Power Supply vs. Radio Receiver Selectivity/Sensitivity

Hi

Thanks for reply .

Thats a sensible easy to understand explanation Thanks.

I have run a valve (tube) car radio that used 200V DC (via a vibrator and transformer), directly on 12V DC. Valves were 6BA6, 6BE6, 6BA6, 6AV6, 6AQ5). The output audio stage wasn't up to it, but the other stages ran OK with same selectivity, but lower sensitivity. It was necessary to reduce the oscillator resistor value so there was enough oscillator injection to the converter.

I used 12V valves 12J8 (12 plate volts) in push-pull and an extra driver stage and transformer, and got quality audio, but needed it reasonably quiet for comfortable listening. Audio output was about 40 milliWatts.

Usually the audio output will suffer first from weak batteries, and will distort. Also there may be a motorboating sound in the speaker, because the battery volts go up and down with the current draw (higher internal resistance in the battery). This causes slow speed oscillation, heard as motorboating. Turn the volume down and you may still hear the weak audio, but no motorboating.

Transistor radios will have similar symptoms.