RF Sensing Electronic Bias Switching for the 144MHz 8877 Power Amplifier

Lance W7GJ supplied me with this idea which was based on an article originally described in March 1975 Ham Radio Magazine by Marv Gonsoir W6VFR.

This simple circuit can be added to an 8877 triode Power Amplifier to reduce the amount of heat produced and also reduce the electricity bill.The circuit works by biasing the 8877 tube on only when RF drive is detected.
During gaps in between morse characters or speech the tube is cut off.

Suitable time constants are chosen to give a soft make (40mS) and a slow decay (140mS).
R1 and C1 determine the keying sensitivity.
R2 and C2 determine the decay time of the switch.
R3 and C3 affects the make and the break.
Here is the circuit diagram of the arrangement I used: