It may be called automatic compressing bandwidth, acb.
Assuming for example that the first stage, used as front end and variable gain stage at the same time is the Pleiades V6 amplifier:
When a side chain DC signal proportional to output above a threshold, is returned to the grid to make the grid more negative ie decreasing gain....
The corresponding reduction of anode current or increase of internal plate or anode resistance will interact with the output or interstage transformer to reduce bass or treble response.
The compressed sound may sound louder and the bandwidth reduction may reduce corresponding bass or treble heaviness due to Fletcher Munson or Voice Effort curves.
A side chain signal can be created in usual known ways such as Altec, RCA BA-6A compressors, limiters or for example:
http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PTechReview/PTechReview-15-1953_54-037.pdf
https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2373180A&KC=A&FT=D&ND=3&date=19450410&DB=EPODOC&locale=en_EP
The reflection that bandwidth can be restricted on purpose while compressing if the coupling transformer inductance is at a critical low enough value came while reading the above brilliant patent by Faltico, Maico (hearing aid) company.
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