Results 1 to 28 of 28
Like Tree4Likes
  • 1 Post By jazbo8
  • 1 Post By jazbo8
  • 1 Post By J M Fahey
  • 1 Post By Wombaticus

Thread: How to Avoid the Drudgery of Drawing Push-Pull 6L6 Composite Load Lines?

  1. #1
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Posts
    2,522

    How to Avoid the Drudgery of Drawing Push-Pull 6L6 Composite Load Lines?

    I need to draw-up a composite load-line chart for a push-pull 6L6 setup that will run at plate voltages of about 530 VDC. Instead of doing things the old fashioned way (cutting and pasting printouts of the tube characteristics) or a more modern way (photoshop), I'm wondering if anyone has gotten really high-tech and converted the 6L6 characteristic data into a machine-readable format.

    Steve Bench has a spreadsheet > here < that does the calculations for a theoretical tube, but it contains no data for a real tube like a 6L6. This spreadsheet looks like it would be a very handy tool once it was populated with the right data.

    So I've been wondering...Has anyone converted the 6L6 characteristic curves into a machine-readable format? If so, that would make the process of drawing composite load lines a whole LOT easier.

  2. #2
    Senior Member jazbo8's Avatar
    Join Date
    Jun 2011
    Posts
    372
    Prof. Fawcett just posted this recently over at AX84.

    Jaz

  3. #3
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    I use LTspice to generate plate curves from tube models, which is useful because then you can generate curves for tetrodes and pentodes at screen grid voltages other than the ones in the published curves. Next, I just draw load lines on those curves. This all can be done in LTspice.

  4. #4
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    2,433
    Don't the curves need to overlay such that the bias point is the same spot on both graphs? this would mean you would need to redo it each time bias or B+ changes.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  5. #5
    Senior Member jazbo8's Avatar
    Join Date
    Jun 2011
    Posts
    372
    Quote Originally Posted by FredB View Post
    I use LTspice to generate plate curves from tube models, which is useful because then you can generate curves for tetrodes and pentodes at screen grid voltages other than the ones in the published curves. Next, I just draw load lines on those curves. This all can be done in LTspice.
    Provided that the pentode (or tetrode) Spice model has relatively accurate screen current vs. plate current characteristic. Otherwise, the curves will not be scaled properly. Many of the models that you find on the net, do not meet this criteria.

    Jaz
    J M Fahey likes this.

  6. #6
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    I just graph out one tube, and yes if you change B+, screen voltage, or bias have have to run the sim again, although that's as easy as a mouse click once you have things arranged.

  7. #7
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    Yes there are limitations concerning the screen grid current and function, especially as one moves away from the screen voltage, which was used when the models was created. I hold the result of the screen models with particular skepticism, however until someone comes up with better models that's all we have do use.

  8. #8
    Old Timer
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    5,129
    I'd rather print, cut and paste and just use it the old fashioned way.
    Much faster than the alternatives (Photoshop, etc.) and much more trustable than SPICE models simulations.
    Personal opinion, others may differ.
    But I have been doing it for over 40 years and it's widely known that old dogs hardly learn new tricks.
    Juan Manuel Fahey

  9. #9
    Senior Member jazbo8's Avatar
    Join Date
    Jun 2011
    Posts
    372
    Quote Originally Posted by FredB View Post
    Yes there are limitations concerning the screen grid current and function, especially as one moves away from the screen voltage, which was used when the models was created. I hold the result of the screen models with particular skepticism, however until someone comes up with better models that's all we have do use.
    Yes, good pentode models are hard to find, the ones from Ayumi seem to be the most accurate I have found so far, and is based on the physical model (unlike Koren's or Duncan's), take a look at his methodology, and try a few of his models to verify for yourself. They are the only models that I use now.

    Jaz

  10. #10
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Posts
    2,522
    Quote Originally Posted by loudthud View Post
    Don't the curves need to overlay such that the bias point is the same spot on both graphs? this would mean you would need to redo it each time bias or B+ changes.
    That is *exactly* the "drudgery" that I was referring to. Cutting and pasting a new chart every time that you make a significant change can be somewhat of a PITB. I guess I fall into the category of old-dog willing to learn a new trick. Like JMF said, cutting and pasting is guaranteed to provide the right answer. I'm just worried about trusting modelling software that may or not be accurate.

    The idea of Spice models is interesting. Being an old-dog, I don't know how to use Spice. Tried it once and never got it working for lack of a good tutorial. I'm not sure that it's even worth the effort of learning Spice if the models are in any way less accurate than paper, scissors and tape.

  11. #11
    Senior Member
    Join Date
    Apr 2010
    Location
    London
    Posts
    193
    To find the optimum loading, presumably you also need to calculate the HT and screen supply under full loading. Is this included in the pSPICE models mentioned above, or would you have to add this to the models?

