Excessive drive to the PA results in distortion of the signal, which in turn creates unwanted components non only in-channel, but also adjacent to the channel.
To be effective in limiting distortion, an ALC system has to act very quickly (amongst other things).
The ALC detector in external PAs is often slow in response, and the external ALC interface on many transceivers is often heavily filtered (again slow in response)… yielding a control loop that is very slow responding.
Such a control system can readily be adjusted under constant carrier conditions to limit drive to a level that is within the linear range of the PA, and that holds grid current to a safe value, but the dynamics of the control loop may mean that it is essentially ineffective in properly avoiding transient overdrive and distortion on SSB telephony.
So, it may be prudent to connect the external PA ALC up to reduce the risk of melting the grid out of a PA bottle for isntance, but it should be regarded as ineffective in preventing transient overdrive until proven otherwise.
In any event, practical ALC circuits invariably exhibit some level of transient overdrive, and using ALC to control very high IF gain (typically operating at high ALC meter deflection, the top of the ‘safe’ zone) exacerbates the problem. ALC usually does not make a good audio compressor.
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