In older commercial grounded grid amplifiers, it is common to see a cahngeover relay used to switch between a zener diode or similar bias regulator and a bias cut off resistor.
My question is, why did they do it like that?
To me, it is bad engineering, in that during the change of state between transmit and receive, there is a short period where there is neither cut off nor idle current bias applied. OK, it is a very short period but it happens.
In my view, it is surely better to have the bias cut off resistor permanently in series with the bias regulator and simply have the relay contact short the resistor when the amplifier is keyed. That way, that small period of open circuit bias can never occur. Why couldn't the designers of these old amps see the simplicity and sense of doing this?
73, Alek VK6APK.
