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 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:
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.

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

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    1. Another Triode some people like ,is the Russian 6S19P .It is intended as a small power supply pass element.

    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?

      1. Thank you so much for adding these models!
        If it’s not to much to ask for, is it possible to also add the ECL80?

        Maybe a feature request is to add the whole ECL8x family (ECL80, ECL82, ECL83, ECL84 and ECL85(or 805) )
        These are all great tubes to use.

        You’re work is really appreciated!

        1. In the meanwhile I added ECL80 (triode section and pentode section). The others are in list. Each of these tubes contains in fact one triode and one pentode. So, I need some time to add them all. 🙂

          1. Thank you so much, your work is really appreciated!
            Very interested because on paper the EL91 and the ECL80 should be very close to each other (I am talking about the power Pentode), but in these graphs they are pretty different?

          2. Did you check that you are using the same screen voltage in both cases? My impression is that using the same screen voltage (for instance 250 or 200) you get very similar graphs for the two tubes.
            I also double checked comparing my graphs with the datasheets (http://www.r-type.org/pdfs/ecl80.pdf and http://www.r-type.org/pdfs/el91.pdf) and my impression is that they overlap quite nicely.
            Can you be more specific on the differences you found?
            Thanks again for your feedbacks

          3. I double checked and you’re actually right, they match pretty nicely indeed!
            One small thing, but I think the Iq is the bias current per tube (and so the cathode resistor as well).
            This is a little confusing with a PP and I think people can easily take the wrong value maybe?

          4. That’s ok! Thanks.
            For what concerns Iq. All values are referred to a single tube. So Iq is the current in one tube, also in PP configuration. The only exception is the load, whcih in PP is referred to anode to anode load.

  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!

  4. Very nice calculator! This may be the single most useful thing I’ve ever found for amp design. I am very thankful. If you could, it would be very helpful to me if you can add the EF86 to this.

  5. It would really be interesting to see how a 6BQ6GTB behaves as a UL pp amp at B+ of 200V , since the DC voltage for G2 for the GTB is also 200V . Would you be so kind as to show us ?

  6. Oh ok , I missed the part that 6BQ6GTB in your pending list. But why do you show 6JQ6 limited to 140v on g2 when the maximum is +330 volts ? And could the 6JQ6 handle 350v peak on g2 is a UL configuration ?

    1. Hi,
      You can use any value for g2 in pentode mode. Just type a new value in the corresponding field.
      However, the value is locked in ultralinear mode and triode mode.
      In fact, in pentode mode, g2 is not fixed and it is automatically and dynamically computed according to the ultralinear tap percentage.
      In triode mode, g2 is equal to Va (equivalent to pentode mode with 100% ultralinear tap).

  7. Dear Guiseppe, I like this very well. I wonder if you could add some more triodes. I am using 45, 2A3, 101D, 205D types. Your 2A3 simulation is spot-on to a real amplifier I have built.

    kind regards,
    ciao, Adam

  8. Dear all,
    thanks for using this calculator and thanks for suggesting additional tubes.
    The pending list of tubes that will be added now includes:

    ECL84, ECL85
    kt120
    EL86, 6AH6, A2134, 12E1
    GM70, GU50
    6GU7 6CG7 12BH7
    45, 101D, 205D

    Hope to be able to add them all in a couple of weeks.

  9. Thank you again for adding all these wonderful new tubes!
    One small thing, the ECL85/805 (and PCL805) has a anode dissipation (Wa) of 8W instead of 7W

  10. Great tool !
    i’m looking for a tool that uses spice models and gives me Ia for a given tube, plate voltage, screen voltage, grid voltage… That would be usefull when testing tubes. I think what you did could do that easily ?

    1. My tool does exactly this … even if, it does not actually start from Spice tube models. However, my model and parameters can be used to build Spice models, provided grid current, screen current, and internal capacitance are modelled as well.

  11. Hello, Giuseppe. Many thanks for Your job!
    Can You add 6JN6 for Yours sim?

    With a best regards, Dmitry.

  12. would you be so kind as to add 6П3С to your list . Thanks , and thanks for adding 6JQ6 which in some ways seems to be better than the 6BK5 tubes.. As it seems the 6JQ6 can put out 9.89Wrms as a UL PP amp , which is better than the 6BK5 UL pp amp. That was quite surprising.

  13. I have a request for an oddball, the QQE03/12 (or CV2798)
    Originally this tube was created for transmitters, but I have actually seen some very interesting circuits for audio amplifiers.

    Maybe also ECL81 and ECL84 to make the list complete? 😉

    Thank you so much for the great work!!!!

  14. Added also these to my todo list. By the way, I considered ECL84 also before. However I did not find anode characteristics graph for the triode section. I just have it for the pentode section. Does anyone have it?

      1. Thank you!
        I am starting to feel a little guilty, but of all great output tubes, I would like to ask for the 5881 as well.
        Thanks for all the great work! It’s really appreciated!

  15. Thanks for all the updates so far!
    I have been really spending a lot of hours tinkering with some tubes now!
    I saw the very interesting 6N7 tubes (which can be used in a cute little amp), so I thought maybe it’s a nice idea to also add the 6SN7.
    I will be hunting down some more interesting high power double triodes 😉

  16. Hi Giuseppe, I just discovered your website and have been playing around with the load line calculator. This calculator is GREAT! Thank you for providing such a powerful tool and making plotting load lines so easy. I recently learned how to plot manually and your calculator helped me check my work. A great advantage of this calculator is being able to adjust the screen voltage, as we know the prints only provide Vg2=250v and 300v if you are lucky.

    I have a request: If possible, it seems advantageous to be able to print out calculated load lines. This could be used for build/repair documentation and comparison of various plots.

    Thanks for your hard work. I am looking forward reading your book that I will be purchasing soon!

    1. Hi,
      Many thanks for appreciating my work.
      I am glad discovering that it is becoming useful to many vacuum tube enthusiasts and professionals.
      Adding the possibility of printing the calculated graphs might be a nice feature. I’ll see how I can add it to the calculator.
      Thanks again 🙂

  17. I found another interesting one; EFL200/PFL200 (same as 6Y9, 17Y9 and Russian 6R4P)
    Double pentode with max 5W.

    Btw, would it be possible to show the power dissipation of the current bias setting?
    Of course this is easy to calculate by hand, but it helps if it’s shown right away.
    (ideally in combination with percentage/ratio compared to the total max power)

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