I am indebted to you for taking such time and trouble, not only to explain how to create this circuit, but to also to explain how it works and foresee issues I hadn't even considered, and even create a schematic! My thanks too go to WB6BYU. I don't have that particular comparator, but have ordered 4 and hopefully they will arrive tomorrow. I only have LM339's here. I am not savvy enough to know if they are suitable.
I took the liberty to Google your call sign and I am flattered and honored that a gentleman with such a CV is willing to share freely such hard won and valuable knowledge with the ham community, it's much appreciated. I attach a link to the schematic for this LF amp, in case it's of interest to people as to what I am up to and why. Whilst 90% reliable, when it has "done something funny" the single trip LED left me guessing as to which of the 3 protection circuits had caused the trip. No drive signal, over current to the PA FET's, or high SWR, so I am rebuilding it with a separate LED fed via a set / reset flip flop, and where needed, an inverter, for each fail scenario. Then I will hopefully have an LED stay on showing WHAT tripped. I changed from a Hall effect device looking at the end of the choke ferrite to one of the current sensing modules now commonplace, and a comparator to allow accurate setting of the trip voltage from the sensing module, and I have the no signal scenario linked to a flip flop and separate LED. In fact from the original circuit I have fiddled about and deleted unneeded stuff like the keyer circuitry.
Before embarking on improving the high reflected power trip I had to feed the flip flop from the bridge side of the 47k resistor, just before the voltage limiting diode D3. I am unsure just what the 47k resistor does, is it current limiting?
Again, thank you most sincerely for all your help!
Original build document and schematic here:
http://www.chriswilson.tv/bigtxv2.dochttp://www.chriswilson.tv/136bigv2mods-6.jpg shows a few deletions and where I have tapped for individual fault cause LED's