Fundamental Pentode Question

[Troncones]

Using a 6BQ5 and a 5K/8 OPT with 200 Ohm resistance and a bias of about 50mA as a reference is my thinking correct?

As current swings from 30mA to 70mA the DC plate voltage changes 8 V. At the same time the AC created at the Primary is 200V P-P.

 

[SY]

As soon as you say "swing," you mean AC. If the tube is swinging symmetrically about the idle current point (which in the linear region is a good approximation), the DC component doesn't change, the AC component of voltage is current swing times load resistance.

 

[Troncones]

So regardles of the OPT DC resistance as the current "changes" 40mA the DC voltage remains the same?

 

[smoking-amp]

The DC measured winding resistance is effectively in series with the AC reflected load. So your calculation of voltage "swing" would be correct. Just that it is AC swing, not DC swing. DC is non varying by definition. Only the V swing across the reflected AC load makes it to the load though (minus yet another secondary resistance component, and a small loss in the steel which shows up in the aprrox. 90 deg. magnetizing current for the core). So the +/- 20 mA AC component is said to be superimposed on the 50 mA DC component. Causing a 208 V P-P AC swing at the plate. Only 200 V P-P of which heads toward the load (minus another approx. 8 V P-P for the reflected equiv. secondary DC measured resistance loss).

It gets more complicated when you consider the distortion from the tube's non-linear gain. That +/- 20 mA component is more likely +22/-18 mA and the assymetry gets worse as the signal gets larger. This distortion caused shift is often referred to as a shift of the DC op. point (the 2 mA shift) if it is maintained by the larger signal envelope for some period of time. It really is an AC envelope (ie. lower frequency than the nominal signal) related shift. Frequencies lower than the nominal signal range are often referred to as DC shifts, conceptually helpful as long as everyone understands the intended meaning.

 

[Troncones]

Thanks for your helpful input.

T