As discussed above, you can easily determine the plate current (if the unit is operating) by measuring the voltage across the cathode resistor R22 or R24, since the cathode current
Ik equals the plate current
Ip with negative
Vgk. The plate and cathode currents are defined as positive in normal operation. That current will depend on the plate-cathode voltage
Vpk and the grid-cathode voltage
Vgk. In turn,
Vgk = -
Rk x
Ik.
Calculating the tube's plate current is a good exercise in reading tube data sheets, if you are going to change the plate supply voltage
Vbb by using a Zener regulator that will reduce it to the Zener voltage.
Take the characteristic curves, which is a family of curves for
Ip vs
Vpk at one value of grid voltage
Vgk for each curve.
You need to add two "load lines" to the graph:
1. The actual plate-cathode voltage is reduced by the voltage across the cathode resistor:
Vpk =
Vbb - (
Ip x
Rk). Plot this as a locus of possible pairs of
Vpk and
Ip for your design values of
Rk and
Vbb and get a straight line on the
Ip vs
Vpk axes.
2. Since the grid is grounded (not connected to cathode), the grid-cathode voltage is
Vgk = -
Rk x
Ip, remembering that the cathode current
Ik equals the plate current
Ip. Plot this as a second locus (not necessarily straight) of one point on each curve of constant grid voltage that corresponds to the current that gives that voltage.
Where these two added loci intersect, that is the operating point: the actual plate-cathode voltage, plate current, and grid-cathode voltage determined by the tube, the supply voltage, and the cathode resistor.
In your case, the cathode resistance is the sum of R22 or R24 plus half the value of the pot R23, for each of the two triodes.
The gold mine for tube datasheets contains
https://frank.pocnet.net/sheets/093/6/6SN7GTB.pdf . The curves are on page 4.