Self biasing without a circuit board?

Discussion in 'Tube Audio' started by Nixxuz, Jun 14, 2018.

  1. Nixxuz

    Nixxuz AK Subscriber Subscriber

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    I'm just wondering if this is a thing, because it doesn't sound like a thing. But a friend was like "well, old tube radios didn't need to be biased, and they didn't have circuit boards".

    Educate me people.
     

     

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  2. maxhifi

    maxhifi AK Subscriber Subscriber

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    Google "cathode bias"
     
  3. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    That is right. The cathode of the tubes were biased with a resistor. The circuit board really has nothing to do with biasing a vacuum tube. Essentially there are 2 types of bias, cathode bias and voltage or fixed bias. Cathode bias uses a resistor or resistance to establish the voltage drop between cathode and anode which the tube needs to operate.
    You can also achieve the same thing by imposing a negative voltage on the input grid to control the flow of electrons from cathode to plate. The amount of electrons flowing or the current, is an important factor for the tube and the circuit it is used in.
     
    Last edited: Jun 15, 2018
  4. gadget73

    gadget73 junk junkie Subscriber

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    Right, "self bias" usually just means a cathode resistor. It takes care of itself provided the tube isn't shot. There are probably more complex modern methods that automatically adjust fixed bias, and that might involve a circuit board.
     
  5. Nixxuz

    Nixxuz AK Subscriber Subscriber

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    A guy was stating the Primaluna Dialog has no circuit boards and is self biasing. I had just never thought about it before and assumed it had to be a function that would require some sort of monitoring circuit.

    "The More You Know!"
     
  6. kward

    kward AK Subscriber Subscriber

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    The Primaluna gear markets their stuff as adaptive autobias or some such thing. I don't know how it works but the marketing writeups I've seen about it do lead me to believe it is monitoring the output stage somehow and correcting bias as needed not just at idle but all throughout the power curve to offer lowest distortion output at any power level. The pictures I've seen of their amps do show circuit boards that implement that circuit.

    What has been termed here as "self bias" or "cathode bias" is orders of magnitude simpler than that with just a single resistor and bypass cap in the cathode circuit of the output stage.
     

     

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  7. Dandy

    Dandy Super Member

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  8. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    This may simply be using an IC like the LM317 which is a voltage regulator as a constant current source. Apart from the IC and a resistor for setting the voltage, it has the same number of components as cathode bias, which is a resistor and a bypass cap. There may also be some over voltage protection of the lm317 via a diode.

    "self biasing" is another term, along with auto bias, for having a the tube biased by a resistor. Although this statement seems to be at odds from Kev's assessment.
    Nonetheless, in order to learn how vacuum tubes work it is best you read about from a tube manual like the RCA Tube Manual or the ARRL Radio Amateur's Handbook which in layman's terms describes the workings of a vacuum tube. There are many available for sale on the usual sale sites.
     
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  9. kward

    kward AK Subscriber Subscriber

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    Right I think this is the article I have seen in the past.

    Looks like the owner must tell the circuit what type of tube is in the socket, by flipping a switch. From there, the circuit appears to be able to automatically adjust the negative voltage on the grids to keep the tube biased optimally throughout the entire power band. And apparently it can do that for triode or pentode mode (flip of another switch). Indeed, quickly re-reading that article, especially the THD+Noise graphs, it's clear the adaptive autobias thing works pretty well.

    But what complication in that autobias board! Still can't tell if the autobias thing is working continuously in the analog domain or if it's sampling the tube's operating characteristics in the digital domain. Either way it seems to be compensating pretty well for output power fluctuations to keep the output tubes optimally biased anywhere in the power band.
     
  10. Dandy

    Dandy Super Member

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    It would be cool to see a schematic. If it works as well as reported, perhaps we should all try it.
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    I'd expect its basically a servo circuit, monitoring cathode current and adjusting grid voltage accordingly.
     

     

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  12. Hotrodster

    Hotrodster Member

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  13. kward

    kward AK Subscriber Subscriber

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    It might be like that but I would guess the PrimaLuna circuit is much more sophisticated. For one thing, the board in the link you provided doesn't appear to have any active parts (no transistors). But the real give away is that it says it manages bias independent of drive signal (as if this is a benefit, wherein actually I think it is a detriment--unless you're doing full blown regulation of plate, bias, and screen). So it seems this board is simply looking at an average of DC conditions so that as the tube ages you don't need to readjust quiescent DC conditions. I guess that's partially useful.

    But the real advantage here I believe is that PrimaLuna's circuit does not do full blown regulation and IS dependent (indirectly) on drive signal so that it automatically adjusts bias not only under DC conditions but also under AC conditions (meaning under the conditions of music playing and at ALL power levels the amp is capable of delivering).

    But what do I know. I'm just speculating given the marketing writeup and not having seen the schematic. I do agree with Gadget and expect it's some sort of servo monitoring "output conditions" of the tube in real time. Still wonder if it's analog or digitally sampled though. Looks like there might be an ASIC or custom programmed processor on that board.
     
    Last edited: Jun 15, 2018
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  14. pavlikkkk

    pavlikkkk New Member

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    Hello,
    a description of how it works here:
    https://www.audioamp.eu/en-clanky-1.html

    There is a lot more to describe in the forum:
    http://dynacotubeaudio.forumotion.com/t3627-auto-bias-board-on-roy-s-website

    Pavel
    www.audioamp.eu
     
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  15. Palustris

    Palustris Well-Known Member

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    There is nothing new about using servos to bias output tubes. There was 'how to' article in Glass Audio back in the '80s and conrad Johnson design has been doing it since probably the early 1980s with their MV 45 and MV 75.

    cj MV75.gif
     
  16. maxhifi

    maxhifi AK Subscriber Subscriber

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    That one isn't a servo, that's just a comparator which turns the LED on when the bias is over a certain level. That's a tool for setting bias, but you still have to bias the amp manually.
     

     

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  17. kward

    kward AK Subscriber Subscriber

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    Thanks for sharing that link, but this doesn't appear to be a description of the Primaluna circuit.
     
  18. maxhifi

    maxhifi AK Subscriber Subscriber

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    Here's a simple automatic bias circuit from a traynor guitar amp. Not an ideal circuit but it would definitely keep operator error form ruining tubes and blowing fuses.

    Screenshot_20180616-102530.png
     
  19. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    As interesting as this discussion has become, we seem to have strayed way off the OP query. I don't think any of the "old radio" mentioned in the first post had something close to any solid state device.
    Anyway, i am finding that that type of bias circuits which have LED or ICs become problematic after about 10 years or so. That is my experience from repairing those type of amps. I am currently working on a Sonic Frontiers amp with a bad either led or pot in the bias circuit.
     
  20. pavlikkkk

    pavlikkkk New Member

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    I do not think so. For nearly twenty years, Tentlabs has been selling similar
    auto-bias modules, which work on the same principle ...
     

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