Universal loadline calculator for vacuum tubes

This calculator allows you to trace the anode characteristics graph, and to compute and tune loadline and operating point of various vacuum tubes.

It considers both reactive loads and resistive load.

You can:

  • choose among most common 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 load type as reactive or resistive
  • set the Power Supply V+ voltage, the screen voltage, the screen tap, the quiescent bias current.
  • set the output headroom

The calculator:

  • plots the anode characteristic graph, for the chosen operation modality,
  • plots the reactive loadline for reactive load,
  • plots the DC and AC loadlines for resistive load,
  • highlights the operating point,
  • highlights the needed output headroom, if set,
  • estimates the grid bias voltage,
  • estimates the expected power at the load when max Grid Voltage is the maximum displayed, when max Grid Voltage is 0, when operating in Class A or A2, and at the selected output voltage headroom.
  • estimates the Harmonic Distortion (HD) in the operating range corresponding to the selected output headroom

Did you buy my book? Remember to leave a review on amazon!

Select a Tube: Check full list of available tubes
Operating mode:
V+ (V): Grid Bias Voltage (V):
Quiescent Operating Point: Iq(mA):
Output Power (W):

Load (Ohm):
Next stage AC Impedance (Ohm): HD%:

Screen Voltage (V):
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 anode characteristics curves, are those discussed by Norman Koren, in his web site.

163 thoughts on “Universal loadline calculator for vacuum tubes

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  1. Very useful calculators! Thanks for your hard work. Any chance of adding 6l6gc and perhaps 6v6 to the list?

  2. I appreciate your work here! Perhaps you should do a Patreon? I would certainly donate to this project. I use 6sl7 and 6sn7 quite a bit for preamp and phase inverter duty. Would love to 6v6 on here as well. Thank you!

      1. Great! Thanks! This is definitely going to in handy. Because the 6AK6 can be used as a power valve, could you also add it with a reactive load? Thanks in advance .

  3. How difficult would it be to implement parallel valves? Could it be as simple as maybe a tick box to double the current scale and push the load line down?

  4. Hi Giuseppe, I don’t know if it’s easy to do but having some double triodes on reactive loads would be a hell of and add-on ! I’m thinking 6SN7 push pull, 12AU7 push pull… Maybe it is possible since you have the models? That would be nice!
    Your simulator is so much quicker than opening Orcad or another spice simulator.
    Thanks for the amazing work !

  5. Thanks Giuseppe for this amazing calculator. I was just checking it against a 2A3 data sheet and they match beautifully. Any chance of adding the JJ 2A3..?

  6. Great job! My congratulations! I would also like to see EL86, 6AH6 and other, less common tubes, such as A2134.

    Best regards!

  7. Hello Giuseppe, greetings from Greece

    This online curve calculator is a really nice work, congrats!
    Can you please add GM70 & GU50 to your list with positive grid curves

    Keep going with all good fortune


  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
    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. Very nice tool Giuseppe!!! When I compare tubes it seems for example EL34 use 0V as Control Grid voltage when this calculates Power so Anode Current comes lower and Anode Voltage higher than for example 6L6GC which use +15V Controd Grid voltage. I just bought your Kindle book which I hope explains how it is possible to use CG positive values…

  10. Sorry Giuseppe! Now the previous comments did open for me to read and you have already pointed to chapter where it is explained.

          1. Thanks I seen.
            6N7/6N7P/6H7C are Same Tube ,6N7 is US Model,6N7P is China Model,6H7C is Soviet Model. It’s use for the low power Class-B PP.

  11. Hello, Giuseppe! If possible, please add E180F and EL803S. I will be very grateful!

  12. Buongiorno Giuseppe, complimenti per il tuo lavoro!!! Potresti aggiungere la 845? per la 211 e 845 sarebbe interessante qualche valore di griglia positiva!

    1. Tra l’altro, considera che per la 211 ci sono già le curve con valori positivi di griglia.
      By The way, note that 211 already has positive grid values available.

  13. Ciao Giuseppe,
    could you please also add 12AY7 and ECC808? Thanks for all you do, it’s really very useful

        1. Hi Tomislav,
          here is my pending list: EFL200/PFL200 , 2AY7 ECC808, 3A5 (DCC90), PX25, 300BXLS, T100, 12AT7/6072, 5902/6p30b, 6n16 and 6n17, 6C41, 6U8A, 6528.
          Just a few, before your 3A5. Be patient 🙂

    1. 4P1L is ready as well. However, be carefull. In my model, triode mode is with G3 connected to to ground. I saw some curves, obtained with curve tracers, where 4P1L triode mode was plotted using G3 connected to anode, resulting in much higher current. Take this into account, if you compare my curves with those.

  14. Hi Giuseppe.
    Why I cannot set both Iq and Vq using my “own” values?
    If I set Iq, the tool automatically calculates Vq and related Grid Bias Voltage or if I set Vq, the tool automatically calculates Iq and related Grid Bias Voltage.
    My desire is to put my pair of Iq and Vq values, then obtaining Grid Bias Voltage from tool.