    I now determine optimum loading experimentally.

    Firstly I "guestimate" the loading.

    I then employing a very large wirewound resistor with a terminal that can be clamped along it's length to vary the resistance to find the optimum loading for maximum power.

    Of course this approach is not ideal for one off designs as if your 1st estimate is a long way off you end up having to buy another output transformer.

  12. #12
    Senior Member jazbo8's Avatar
    Join Date
    Jun 2011
    Posts
    372
    The pentode/tetrode models only model the tube itself, any external components still need to be added to complete the circuit analysis, just like in the real thing. Of course, nothing beats actual bread-boarding which needs to be done even SPICE is used in the initial stage of the design process - but it can help you eliminate some of the guesswork involved.

    Jaz
    J M Fahey likes this.

  13. #13
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    Quote Originally Posted by jpfamps View Post
    To find the optimum loading, presumably you also need to calculate the HT and screen supply under full loading. Is this included in the pSPICE models mentioned above, or would you have to add this to the models?

    I now determine optimum loading experimentally.

    Firstly I "guestimate" the loading.

    I then employing a very large wirewound resistor with a terminal that can be clamped along it's length to vary the resistance to find the optimum loading for maximum power.

    Of course this approach is not ideal for one off designs as if your 1st estimate is a long way off you end up having to buy another output transformer.
    How do you determine which loading yields maximum power?

    Would you need to have two variable resistors to determine the optimal load for a push pull output stage?

  14. #14
    Old Timer
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    5,129
    Yes, that's the way.
    No need for two resistors, just a variable one.
    I took the SS trail *long* ago, around 1972, but still make the occasional tube amp for those who *insist* on the orange glow.
    And experimenting as you say I found that the standard run of the mill 6L6 I can buy here over the counter are happier with 5500 ohms plate to plate instead of the clasic 4000/4500 .
    Suppose they are somewhat lackluster in the emission area, plus I don't beat them to death, use moderate 420/430V +V and 1K screen resistors instead of 470r.
    End result?: 40 W RMS , quite real by the way, not cheating by allowing some clipping but *just* reaching the edge and then backing a little.
    Perfect power for regular Club work with 1 or 2 12" without drowning everybody else and enough for Heavy Metal headbangers in large stages *if* driving a good 4x12".
    Of course, the audience gets the miked PA version of this sound.

    Can you get more power?
    Yes, by using NOS or premium/selected tubes, raising +V, loweringscreen resistors, the works.
    For my reality, that increases costs *a lot* for meager 5/10W extra.
    Simply not worth it (here).

    By the way, I wind my own transformers, so this impedance correction is achieved simply by winding the original, Fender specs primary, but lowering secondary from 60 turns to 50.
    Works like a charm and , as I said, run of the mill tubes last a lot.

    EDIT: I see you intend to use 530V +V
    All I can comment is "OUCH !!!!"
    Juan Manuel Fahey

  15. #15
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Posts
    2,522
    Quote Originally Posted by J M Fahey View Post
    EDIT: I see you intend to use 530V +V
    All I can comment is "OUCH !!!!"
    Yes, OUCH! I wouldn't have designed the amp that way, but the power supply iron is what it is, and the creeping rise in wall voltage here in the US isn't helping either.

    I have given thought of things like bucking the wall power to vintage levels, and zener trickery, but before doing things like that I'd like to crunch some numbers for 530 VDC.

  16. #16
    Old Timer
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    5,129
    OK, as the Beatles said, "Let it be".
    Although we all know what happened after that record.
    Try to at least regulate the screens down with a nice MosFet and keep them at not more than , say, 400V.
    And .... good luck!!!
    Juan Manuel Fahey

  17. #17
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    What do you mean happier?

    I believe my RC30, which is hiding at the moment, lists the recommend P2P impedance for a pair of 6L6 tubes as 6K. That's close to your 5k5 figure.

    I presume guitar amp designers lower the primary impedance to get more watts out of a lower supply voltage. At least that's what my sims suggest.

    Could you post a schematic of how you'd use one resistor to determine the optimal impedance for push-pull pair of tubes.

    Wind you own transformers aye? Awesome.

    I've looking into doing that. What winder are you using?

