24-07-2017, 04:06 PM
I'm a great believer in frequently going back to beginnings. When I first started the basic radio theory, I had Chas Miller's tutorial. Chas taught his students to measure grid bias with + meter probe on chassis and - on grid. As I progressed, I was taught that the grid of a triode is always in negative values. Then the alternating signal would add or subtract from this negative bias. Yet now the time has come to dig deeper. To fully understand modulators, voice amplifiers and drivers, it pays to understand the amplification stages. Sure, I've come across push-pull before but there is a bit more to it. There is the phase aspect and the effects on impedance (driver stages).
Class A stage was the one that had me most baffled. Rules so far are, the output waveform should be in phase with the input. The triode should be drawing anode current throughout the cycle (360 degrees) but the grid never goes into positive values. The emphasis is on voltage amplification so the anode load will be very high. Oh, and resistance capacity coupling is used.
The confusing bit too is when "grid current" is referred to. Not "anode current". When grid current per se is drawn, power is lost in the grid. In class B, each triode rests totally from any grid current draw while its partner triode draws into grid positive values.
Maybe a simple way of putting it is when the grid doesn't draw current, the valve is more power efficient. Really though I need a bit more time to digest it. It has no bearing on practical engineering at all but I like to progress in stages.
Class A stage was the one that had me most baffled. Rules so far are, the output waveform should be in phase with the input. The triode should be drawing anode current throughout the cycle (360 degrees) but the grid never goes into positive values. The emphasis is on voltage amplification so the anode load will be very high. Oh, and resistance capacity coupling is used.
The confusing bit too is when "grid current" is referred to. Not "anode current". When grid current per se is drawn, power is lost in the grid. In class B, each triode rests totally from any grid current draw while its partner triode draws into grid positive values.
Maybe a simple way of putting it is when the grid doesn't draw current, the valve is more power efficient. Really though I need a bit more time to digest it. It has no bearing on practical engineering at all but I like to progress in stages.
