Loadline calculator for power stages with reactive load

This calculator allows you to compute and tune loadline and operating point of tube amplifiers’ power stages.

You can:

  • choose among most common power vacuum tubes (I am slowly adding more),
  • set the operation modality, among ultralinear, pentode, and triode mode,
  • set Single-Ended or Push-Pull configuration
  • set the Power Supply V+ voltage, the screen voltage, the screen tap, the quiescent bias current.

The calculator:

  • plots the anode characteristic graph, for the chosen operation modality,
  • plots the loadline,
  • highlights the operating point,
  • estimates the grid bias voltage,
  • estimates the maximum expected output power,
  • estimates the needed cathode resistor, if you want to use self-bias
Single Ended
Select a Tube:
Operating mode:
V+ (V): Grid Bias Voltage (V):
Quiescent Operating point: Iq(mA):
Max Output Power (W):
Load (Ohm):
Screen Voltage (V): Cathode resistor (Ohm):
UltraLinear tap (%):

If you are not familiar with loadlines, their purpose, and their use, I suggest you checking my guide: Section 3.2 (for resistive loadlines), Section 4.1.3 (for reactive loadlines), and Section 4.2.2 (for AC loadlines).

The underlying model and equations, for tracing tube characteristics curves, are those discussed by Norman Koren, in his web site.

16 thoughts on “Loadline calculator for power stages with reactive load

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    1. When you use two parallel tubes, each tube sees twice the load. So, when you use two parallel tubes in your project, you should simply use a load that is half the one that you test here (without parallel tubes). If you use three parallel tubes, each tube sees a three times larger load, so you have to use one third load.

    1. The only value that is not referred to a single tube is the load, which in case of PP is from anode to anode. Clearly, even if all other values are per tube, they must be the same between the two tubes. And, yes, it is two tubes!
      Estimated max output power is at the speaker, with an ideal transformer.

  1. Thanks for your reply and clarification. I was wondering what arithmetic I can do on my end to extrapolate to a 4 tube push/pull amp from the 2 tube push/pull results given here?

    1. In a 4 tube push/pull the load seen by every tube is doubled, with respect to the transformer impedance. So, if the transformer is 4k, from anode to anode, with 4 tubes it is as if it were 8k. In other words, if you use 4 tubes and a 4k anode to anode transformer, 8k is the value that you have to specify as load in the calculator. Viceversa, if you want the 4 tubes to operate along a loadline corresponding to a 8k anode to anode load, you have to use a 4k anode to anode transformer.

  2. Very well done!
    I was actually making something similar myself. lol
    May I give some suggestions?

    What I always like to calculate, is what the ratio of Class-A vs Class-B power is compared to the total output power.
    This really helps with designing a certain output stage.
    In case of guitar amps, it really helps designing a certain tone of the output stage.

    Also I think it would be nice to give a notification if certain limits are being reached, like maximum power dissipation, V+ etc

    Would is be possible to add EL91 and EL95 tubes at a certain point?
    Or maybe that users can make their own list?

  3. It’s possible to have the GU81 as pentode (there is a graph of it as triode but i dont know really how close it is) or a way to plug in datasheet numbers into the formula to plot custom graphs? many thanks!

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