  18. #18
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    2,433
    If you look through the RCA tube manual, you see all sorts of strange plate to plate impedances. I think what they did was mockup the amp with something close for an OT, and use a variable resistance load, being carefull to calculate the power out. Then back calculate what the primary impedance was.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  19. #19
    Old Timer
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    5,129
    Quote Originally Posted by FredB View Post
    What do you mean happier?
    They drive that load easier than any other

    I believe my RC30, which is hiding at the moment, lists the recommend P2P impedance for a pair of 6L6 tubes as 6K. That's close to your 5k5 figure.
    Cool guys

    I presume guitar amp designers lower the primary impedance to get more watts out of a lower supply voltage. At least that's what my sims suggest.
    Yes, problem is sims presume the original tubes, what we call NOS today, and made by the original manufacturers.
    What you can buy over the counter for a reasonable price simply is not the same.
    Could you post a schematic of how you'd use one resistor to determine the optimal impedance for push-pull pair of tubes.
    No need for that.
    Just set your adjustable resistor to , say, 8 ohms, plug it into your 8 ohm tap and measure.
    Then raise by 10% steps until you get to 16 ohms and lower same way to 4 ohms.
    Draw the curve of power vs. impedance and select the best.
    It's patience, not rocket science .

    Wind you own transformers aye? Awesome.

    I've looking into doing that. What winder are you using?
    An old '56 vintage one, bought from a supplier when he retired.







    This is one of my commercial amps, a Marshall 1987 clone, using those transformers:



    With basically the same set, I can build Fender Bassman, VOX AC50 or Ampeg B15 clones if the customer so desires, so it was a good investment.

    Of course, not for a couple ones, but I did wind quite a few in a hand cranked machine at the beginning, so it can be done.
    Still have it, for prototype work which does not justify setting the automatic one up.
    Attached Thumbnails Attached Thumbnails 731065648_2cd3736447_z.jpg   730961652_b33ebb63dc_z.jpg   731065672_638c648ac7_z.jpg   711031199_53a17e5c44_b.jpg  
    Last edited by J M Fahey; 03-09-2013 at 12:59 AM.
    frus likes this.
    Juan Manuel Fahey

  20. #20
    Senior Member
    Join Date
    Aug 2010
    Location
    Florida Space Coast
    Posts
    122
    Sweet winder.

    I have someone who will sell me an old one that can stick wind up to ten windings at a once for $150. Although it's halfway across the country from me and it weighs several hundred pounds. Shipping would cost prohibitive.

    I'm still debating getting a hand cranked one with or without a wire feeder. The one with the wire feeder is like 5x as much.

    I've done that reiterative output verse impedance study with sims. I need to pick up some more power resistors of various values. I have one 10 ohm 250W one that I use for testing, and some smaller ones.

    How do you pick the point at where to measure the output, where the wave form starts to flatten?
    Last edited by FredB; 03-09-2013 at 03:28 AM.

  21. #21
    Old Timer
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    5,129
    For just one or two, buy a hand cranked simple one, use good light and glasses (even if you don't ned them, consider them binocular "loupes", I use scheap street bought +4 ones), put some nice background music and go ahead, hand guiding.
    Wire sort of self guides following the last turn.
    Juan Manuel Fahey

  22. #22
    Senior Member Wombaticus's Avatar
    Join Date
    Apr 2010
    Location
    Richmond, VA
    Posts
    281
    Quote Originally Posted by bob p View Post
    That is *exactly* the "drudgery" that I was referring to. Cutting and pasting a new chart every time that you make a significant change can be somewhat of a PITB. I guess I fall into the category of old-dog willing to learn a new trick. Like JMF said, cutting and pasting is guaranteed to provide the right answer. I'm just worried about trusting modelling software that may or not be accurate.

    The idea of Spice models is interesting. Being an old-dog, I don't know how to use Spice. Tried it once and never got it working for lack of a good tutorial. I'm not sure that it's even worth the effort of learning Spice if the models are in any way less accurate than paper, scissors and tape.
    Hi Bob,

    For the load line sims I posted on AX84 that Jazbo8 was referring to, I really made an effort to make sure that everything is plug-and-play, with very little understanding of LTSpice required.
    If you download and install LTSpice, then put the models in the correct folders, it should be no harder than opening up the file with sim and plugging in your values --- it's clearly indicated where to punch in the voltages and bias % you wish to target. The only thing you need to really know about Spice is where to find the "run simulation" button. Basically, you press on the button with a picture of a little man running, and it's done. As others point out the results will only be as good as the model used, but I have obtained highly satisfactory results from Stephie Bench's 6L6 model in particular.

    Keep in mind that this is an approximate science anyway as the datasheets correspond to a more-or-less mythical "bogie tube", and real world tubes commonly vary +/- 20% or more.

  23. #23
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Posts
    2,522
    Quote Originally Posted by Wombaticus View Post
    Hi Bob,

    For the load line sims I posted on AX84 that Jazbo8 was referring to, I really made an effort to make sure that everything is plug-and-play, with very little understanding of LTSpice required.
    Thanks for posting!

