Dynamic biasing

The picture above shows the entire biasing system which consists of two systems. Q2 is a 2N3055 that I discussed in the standard bias system. ZD1 is the zener diode that sets the bias level of that system. S3 is a "defeat" switch that enables and disables the dynamic bias circuit. Disabling the dynamic bias causes the standard fixed bias system to operate as normally would be done. With S3 open the dynamic bias circuit is active and reduces the idle current of the tube(s) to near zero between syllables.

Control signals are derived from the RF input sample taken at the same point where ALC sample it taken or on the input side of the T/R relay. A 220µF Silvered mica cap couples the RF to a 1K ohm 3 watt resistor, then through a voltage doublers circuit formed by D1 & 2. Q1 is a Darlington-pair.  The SCR is a 1.5A 100V type. ZD2 is a 75V 1 watt zener diode. The B minus line runs along the bottom of the picture and is connected at the far right. The cathode of the tube is connected at the point indicated.

This circuit can reduce the "standby" heat output of most tubes by up to 1kW, depending on the tube standard idle current levels.

If you build this circuit and can't find exact parts, feel free to substitute to the next closest type.  ZD1 can be whatever the user wants that fixed bias level to be. The "defeat" switch can be labeled "CW" for closed position, and "SSB" for the open position. It is best to use fixed bias for carrier type drive signals, and dynamic for SSB signals.

This circuit can only be used on triode tubes.

How it works; The dynamic bias circuit works as an analog switch, driven by a modulating signal of RF in the amp. The RF signal can be a carrier or a voice signal. Since a carrier would keep the switch on all the time, defeating the circuit would be better for efficiency. Dynamic bias really works better on a voice (SSB) type signal. The sampled RF signal is rectified and then acts as a modulated on-off switch, controlling the Darlington transistor which acts as a high current flow controller proportional to the voice signal. The SCR acts as a high speed "crow-bar" circuit that prevents over-shoot of the bias. The two 50K ohm resistors act as "cut-off" bias on the B minus line. Maximum cathode current is limited by the SCR, and minimum cathode current is limited by the cut-off bias resistors. Should there be fault type cathode current, enough to open the cathode fuse, the system would be "saftey" biased to near cut-off levels, until the fuse it replaced. If this happens, the source of the fault current should be investigated.

Between voice syllables the system biases the cathode current to near zero.

This circuit and a full description can be found in William Orr's Radio handbook 23rd edition.

If you have any questions, please e-mail me.