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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    CAUTION - TUBE AMPS CAN USE 500+ VOLTS DC POWER WHICH CAN KILL YOU. BE CAREFUL, ASK QUESTIONS FIRST.

    Useful links for tube and tube amp information are located here.

    Explanation of how a tube amp works is continued here and here.

    What are the parts used in a simple triode (three part) vacuum tube?

    [​IMG]

    Anode – the piece of metal in a tube, also called the Plate, which is positively charged (B+ DC voltage) and to which the negatively charged electrons flow. The electrons flow out of the tube via the anode (plate) to generate an electric current. In a small signal (preamp) tube, the anode current is fed to the control grid of the power tube. In a power tube, the anode current is fed to the output transformer input winding.

    Grid – also called the control grid. It is placed between the cathode and anode. The grid uses a negative bias voltage. The AC signal from your source (e.g., CD player, turntable, tuner) is applied to the grid. The grid acts like a valve inside the tube that controls the flow of electrons from the cathode to the anode. This is why vacuum tubes are also sometimes call Valves, or thermionic valves.

    While we are using the water hose analogy to understand electricity, think of electrical voltage as being the pressure inside the water hose (wire), electrical current being the flow rate of water through the hose (wire) and a ball valve or nozzle on the end of the hose being an electrical component, like a resistor or rheostat in the electrical circuit, that controls the flow rate of the water through the hose (wire or circuit).

    If a tube didn't have a control grid, the signal leaving the tube would be at the maximum all the time. The control grid regulates the flow of electrons from the cathode to the anode. Higher electron flow equals more amplification.

    Cathode – the piece of metal in a tube that surrounds the heater, but does not touch it (an indirectly heated cathode), or is located nearest the heater. The cathode emits electrons when heated and DC power (B+ voltage) is applied. These electrons generate a current as they flow from the cathode to the anode.

    Heater – the filament (piece of wire usually wrapped around a rod) that heats the cathode so the cathode can emit a cloud of electrons. Electrons are negatively charged. The heater is what you see glowing inside a vacuum tube you amp is turned on. The heater is also of course what causes the tube to be hot to the touch. Heaters typically use AC power (but can use DC power), 6.3 volts for 6DJ8 and similar tubes, 12.6 volts for 12AX7 and similar tubes.

    Getter – usually a halo shaped piece of metal wire near the top of the tube, but it can also be on the side or bottom. Other shapes are also used: squares, “D” shaped, windmill, dishes, etc. The getter is coated with a highly volatile powder before being installed in the glass envelope. The getter has no effect on how a tube operates in a tube amp. After it is flashed (see Getter Flash below), it serves no purpose.

    Getter flash – after the tube is built, all the air removed and sealed to create a stable vacuum in the tube, the getter is flashed by heating it and the volatile powder explodes and created the getter flash. Getter flash appears as a silvery mirror-like coating on the inside wall of the glass envelope near the getter wire. Getter flash turns powdery white or light brown as a tube ages. This is typically caused by a slight loss of vacuum in the tube and the air that leaks in reacting with the getter flash. Loss of getter flash can indicate that a tube is going bad. If the getter flash is 100% powdery white, the tube is probably bad.

    Glass envelope – a glass bottle (or sometimes metal) is used so that the air inside the tube can be evacuated to create a vacuum inside the tube. A vacuum is required to keep the components inside the tube from burning up rapidly.

    Base and pins – the base, usually plastic, gives the bottom of the tube and pins mechanical strength and reduces the potential that the tube will be damaged during installation and removal. Most small signal tubes(e.g., 12AX7s, 12AU7s, etc.) and some power tubes (e.g., 7868s) don’t have a plastic base. The pins are the electrical connection between the components inside the glass envelope and the tube sockets in the amp, preamp or tuner section of the tube amp.

    If you want to know what each pin of any tube does, check out the Duncanamps.com searchable database: http://tdsl.duncanamps.com/tubesearch.php

    [​IMG]

    Types of vacuum tubes?
    Tubes like the 12AX7, 12AU7 and 6DJ8, all commonly used in preamp sections of a tube receiver or integrated amp, are all dual triode tubes. This means they are actually two tubes in one glass envelope. Preamp tubes amplify the signal only a little bit in comparison to the amplification achieved with power tubes.

    There are many other kinds of vacuum tubes. For example, a pentode power tube like the EL84/6BQ5/7189 contains all three components described above (cathode, anode, control grid), but adds two more grids (screen grid and compressor grid).

