How does a tube amp work?

Discussion in 'Tube Audio' started by BmWr75, May 15, 2012.

  1. BmWr75

    BmWr75 Addicted Member

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    Your picture is not showing up for me.
     
  2. bobrown14

    bobrown14 Super Member

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    Ok take another looky

    Cheers,
    Bob
     
  3. mike1259

    mike1259 Magnaphile

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    Noblesse Oblige Guys!!!:tresbon:
     
  4. BmWr75

    BmWr75 Addicted Member

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    I can see it now Bob. Thanks.
     
  5. 68custom

    68custom Addicted Member

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    wow this is a great thread! I am trying to do a crash course in tubes and I may be just dangerous enough now.:) just kidding! please keep it coming!
    any one suggest any other online resources?
     
  6. BmWr75

    BmWr75 Addicted Member

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  7. BmWr75

    BmWr75 Addicted Member

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    Continuation of Post#2

    This thread starts here.

    What is a diode?
    [​IMG]

    Echowars explains how to test diodes and transistors here: http://www.audiokarma.org/forums/showthread.php?t=43186

    A diode is a two-terminal electronic component with asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other.

    The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking current in the opposite direction (the reverse direction).

    So, a diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current (AC) to direct current (DC) —this type of diode is one type of rectifier.

    Diodes can have more complicated behavior than this simple on–off action. Semiconductor diodes do not begin conducting electricity until a certain threshold voltage or cut-In voltage is present in the forward direction (a state in which the diode is said to be forward-biased). This is used in tube amps to soft-start the bias voltage on tubes.

    Bias diodes in a Sherwood S5000 II tube amp. The flat side in the anode and the rounded side is the cathode.
    [​IMG]

    Various types of diodes and the associated diagrams.
    [​IMG]
     
    Last edited: Nov 30, 2013
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  8. Perkinsman

    Perkinsman Active Member

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    This is great, thanks for taking the time to put it up. I have a few questions. If a rectifier converts AC to DC, how do the heaters get powered since I've read that they operate on AC? Is there a specific secondary winding exclusive to heater power? When taking voltage measurements off the pins (except for heater pins which I assume is always set on AC), then is your DMM supposed to be set on DC voltage?
     
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  9. BmWr75

    BmWr75 Addicted Member

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    The power transformer only steps the incoming AC voltage up or down to meet the power needs for circuits in a tube amp. The rectifier is downstream of the power transformer and converts AC to DC voltage.

    Yes, there is a low voltage (6, 12, etc.) AC power source in amps that run AC powered heater tubes. Yes, keep your DMM on the DC voltage setting for all other DC power measurements in the tube pins.
     
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  10. bobrown14

    bobrown14 Super Member

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    Sometimes tube amps will run heaters on DC (converted from AC). Usually the amps running DC will be TRIODE output amps with low power. "Sometimes" DC heater power will make less noise, so some builders will run preamp tubes heaters on DC and if the output tubes are triodes run those heaters on DC as well.

    You do know that you need to be EXTREMELY careful poking around there. Even though the heaters may have 5 volts give or take they will have SEVERAL amps on them, enough to kill you. Use clip leads, attach with power off, power on take voltage reading and power off. Be mindful of capacitors under the hood they hold 400-500V (stored) on them as well even after power off. Safety first...

    Cheers,
    Bob
     
  11. valve dude

    valve dude Member

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    love bring the EHT on
     
  12. Joseph Jove

    Joseph Jove Member

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    The most informative thread ever. My I.Q. has risen by several points.
     
  13. BmWr75

    BmWr75 Addicted Member

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    Thanks Joseph. Glad you found the thread informative. I certainly learned a lot pulling all the information together from other AK members and web sources.
     
    tony taylor likes this.
  14. VYNULADIKT

    VYNULADIKT Active Member

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    A comment on lethal voltages

    Great thread. I always wanted to have a better understanding of tube circuitry and this helps.

    The last thing I want to do is be a nit pic or naysayer but I feel the need to comment on this post:



    You do know that you need to be EXTREMELY careful poking around there. Even though the heaters may have 5 volts give or take they will have SEVERAL amps on them, enough to kill you. Use clip leads, attach with power off, power on take voltage reading and power off. Be mindful of capacitors under the hood they hold 400-500V (stored) on them as well even after power off. Safety first...

    I fully agree that the voltages present in tube equipment are lethal. Yes, always use clip leads, one slip could be your last. What I take issue with is the statement "5 volts give or take they will have SEVERAL amps on them, enough to kill you". This is a misconception that I have seen frequently. The argument back and forth of, is it VOLTS that kill you or AMPS that kill you.
    Here is the physics fact of the matter. To have current flow requires enough voltage to overcome the resistance in a circuit. The human body (skin) has considerable resistance, 200k to >500k hand to hand, less if sweaty or wet.
    Death CAN occur with as little as 70 milliamps AC or 300-500 milliamps DC. Entirely possible with B+ tube circuitry, or any other circuit in the 80v+ range, but not possible with 5 volts. The math, 5volts\200,000ohms is only 25 micro amps, far from lethal. Now this would look like it would take several hundred volts to push a couple hundred milliamps through the body but an interesting thing happens once current starts to flow. The resistance of the skin breaks down and the current continues to increase. Also you can hang onto a neon sign transformer that has 20,000 volt output but is limited to 20mA current with little more than a tingling up your arm. Current flows but not a lethal level due to the limited output, unless you have a serious heart condition or pacemaker. Not that I recommend this. I just wanted to clear up this common misunderstanding of the physics involved, it takes voltage to create the lethal current.
    Some interesting data here http://hypertextbook.com/facts/2000/JackHsu.shtml
     
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  15. Dr. Humboldt

    Dr. Humboldt AK Subscriber Subscriber

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    Thanks for the info! This really helps me -- I'm about to begin my first recap job -- a compact Stromberg-Carlson ASR-433.
     
  16. trudie

    trudie Ted

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    Great INFO!!! I'm reading up on threads prior to rebuilding a Heathkit AA-100
     
  17. Dandy

    Dandy Super Member

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    Good to see that you have found Gordon's 'end all' thread. That is the one to follow. Take careful note of his advice on handling the old PCBs and checking components.
     
    trudie likes this.
  18. Dr Morbius

    Dr Morbius Active Member

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    This is one super thread! Many, many thanks!!!
     
  19. babblefish

    babblefish Active Member

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    Great thread. May I also suggest these websites?:

    http://www.valvewizard.co.uk/index.html - he talks mostly about guitar amps, but much of it applies to audio too.

    http://www.thermionic.info/ - just a general collection of articles and application notes on tube design, though much of it dates back as far as the 30's. Still good reading.
     
  20. avahifi

    avahifi AK Member

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    Tube amps work by forcing electronics across a open connection, a nice vacuum insulator, by putting a lot of heat and voltage across the connection inside the tube. This is kind of ironic as compared ot 6 gauge welding cable for speaker wires with very very very low resistance to electron flow. If the first condition produces great sound, why do we need the second condition in series with that?

    Just wondering?

    Frank Van Alstine
     

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