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:
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):
Resistive
Reactive
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.

310 thoughts on “Universal loadline calculator for vacuum tubes

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  1. 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 .

  2. 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?

  3. 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 !

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

    Best regards!

  5. 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

    Cheers

  6. 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.

  7. 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…

          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.

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

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

        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 🙂
          Thanks.

    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.

  10. 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.
    Thanks.

    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).

  11. 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.
    Regards

  12. THANK YOU GIUSEPPE.YOU SAVED US FROM A LOT OF CALCULATION TIME.THE TUBE 6U8A IS A VERY HANDY TUBE COULD YOU PLEASE ADD IT TO YOUR LIST?

  13. 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.

  14. 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.

  15. 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

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

    Best regards, Dmitry.

    1. D3A is now available. I did not find enough data in datasheets to find parameters for 6E5P , 6E6P , 7721: I need either penthode mode grahs with various G2 or pentode mode with one G2 and a triode mode graphs

  17. 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.

  18. 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 ?

  19. 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?

  20. 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?

  21. 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

  22. This tool shows promise!
    A couple of questions..
    -Is V+ meant to be the power supply (from the rail) or the plate voltage (after DC voltage drop across the OPTs primary)?
    -Does this tool support cathode biased implementations or just fixed bias?
    Thank you

    1. Hi,
      V+ is power supply.
      Grid voltage, in real circuis can be obtained using fixed or cathode bias. However, from the anode characteristics graphs point of view there is no difference.
      I am planning to add computation of the cathode resistor in future releases. This is already available for the power stage loadline calculator.

  23. Fantastico Giuseppe!! Veramente bello. Per chi come me si appassiona a ricavare il massimo da apparecchi vintage, sarebbe bello se ci fosse la 7591, anche se e’ fuori produzione. In ogni caso, grazie! E complimenti.
    Giovanni

  24. Hello Giuseppe

    Thank you for investing the time into the calculator and making it available to us.

    Is the 13E1 tube going to be added at one stage?

    Cheers
    Stephan

  25. Hi Giuseppe,
    Hopefully you’re doing well!
    Your little calculator is still highly appreciated!!

    I came across two interesting tubes, ECL113 and ECL81.
    I think they are actually very similar(identical)?

    Thanks as always!

  26. Ciao Giuseppe! This is brilliant. If you want some ideas for the pending list, let me suggest ECC82, EF80, EF184 and JJ’s ECC99.
    Cheers,
    Andy

  27. Hi Giuseppe, this tool is awesome! I love the wide range of tubes you cover. There are valves here that you just won’t find on another calculator. Would it be possible to add 6AT6/6AV6 tubes to the list?

  28. Ciao Giuseppe, great tool! May I ask what you use as condition for max g1? Is it g1=0 or the “knee” of the output characteristics? Grazie in anticipo.

    1. Hi Emmanuel,
      Not sure I understand correctly. I guess you mean how do I compute power for max g1, right? What I consider max g1 is the max g1 used in the corresponding tube datasheet, which is the maximum g1 I display (not necessarily 0).

  29. Very nice tool and it looks as if it is quiet accurate. It would be great to have the grid lines labeled and if it would be possible to adapt the voltage and current scaling.

  30. Hi Guiseppe. I’m a total noob in building tube amps. And I don not understand what your Output Power numbers mean. For instance for KT120: at class A/A2:48.99 at headroom:5.76. How much power I’ll get from an output transformer? On the one hand, headroom power for me means a variable part of a current (which is a signal). On the other hand KT120 is a tube that is capable to generate up to 70W of power into a loudspeaker…
    I’m confused.
    Thanks.

      1. I think I understand. Numbers in the power section mean how much a tube works in A or AB classes. This is why I saw about 48 W of a heat, and only 6 W into a loudspeaker.
        Thank you very much for a great tool you’ve made for all of us!

  31. Giuseppe, I am missing the possibility to connect to tubes same type in parallel.
    This, to see if the max allowable plate dissipation can be compensated by parallel connection.

    1. Hi,
      when two tubes are connected in parallel, they basically see twice the load. So, you can simulate two parallel tubes by indicating a load twice the real load. Or viceversa, the real load that you use in your build, when you have two parallel tubes, is half the load specified in the calculator.

  32. Great stuff…but could you check the curves of the 211 please ? Your 211 curves show e.g. 112mA @400V while in reality it is 80mA etc…they are quiet different from the data sheet

  33. The 6AK6 model will not let you go in P-P and in SE the B+ is given as 330V rather than the chart which gives it at 180V (which is datasheet value)

    1. Hi,
      in order to go in PP you should specify reactive load.
      After that, you can set 180V as B+.
      In resistive mode, V+ corresponds to the voltage applied to the load, which from 330V is reduced to 180 at the anode.

