Tube biasing questions

Discussion in 'Shock Brother's DIY Amps' started by Hyakuya, Sep 21, 2021.

  1. Hyakuya

    Hyakuya Tele-Meister

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    Hi all

    As part of my recent Bassman Micro build I added a little circuit to adjust the bias on the EF80 output tubes along the lines of this but with 2 pots so I can adjust each tube indipendently.

    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Adjustable_Balanced_Bias

    The idea being that EF80s weren't really designed for guitar amps and finding a matched pair would be unlikely so I could balance whatever I got.

    It didn't work and I removed it but on thinking about it I've realised that I may have made a very rookie error. The amp is cathode biased by design with the tubes sharing a 300R cathode resistor which I'm now thinking probably means that I can't apply this type of circuit to adjust each tube.

    Is it time for the pointy hat with a big D on it..
     
  2. 2L man

    2L man Tele-Holic

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    I think there can be cathode resistors on grid bias circuit but it can not be shared. Cathode resistors are not needed for grid bias operation but often we install 1 ohms to make easier to measure cathode current just taking voltage loss over them.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Could you not use a separate cathode resistor circuit for each power tube and match operation with different resistances??? It would be a process to attain a match through measuring each tube’s biasing numbers and doing the math.
    However, how do you know you do not have a suitable match? In such a situation, I would judge the sound. If it sounds good And I were still curious, I would judge the heat of each tube. The touch method is valid to judge drastic different cases, imho, but an infrared heat sensor would be more accurate, right?
     
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  4. Hyakuya

    Hyakuya Tele-Meister

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    That did occur to me but I was hoping to be able to adjust the grid current for each tube and bias that way rather than swapping resistors in and out.

    After having a look about the interwebs I found a nice piece over on el34.com (https://el34world.com/charts/Biascircuits.htm) and combining that with the info on Mr Robinette's site I've adjusted the bias circuit a bit to get this...

    upload_2021-9-21_16-56-56.png

    With this I can see the DVM reading on the grid (pin 2) change as the trim pot is turned so it's doing something. What I don't see is the reading on the bias probes change so I'm thinking this is due to fact it's cathode biased.

    Next plan is to bypass the cathode resistor and try the bias circuit again....

    I've not done anything with biasing before, always just left amps cathode biased so I'm enjoying finding out what's happening. Every day's a school day.... :)

    Edit: just realised that circuit isn't going to work.... back to the drawing board...

    Better ?

    upload_2021-9-21_17-31-55.png
     
    Last edited: Sep 21, 2021
  5. Wally

    Wally Telefied Ad Free Member

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  6. Hyakuya

    Hyakuya Tele-Meister

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  7. 2L man

    2L man Tele-Holic

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    Both are basically same circuit and both potentiometers adjust both grids so they were poor find.
     
    Last edited: Sep 21, 2021
  8. 2L man

    2L man Tele-Holic

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    I have used circuit on right and it functions. Both these circuits are good because they pull grids negative if pot slider fails.

    Dual Bias and Bias Balance (2).gif
     
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  9. Wally

    Wally Telefied Ad Free Member

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    It does get involved, but if Hot Blue Plates says it can be done..it can be done. It should be easier to separate the cathodes and balance that way. Or…..go basic……judge the heat and the sound.
     
  10. Wally

    Wally Telefied Ad Free Member

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    is this not a fixed bias circuit? Hyakuya is working with a cathode biased amp….unless I have missed something.
     
  11. 2L man

    2L man Tele-Holic

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    Wally, to me it looks like this thread concerns adjustable fixed bias which has also balance adjustment.
     
  12. 2L man

    2L man Tele-Holic

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    Blumlein Garter "cross-coupled cathode bias" is said to keep push pull balanced even tubes have different gains. I have planned to use this soon on Toroidal OT PP amp.

    There are not much documented builds and those few I have found are on HiFi forums. This is one scheme. Do not scare those high cathode voltages because they are strange triode tubes. In practice there come double the voltage loss what normal cathode bias circuit has. There is also need to use bypass capacitors over both cathode resistors on both cathodes.

    blumlein garter-belt bias 2.png
     
    Last edited: Sep 21, 2021
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  13. Wally

    Wally Telefied Ad Free Member

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    @2L man, it is confusing. Hyakuya tried to implement the fixed bias adjustment circuit he found at rob Rob’s site into a cathode biased circuit.

    This is from his first post…
    I knew that you had missed that part of his post since your advice is always excellent and well expressed…and almost always beyond my understanding. I appreciate everything you have posted.
     
  14. 2L man

    2L man Tele-Holic

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    I did read that part but I did not understand it was a conclusion when on his post #4 there are two more inoperative balance/bias circuit layouts.
     
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  15. Ten Over

    Ten Over Tele-Holic

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    I wonder what would happen if you varied the DC grid voltage like this:
    Bias EF80 Micro.png
     
  16. printer2

    printer2 Poster Extraordinaire

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    Marshall made an amp that is fixed and cathode biased.
     
  17. Hyakuya

    Hyakuya Tele-Meister

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    more thinking out loud than a conclusion as were the dodgy circuit ideas in #4.... :confused:

    For my part, I've decided to stick with the single cathode resistor for now, I have a small box of EF80's in the post so I will test them out and find the best match I can.

    Maybe later down the line I'll play about with splitting the cathodes to seperate resistors or going fixed bias and using the circuit on @robrob website to adjust it.

    The ideas being posted here are fantastic and why this forum is so valuable but they are also way over my head and experience.
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good plan, keep it simple, keep it real. The note I like best here is the one from @Wally that some mismatch sounds good, nicer harmonics, richer OD. So make sure to judge by ear more than multimeter. :)
     
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  19. Ten Over

    Ten Over Tele-Holic

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    So did Fender. AB568's, AC568's, AC668. Nobody ever seemed to like them.
     
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  20. Ten Over

    Ten Over Tele-Holic

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    Here is a variation on the theme that may be more familiar to you guys:
    Bias EF80 Micro 3.png
     
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