Care and Feeding of Vacuum Tubes (Valves)

On 03/14/2013 03:16 PM, Tony Duell wrote: Although there were some low-voltage space-charge tubes for automotive use, just about every auto radio that I've ever seen that uses those throws in the towel and uses a good old power transistor for the final audio stage. I recall building a code-practice oscillator that used nothing more than a 6.2VAC filament transformer for its power supply. The tube used, IIRC, was a 6SL7 dual triode. It did make use of an interstage transformer for the audio feedback loop and drove a pair of high-impedance headphones for a somewhat audible tone. Generally speaking, however, it makes design a lot easier if you employ a real high-voltage plate supply. As far as audio goes, it's my impression that solid-state amplifiers tend to use a lot more negative feedback than did even the ultra-linear tube versions. But then there are those who believe that any feedback is a crime against nature and run class A power triodes with zero feedback for a couple of watts of power. --Chuck