  34. Hello Giuseppe, from Finland.
    Your work is very valuable, thank you.
    F2a audio tube mihgt be an intersting to add, not for only me but for more commonly. I have realized several amps with those with good success.

    Best regards, Mauri

  35. Great stuff, but I guess I need some more advise on PSE…if I just double the primary load as you suggested…well…how do I get the correct power and distortion figures ?

    One more: Could Ou as well integrate the 813 in triode and the 814 in triode ? I can send you links for the curves if you dont find them, but Andrea (Audiodesignguide) has the 813 and Bartola the 814…

    Thanks so much for your great work !!,

    1. If you use two parallel tubes and, in the calculator, you set a load value twice the real load you are using, then power delivered by the two tubes will be twice than the one computed by the calculator as well. In fact you will have twice current at the same voltage.
      THD should remain invaried. So, resuming, twice power at practically the same THD.
      I also added 813 and 814 to my vert very long pending list. If you have the datasheets I spare some time to search for them. 🙂

  36. Hi from China
    Really perfect job. Your explanation here is perfect also
    One question about two tubes. 5787, 12B4 also. Are they in your pending list?
    Thanks a lot
    Nikos

      1. I think that still doesn’t completely answer the question?
        Usually the anode (plate)-cathode voltage is given.

        Or in other words, the supply voltage WITHOUT the bias voltage (when self bias is being used)

        1. The plate-cathode voltage is the horizontal axis of the graph. V+ is the voltage given to the load. In case of reactive load, V+ corresponds to the quiescent plate-cathode voltage (the red spot corresponding to the operating point). With a resistive load, quiescent plate-cathode voltage is lower than V+, since voltage drops through the load.
          In addition, note that the anode characteristic graph and the loadline are drown considering fixed bias (not self-bias).

  37. Devo dire che è molto interessante il suo lavoro soprattutto per me che sono alle prime armi, interessanti anche le risposte alle domande che mi hanno ulteriormente chiarito le idee. posso esprimere il desiderio di vedere anche le curve del tubo 6P21S. Grazie per il lavoro che svolge.

    Saluti Sergio

  38. Hi, thank you for this fantastic tool. I have a couple of requests if you have time for them: EF184, EF80 and the 5672. Cheers again!

  39. Wow, very nice info and excellent response from you Giuseppe. I found this site by searching for quiescent bias current answers. I’ve come across a formula in a piece written by C.R. Couch. In it he gives a formula Iq = Imax / p but no where does “p” get defined, would you by chance know this?

  40. Hi Guiseppe,
    thanks for the great work, especially covering lots of russian tubes I use regularly 🙂
    It’s really usefull to quickly get a first idea (within the borders of an idealized model ;-)) about operating points, voltages and load impedances. Can you add a model for the 6P1P-EV please? It’s a “near relative” of EL90 and 6AQ5 but not exactly the same, so this would be appreciated…
    I’m looking forward to read your book I`ve just ordered – I can give you a sound feedback if you’re interested 😉

  41. Fantastic job, much appreciated!
    I hope you’ll keep up the good work for a long time.
    My request would be the king of double triodes, the E80CC.

  42. Great, as soon as my daughter gave me the Kindle book from Amazon, I started reading it, it is very comprehensive and detailed. I am waiting when you can add PL504, it is very good for initial tests for me.

    1. I’m a bit confused by Iq. Could you elaborate from which part of the tube data sheets this default gets populated? It is plate/anode current or grid current?

      Thanks!

  43. This tool is so helpful.
    Do you have any info on the new KT170 tube from New Sensor / Tung Sol?
    I am starting an amp build using that tube, and would love to see it added when you have the time.
    Thanks, and have a great day.

    1. Hi,
      I do not have enough data to generate a model. I would need pentode mode graphs with at least two different screen voltages or a grapg in triode mode. Available datasheet only have pentode mode with one single screen voltage 🙁

  44. Hi,
    Thanks a lot for for making this webpage. The information you put here and the calculator are amazing.
    I have one question to pose to you regarding the calculator, if you don’t mind:
    When designing a class AB amplifier, the load seen by the each tube is 1/4 of the impedance of the primary of the transformer for class B and 1/2 for class A.
    So, when I enter the value of the “reactive load”, is that 1/4 or 1/2 of the transformer’s impedance?
    In my case, I have an impedance of 16kΩ, so should I enter 4kΩ or 8kΩ?
    Thanks a lot.
    Best regards,
    Pedro

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