Many HF radios have a setting for "Transmit Delay" to delay the start of RF to a linear amplifier to make sure that the amp's relay is closed before RF hits it to avoid hot-switching the amp. I don't know of any such setting in the TX-500. Is there any reason why it would not have that feature? Perhaps the TX-500 designers didn't think anyone would use a linear amp with it? I am a CW op.
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73, Norm/KC1BMD
Norm - KC1BMD