It looks for me as stupid schematic (Oldtimers welcome!)

[Yansi]

No, I meant this:

When the operating point of a pentode moves out onto the flat part of the plate curve the plate current is effected very little by changes in plate voltage. This makes the pass tube(s) almost immune to ripple and line voltage changes even if there were no feedback regulation. The main effect of load changes is to bring about changes in the output voltage of the rectifier filter and as already explained the pass tube(s) are highly immune to these changes.

Not sure what the heathkit designers did, but having a constant Vg2 lets you optimize the tube characteristic by a lot. If you vary Vg2 from zero up, you can affect the maximum passable current with zero control grid voltage.
Too small Vg2 and you can't pass enough current through the pentode.
Too high Vg2 and you need silly voltage swing at the control grid from 0 to -Vcutoff voltage, from fully open to close the tube completely.  Silly voltage swing at the control grid means you need even more gain in the control loop of the regulator, resulting also in poor regulation.
By changing the Vg2, you can affect the control grid cutoff voltage like 10:1 ratio (from -100V to -10V).

With a triode, you can't optimize the characteristic by any other means, than replacing the tube by a different type.

A little shame usual datasheet do not provide plots with Ia vs. Vg2 at a constant Vg1 and plate voltage, or the cutoff voltage vs others.

[T3sl4co1l]

The effect of the screen grid is to flatten the plate curves, which in the case of the cathode follower, gives good PSRR.

The screen acts as a virtual plate, so that varying the screen grid voltage varies the plate current in the same way varying the triode plate voltage varies plate current.  (It is no accident that "triode mode" behavior results from tying screen and plate together.)  Meanwhile, plate current is less dependent on plate voltage, i.e. the plate resistance is very high.

Another way to look at it is, the cathode current is the sum contribution from control and screen grids.  The screen grid has lower gain, by a factor of mu_g2g1.  If you don't have screen grid curves available for a given tube, then you can translate: an increase of mu_g2g1 volts on the screen is equivalent to 1V on the control grid.

Plate current is then cathode current minus grid currents, which are generally small, and the screen current is normally the most significant.

Screen current however gets extreme at low plate voltages, usually below V_g2 / 2 for beam tetrodes.  This is the main downside to a tetrode/pentode pass regulator: besides the required screen supply (which has to be referenced to the output, so is usually a separate winding and rectifier), you need to be careful not to run into saturation (dropout), lest the screens go toaster-grid on you.  (Some protection can be provided, like supplying screens from a modest value series resistor, and perhaps adding a diode from screen to plate, so that when plate voltage is low, the screen voltage is pulled down; this gives a triode-like saturation characteristic.  Better-than-triode saturation can be had with a more clever protection circuit, but this does require more parts, including preferably a PNP transistor, which kind of begs the question, why the tube in the first place...)

Tim

[001]

Thanx again 

My question is about this situation:
if 2nd grid voltage is about 250v
but  voltage drop at pass tube is 90v

is grid current became greater then anode?
i.e. is minimal regulator drop is limited by 2nd grid voltage?   

[Yansi]

2nd grid voltage of 250V is the silly business I was talking about. 50 to 100V as Vg2 may be more then enough and prevents becoming a toaster grid.

//EDIT: To see on an example:  PL500 as a pass tube:  Maximum g2 dissipation is 5W.  At 60V screen grid voltage, you get about 60mA current there, with zero Vg1 and Va.  That is still safe within those 5W (3.6W actually), without considering any g2 protection resistors.
Yet the tube still passes about 100mA at 20V plate voltage, under the same conditions. More then good enough. (Ig2 falls to about 13mA at this point).

PL500 datasheet so you don't have to search for: https://frank.pocnet.net/sheets/030/p/PL500.pdf

//EDIT2: Note I am not saying that no g2 resistor shall be used, just saying, less is sometimes more. Any series resistor with the screen grid may be beneficial, also as a means for preventing unwanted oscillation of the tube.

[T3sl4co1l]

Good illustration of why 6L6s aren't very good here.  You want high perveance types like sweeps and, well, voltage regulators (most pass reg types are triodes though), so screen voltage stays low and the plate saturation is low as well.

V_g2 / 2 isn't a hard and fast rule, by the way; 6V6 saturates quite low (~30V) even with a high screen voltage (I forget if that was at 150 or 250V or what).  EL34 not so much -- true pentodes seem to perform worse.  Sweeps typically saturate in the 50V range, with screen in the 100-200V range.

An equally useful definition of "saturation" might be, the point where screen current becomes unacceptably high...

Oh, and sweeps have higher screen dissipation too, so you can push further into that region, safely.

Tim

[001]

Great idea! 

[001]

Too small Vg2 and you can't pass enough current through the pentode.
Too high Vg2 and you need silly voltage swing at the control grid from 0 to -Vcutoff voltage, from fully open to close the tube completely.  Silly voltage swing at the control grid means you need even more gain in the control loop of the regulator, resulting also in poor regulation.
By changing the Vg2, you can affect the control grid cutoff voltage like 10:1 ratio (from -100V to -10V).

Can You describe this effect more? (some practic links will be ok too  )
My disturb is about 2nd grid destroy
Is 2nd grid CURRENT rise then 2nd grid voltage more then drop voltage at pass tube?
I.e. low drop = damage with 2nd grid. isnt`it?