    Is LTSpice a commercial Windows application? I understand that SPICE is open source and multi-platform, but it looks like the LTSpice program is proprietary and Windows-only. Please let me know if I'm wrong about that. I couldn't find any LTSpice downloads at linear.com that were anything but Windows .EXE files. If LTSpice is Windows-only, then I'm out of luck because I don't use Windows.

    I have looked at the 6 files that you posted at AX84, using the open-source version of Spice that's included in the Fedora electronics lab, but I have to admit that I don't understand why the files you've linked to have 3 different types of file extensions, or which ones (if not all of them) are needed to perform a simulation. Loading them up using the gspiceui doesn't yield anything executable.

    Can anyone offer some insights?

  24. #24
    Senior Member Wombaticus's Avatar
    Join Date
    Apr 2010
    Location
    Richmond, VA
    Posts
    281
    Quote Originally Posted by bob p View Post
    Thanks for posting!

    Is LTSpice a commercial Windows application? I understand that SPICE is open source and multi-platform, but it looks like the LTSpice program is proprietary and Windows-only. Please let me know if I'm wrong about that. I couldn't find any LTSpice downloads at linear.com that were anything but Windows .EXE files. If LTSpice is Windows-only, then I'm out of luck because I don't use Windows.

    I have looked at the 6 files that you posted at AX84, using the open-source version of Spice that's included in the Fedora electronics lab, but I have to admit that I don't understand why the files you've linked to have 3 different types of file extensions, or which ones (if not all of them) are needed to perform a simulation. Loading them up using the gspiceui doesn't yield anything executable.

    Can anyone offer some insights?
    Bob,

    LTSpice is indeed a Windows application... It's proprietary, but a free download. While it shares much common DNA with other SPICE applications, it's unlikely that the files I provided will work with another spice variant without some specialist massaging.

    The good news, however, is that LTSpice has been developed to be compatible with WINE, which is a compatibility layer that allows many windows applications to run under linux.

    As to the question of the different file extensions, here's the breakdown:

    .asc files -- these are the files with the schematic/netlist for the simulation. These are what you would load from within LTSpice.

    .plt files -- files with this extension tell how LTSpice to format the plotted output from the simulation. If they are in the same directory as a correspondingly named .asc file, they will be automatically loaded.

    .inc files -- these are library files that contain tube models. On a windows machine they would normally go under the C:\Program Files (x86)\LTC\LTspiceIV\lib\sub hierarchy.
    Steve Conner likes this.

  25. #25
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,246
    As you just found out, the only open source part of Spice is the original simulation engine. Everyone and their dog developed a proprietary schematic editor, results viewer, and file format to support it.

    The two versions most commonly used are PSpice student version (as every EE student gets it inflicted on them in 1st year) and LTSpice. They use incompatible file formats.

    LTSpice supposedly runs fine under Wine.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  26. #26
    Senior Member kleuck's Avatar
    Join Date
    Mar 2010
    Posts
    367
    dav1l.png

  27. #27
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Posts
    2,522
    Quote Originally Posted by kleuck View Post
    Click image for larger version. 

Name:	dav1l.png 
Views:	25 
Size:	112.2 KB 
ID:	22385
    My French isn't what it used to be, but there can be some ambiguity when it comes to the use of the term "spice." In the linux world, Spice can refer to the circuit emulator, or it can refer to the Simple Protocol for Independent Computing Environments, which is a protocol for connecting thin-client users to virtualized desktops.

    Red Hat | SPICE in Red Hat Enterprise Virtualization for Desktops

    It looks like the popular front ends for Ubuntu and Fedora are ngspice and gspiceui.

    I've got them, but that isn't getting me anywhere.

    Code:
    [root@8coreamd]# yum install ngspice gspiceui
    Package ngspice-23-3.fc18.x86_64 already installed and latest version
    Package gspiceui-0.9.98-8.fc18.x86_64 already installed and latest version
    Nothing to do
    [root@8coreamd]#
    gspiceui.png

  28. #28
    Senior Member kleuck's Avatar
    Join Date
    Mar 2010
    Posts
    367
    I do not use these software myself, but the best way seems to use Geda, a software suite using ngspice for simulations.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. No-load push/pull pot?
    By Fons in forum Guitar Tech
    Replies: 3
    Last Post: 02-08-2013, 10:10 PM
  2. Will a pair of 6L6's take 3200 ohm load push pull?
    By priesty in forum Theory & Design
    Replies: 6
    Last Post: 08-03-2010, 03:14 PM
  3. Replies: 4
    Last Post: 05-09-2010, 01:05 AM
  4. JJ load lines?
    By lowell in forum Theory & Design
    Replies: 4
    Last Post: 06-30-2009, 04:57 AM
  5. Software for Load Lines
    By Glasgow in forum Theory & Design
    Replies: 3
    Last Post: 08-28-2007, 08:18 PM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •