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 (suppressor tied to cathode) 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 expected output power at max displayed Grid Voltage, when max Grid Voltage is 0, when operating in Class A or A2, and at the selected output voltage headroom.
  • estimates the needed cathode resistor, if you want to use self-bias
Single Ended
Push-Pull

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Select a Tube: Check full list of available tubes
Operating mode:
Ultralinear
Pentode
Triode
PP/SE:
PP SE
V+ (V): Grid Bias Voltage (V):
Quiescent Operating point: Iq(mA):
Vq(V): 		Output Power (W):

Load (Ohm):
Screen Voltage (V): Cathode resistor (Ohm):
UltraLinear tap (%):
Out. headroom (+/-V):

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.

218 thoughts on “Loadline calculator for power stages with reactive load”

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  1. Thank you for creating this. It’s an incredible resource to have. I was wondering if you could add the EL3N tube to your list? Thanks!

  5. Hi Giuseppe,

    Thank you for this very useful tools.

    I am sorry but I am confused.

    If, for example, I am going to use 2A3 in parallel connection single-end and I have 1.25k output transformer, in the simulator, I should use 2A3 with 2.5k load, am I right?

  7. Been using this load line calculator for a while, it’s great. My question is examining 6BQ5 tube in single ended operation, what exactly is meant by the power at G/max, G@0, vs power at headroom voltage, and finally in class A operation? How do all these figures compare to putting a load on an amplifier, putting it on the scope and setting the amp just before the sine wave clips and extracting the power from the voltage measurement?

    1. I calculate an approximate Output power obtained when
      1) g1 swings up the maximum displayed value for g1;
      2) g1 swings up to 0;
      3) g1 stay within the A or A2 class (that is NOT in AB class)
      3) when g1 stays between the desired headroom (which you have to set)
      Whe you use a scope on a real circuit you get the real values at clipping: In most tubes this corresponds to 1) or 2).

  8. I would like to do single end non reactive DC load driving Cap + Pri to ground – e.g. for a 6k6 with a 10K plate resistor driving a DC blocking cap (large enough tp ignore reactance…e.g. 2 uf) and 10k primary. The reactive load would then be ~5K but the DC load is 10K. App is transformer coupled driver stage for PP output stage.

  9. can your simulator work with the fixed bias configuration? I’m talking about a scheme where I drive the bias voltage on the grid with the cathode connetted to the ground. I would like t use a 300B tube. Thank you.

    1. My simulator calculates the needed grid bias voltage for your chose operating point. This is independent from the self or fixed bias modality.
      In your real build you can obtain the calculated grid bias voltage using a fixed bias circuit or a cathode resistor. It is up to your preferences.

  10. Thank you for your reply.
    I need your help because I can’t understand the relationship between the dynamic working point in the load line and the static working point.

  11. Yes, you are right because I was talking about AC and DC load lines, but for a single ended output stage based on KT88. In such final stage the output transformer impedance can be, for instance, 3 Kohm but the transformer primary resistance is 50 ohm. If I calculate the load line and working point based on the AC impedance when my final stage is in idle state (no input signal) the plate voltage, the plate current and the grid voltage are completely different compared with the AC ones. I don’t know if I’m explained clearly, but this is an issue that I can’t understand.
    Thank you for your patience.
    Flavio.

    1. Actually, I am not sure I correctly understood your question.
      However, when you use a reactive load (a transformer), the load is just seen in AC operation (when a signal is applied to the grid). With a reactive load, when you change the grid bias voltage, the operating point simply moves vertically up and down, given that the plate voltage is fixed and there is basically no load (the transformer primary resistance is basically not significant and lead to an almost vertical loadline).

      1. Let me do an example to explain my not understanding.
        When I work with a resistive load I can define the grid bias according to a right load line and I’m sure that in the idle state (no input signal) my plate current is known a stable.
        I can do the same job with a reactive load in a final stage (ie 300B). In this case I can define a load line with a center point at 250V 60mA assuming a primary load of 3Kohm. Now, in this condition, when there is no signal and having a power supply of 400 V, with the grid bias defined by the AC load line, the plate current rise to levels able to break the tube.
        I hope that this time my trouble description is clear.
        Thank you for your time and patience.
        Flavio.

        1. With a reactive load, the load “exists” only in AC operation. Therefore, at idle state, if the power supply is 400V, this is the power supply seen by the valve, given that there is no voltage drop across the load (as happens with a restistive load).
          If you want the idle operating point to be 250V 60mA, the power supply (applied to the transformer) must be 250V as well, given that there is no voltage drop across the transformer at idle state.

  14. Great Tool, i really apreciate working with it. Very well done. Thanks a lot.

    Calculation of pentode screen (g2) current could be interesting.

    Chris

  15. Follow-up to my last question: When the tool calculates a cathode resistor value for PP, is that a resistor per tube, or a shared resistor between the tubes?
    Thanks
    Ken

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