diodes of the “off” LPF branches conducted and the RF current went straight to the ground. 10w on 50 ohm is about 25v rms so that may be the problem.

So using LDMOS and 1:1 output transformer there’s great amplifier reserve capacity but the danger line is also right there.

It’s a bit of heavy duty soldering work to replace the finals but I’d rather know the limits before I take it to the field. I’ll need a supply voltage limiter once I find out a good supply voltage with enough safety margin. The burnt chip smelled bad and the drain contact that I soldered looked like spot welded - I couldn’t unsolder it.