without the use of external electronic circuitry.
All needed is a milli or micro ammeter in series with the anode circuit.
When the needle starts going to the left at signal peaks one knows the tube is driven hot.
Negative bias increase is taking place by the diode (cathode to grid) effect rectification.
Just like the side chain signal circuit of a compressor.
It is very interesting to observe on a U47 the gain going down at very large signal voltages connected to the grid of the VF14. This experiment was done years ago by disconnecting the capsuleand sending varible sinewaves in steps of dBs The gain slope was evidently changed as witnessd form the changing slope of the trace of an XY connected HP electron tube oscilloscope. The signal generator was also an electron tube from Hewlett Packard.
So the possible explanation is:
Increased negative bias by rectification reduces the transconductance and therefore gain.
See also the nearby post on MD21 to K117 to V6 to TC-D5 Pro on generously driving the V6 prepreamp.
The Gramian DP4 connected to the bBC 1:1.8 transformer and then to Pleiades V6 made the anode 100 microampere showing needle (Ia of the EF183 triode connected tube) move slightly at larger voice singing effort.
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