    What does the Power Transformer do in a tube amp?
    Transformers either step up low voltage to higher voltage or step down high voltage to lower voltage. You have a step-down transformer somewhere outside your home that reduces the high-voltage line next to your house from 440 or higher VAC (volts alternating current) to 120 or 240 VAC that is used in your home.

    AC power is supplier to the power transformer and stepped up to a single higher voltage or various voltages via separate windings in the transformer. The primary winding is the power input side of the transformer, secondary winding is the output side.

    Why is a rectifier needed?
    Rectifiers convert AC power to pulsing DC power. The AC output of the power transformer is fed to a rectifier tube or rectifier bridge or diodes that convert the AC (alternating current) power to pulsing DC (direct current) power. The DC power pulses because only 1/2 of the sinusoidal AC power wave is converted into DC power by the rectifier. These DC power pulses are smoothed out by the power supply capacitors (see explanation below). The resultant DC power (B+) is then supplied to the anode and cathode (plates) of the tubes.

    This AC to DC conversion is required because most parts in a tube amp use DC power.

    What purpose do the power supply filter capacitors serve?

    Great link on why old capacitors need to be replaced, and which to replace: http://www.antiqueradio.org/recap.htm

    Power supply filter caps are polar electrolytics (unlike coupling caps which are non-polar film or paper-in-oil caps) and are wired in parallel (coupling caps are wired in series). The pulsing DC power from the rectifier stage is smoothed out by the power supply filter capacitors to supply consistent DC power to the tubes and other components in the amp. The diagram below shows how the amplitude of the pulses is reduced via the filter caps (and regulator if your amp has one).

    One analogy is the power supply filter caps act like the springs on your car that absorb the bumps in the road (pulsing DC power from the rectifier)and deliver a smooth ride (constant DC power) to the passengers (tubes).

    @GordonW further explains how power supply filter caps work in this post.

    [​IMG]

    How does a small signal tube work in a tube preamp or power amp?
    Signal tubes such as 12AX7s, 12AU7s, 6DJ8s are typically used in the preamp section of an integrated amp to amplify the input source signal. You will also see small signal tubes in standalone power amps too, where they are also being used to amplify the preamp signal.

    The AC signal from from the audio source (e.g., CD player, turntable) is fed to the control grid of the small signal tube and modulates the electron flow from the cathode to anode. In simple terms, the AC signal rides on top of the DC power flowing from the cathode to anode.

    How does a power tube work in a tube amp?
    Power tubes consist of a cathode, grid, anode, getter, heater and sometimes other types of grids. The anode from a small signal tube supplies the slightly amplified source signal to the power tube control grid, which controls the audio frequency and volume output of the amp.

    Maximum # of character reach in this post. Explanation of how a tube amp works is continued here.
     

    Attached Files:

    Last edited: Jun 29, 2015
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  2. b brian

    b brian Well-Known Member

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    The framework that Scott is suggesting has given me more information, even with its holes, then the other sites I have been using to understand my new tube mono blocks.
    It gives a start to sound pathway that helps explain the relationship and jobs of each section.
    Even with its holes, it fills in a lot of knowledge for me. Hopefully this will become a sticky for those new to tubes can follow and be a building block for their knowledge.
     
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  3. Kegger

    Kegger Anything can be S "MODed" Super Mod Subscriber

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    This could get lengthily.
    If I get the time before someone else does I'll do my best with this.
    (no guru here, just a guy whos learned a few things along the way)
     
  4. BmWr75

    BmWr75 Addicted Member

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    Thanks Kegger. Sharing your vast helpful knowledge in this area will be greatly appreciated.
     
  5. aedaza

    aedaza Active Member

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    +1 to this thread! :lurk:
     
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  6. Rockyhill

    Rockyhill 148 State Street Subscriber

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    Thanks for posting this Scott. I'm also a paint by numbers guy.
     
  7. buglegirl

    buglegirl In The Direction Of The Singularity Subscriber

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    Hiya,

    Make sure to sticky it Kegger. I know I would be coming back to it myself from time to time.

    Frannie
     
  8. Dandy

    Dandy Super Member

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    If we all pitch in, we could probably identify helpful explanations that Kegger and others have made in various posts.
     
  9. ferninando

    ferninando AK Subscriber Subscriber

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    seems to me that a fundamental knowledge of how a vacume tube amplifier works would be a necessary beginning.
     
  10. BmWr75

    BmWr75 Addicted Member

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    Necessary beginning for what? Say more.
     
  11. ferninando

    ferninando AK Subscriber Subscriber

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    "Necessary beginning for what? Say more."

    For the learning about how tube amplifiers work as you stated in yur question.
     