    1. Hi Federico,
      Once you fix the load and V+, then Vq and Iq are dependent one from the other becouse the Ohm Law: Iq=(V+-Vq)/load.
      So, if you modify one of the two, the other follows from the Ohm Law and cannot be chosen arbitrarily. The result is that Vq and Iq are constrained to move along the loadline.
      The calculator in addition computes the Grid Voltage needed to obtain the chosen pair (Vq,Iq).

  15. Hi. Thanks for that. It’s really helpful solution for designing and understanding valve amps.
    I appreciate your job.

    Is it possible to add there 6c41 tube? I have a datasheet if you need.


  17. Hi Giuseppe,

    Hope you’re doing well.
    I have a question, some tubes (like PCC88 and 6N7) show positive grid voltage curves.
    I am a little confused by these, does this mean that these are still usable?
    The reason why I am asking, is that the ECC88 for example doesn’t show these, while it’s basically the same tube (different heater voltage).
    The result is that the calculated output power is a lot higher??!!??

    1. Positive grid voltages should be used according to the guidelines in the datasheet. When in my graphs I show positive grid curves is because these are shown also in the datasheet. However, for using them carefully check the datasheet. Input impedance changes significantly with positive grid values, so circuits should be carefully designed for this.
      For what concerns output power. In the last version of my calculator, I show various values, corresponding to output voltage swing along the loadline up to: max grid value displayed, grid equal to 0, class A/A2, and power at the specified headroom.

      1. Thanks.

        So practically for a Class-AB push-pull does this means I have to look at the grid equal to 0 (= g1), or is it also possible those valves to a positive grid?

        1. Positive grid is possible both in Class A2 and AB2. Soecifically, class A means that the tube does not cut-off. It corresponds to a headroom around the operating point that does not arrive to tube cut-off. This might happen within negative grid (Class A) or positive grid (Class A2). In Class AB (negative grid) and AB2 (also positive grid), in turn one of the two tubes might cut-off, for a limited time, when delivering high output power.

          1. Thanks, and it just depends on the tube if they can handle positive grid voltages?
            So why isn’t there a positive grid for the ECC88 but there is for the PCC88 ?

            To give a practical example, let’s put the PCC88 in PP mode, with V+ of 150V, reactive load of 15k and bias current of 8mA
            According to this calculator, I should get 2.53W out of a PP PCC88, is that correct? Or am I just missing something?

          2. Hi,
            In fact there was an imprecision in the PCC88 model. The Philips PCC88 datasheet does not mention the possibility of positive grid. So I removed the option of choosing positive grids for this model.
            When I built it I used the E88CC as basis, which is very similar and also allows positive grid values.
            Now it should be fine.

  18. Can you do JJ’s 6V6S? While it is VERY similar to the 6V6GT it does have higher voltage tolerance as well a high plate dissipation.

  19. Dear firends,
    recentely I have been very busy so my list of tubes to add became very long. Sorry for taking so long to satisfy you requests.
    Currently my pending list contains:
    ECC808, 3A5, 4P1L, PX25, 300BXLS, T100, 5902, 6p30b, 6n16, 6n17, 6C41, 6U8A
    6528, 6sk7, 6ba6, 6j7, 6Y6, 811A, 2E22, 6dq6b, 6gw6, 6V6S

  20. Giuseppe Amato , how are you ? With a 807 tube , 4200ohm load, 350V 75mA. Do you think is a good point of work ?

  21. Hello Giuseppe! Thanks for Your jobe)
    Can You add 6BX7, 6AH4 and 6CK4 tubes in yours fantastic collection?

    Best regards, Dmitry.

  22. Thanks you.. and Could be that when i look E180F in triode mode , dont run correctly.. For example, i put 200v y then figure 100v or 150 !! and the current too is diferent to i put.. Santiago.

    1. Hi,
      I was not able to reproduce the problem. Can you send me a screeshot?
      However, note that Vq and Iq cannot be chosen arbitrarily. They depend one fron the other and the operating point MUST stay on the loadline.

  23. Hello Giuseppe, Thank you so much for your excellent work. I notice when a certain tube is selected, the calculator starts up with defaults to a certain V+ and Iq. Are these the optimum operating conditions you or the data sheets suggest ?

  24. Thank you Guiseppe for a great tool and generous contribution of time.
    I know you have a long to do list, but could you add the sub-miniatures 6021 and 6111?

  25. Hey Giuseppe,

    Hopefully you’re doing well.
    I was wondering if it would be possible to calculate the gain from the load-lines as well?

  26. Dear friends,
    Sorry for not being reactive during last months.
    The lockdown, and the consequent “work from home”, absorbed me and I did not have enough time to dedicate to hobbies.
    When you are in office, the moment you lock the door and return home makes a clear distinction between your job and family/hobbies/entertainment time.
    Doing office work from home makes this distinction fuzzy and you end up dedicating less time to hobbies and entertainment.
    Hope next months to be able to dedicate more time to this

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