600 PA project : Warnings.
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and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.


==>> Always FIRST press F5 <<==

1. A magnetic loop antenna has a narrow VSWR bandwidth. When the TX is accidentally mistuned to another frequency than a narrow band loop antenna is, you can harm the PA.
2. When using SDR software, you can easily jump to an unwanted frequency, by accidentally clicking on a wrong place on a screen.
Use a foot switch to activate the TX. 
3. This balanced broadband amplifier can show nearly 600W output on the meter, by driving it with more power than 1Wrms. But this is not advised.
Why ? A single ended NXP test circuit enables 320W max output and 80% efficiency. These results are only possible due to the use of a resonating output matching circuit. But a balanced broadband amplifier with broadband output coupling (like this A600) is NOT able to deliver 640W without severely overdriving it. Resulting in strong IMD, and a higher chance of damaging the power FETs. Max. undistorted and safe output is 400Wp.

Maximal drive power.

Driving this amplifier with more than 2.6W will damage the gates of the FETs. 
Limit the input power to the A600 input transformer Tr1 to 1W peak input power by adding an attenuator between the PA input relay and the the PCB input.

Also : replace D6 by a fast +/-5v bipolar surge arrestor diode CDSOD323-TO5SC . 
This should limit the FET input voltages to a safer 12Vpp. (Damage can occur when gates are driven with 16Vpp).