  12. ebacon

    ebacon Super Member

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    +1 for a wiki like this. I am about to embark on the tube venture and am also looking for friendly info.

    My questions are so simple they are too stupid for experienced people to write about. For example what are the dashed lines within a tube schematic symbol? The check mark shaped lines? The solid lines?

    I earned my electrical engineering degree in the 1980s and never learned that stuff. Instead I learned the digital schematic symbols such as: a bubble means inversion, a small triangle within the symbol means edge triggered, etc. Typical JEDEC conventions.

    I'm still toob stoopid.
     
  13. Skylab

    Skylab Altimeter reading zero Subscriber

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    Hiya Scott,

    I'm far from an expert, but one of the most valuable things I've learned about tube amps is the relationship between the total current draw of the tubes, and the power transformer's capabilities. A power transformer is going to have the ability to provide a specified total amount of current in Amps to the heaters of the tubes that are being used. It's important not to have the tubes draw more heater current than the transformer can safely and comfortably supply. Unlike SS devices, tubes will draw their full heater current completely irrespective of the audio signal. You can thus read data sheets of the tubes that are used, add up the heater current demands, and look to see that the power transformer is rated adequately for that total demand. This information can also be used to determine whether a given amp might be able to safely supply additional heater current to a substitute tube (although there are obviously other considerations there).

    Also the B+ High voltage demand from the complete circuit needs to be added up so you can rate that aspect of your transformers needs as well.

    Edit: an incredibly helpful book on this subject IMO is "Beginner's Guide to Tube Audio Design" by Bruce Rosenblit. I've read it several times, and I still can't really say that I fully understand everything, but the way that it explained the basics was invaluable to me. After doing so I was able to read tube data sheets and actually understand what the parameters there mean, which is really useful.
     
    Last edited: May 15, 2012
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  14. LexDM3

    LexDM3 AK Subscriber Subscriber

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    Same here. I got my MS in Electrical Engineering in 1979 and worked almost completely in the digital domain. I am pretty good on analog SS circuits but am just learning tubes. The good news is that I'm a neighbor of Audiodon's and about to take on a Fisher KX-100 project with his guidance. I just made a big copy of the schematic and want to understand the theory of how it works before diving in. The idea of a "How (Tube) Stuff Works" sounds great to me. Thanks for starting it, BmWr75.
     
  15. Kegger

    Kegger Anything can be S "MODed" Super Mod Subscriber

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  16. Dandy

    Dandy Super Member

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    Here's an example of a recent helpful explanation by Kegger for bias http://www.audiokarma.org/forums/showthread.php?t=442251&page=2 post #17.
    ----------

    Just for those reading that may not understand how an "adjustable bias" amp works.
    (also known as fixed bias quite often, like an ST-70 for example)

    There is an negative voltage applied to grids of the output tubes, the more Negative
    you make this voltage the Less current the tube draws and you check this current in
    the Cathode of the output tube by measuring the voltage dropped across the 10ohm
    "current sense" resistors generally found there, using ohms law to figure that current.
    (say .5v dropped across an 10ohm resistor will give you 50ma of current .5/10 = .05)

    Now the coupling caps(in this case C5 and C7) couple the driver stage to the output
    tube grids to drive them, they block the DC voltage at the drivers stage from getting
    to the output tubes grids, if those caps fail or "leak" some of that DC voltage will get
    through and mess with the Negative voltage you are using to bias the tube, so you'll
    have to turn up the Bias Voltage(more negative) to try and compensate for it.

    [​IMG]

    ---------------
     
  17. GordonW

    GordonW Speakerfixer Subscriber

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    I'd suggest that this thread (especially the posts delineating the five or six common basic circuit designs used on most tube amps) is a good basis for expansion. It would need to be fleshed out, for a wiki thread, but it's a good foundation, I think:

    http://www.audiokarma.org/forums/showthread.php?t=200317

    I'd be glad to help flesh out such... but I would like to have some co-authors to help fill out info on some of the details, especially if we get into some more off-the-wall variations I may not be familiar with...

    Also, if there's someone who has read the Crowhurst books in more detail than I have (I'm going through "High Fidelity Amplifiers" now), that could be a big help. A discussion on feedback, including the reasons for it, the pitfalls and problems that can result from trying to implement it, and how to make it work, could be a BIG help, to those people scratch-building, and those working on, or modifying, amps. It's probably the most common stumbling-block, that causes trouble with amp operation, when things need to be changed out due to blown/burned/missing parts having to be retro-fitted with different parts. I'd suggest Dave Gillespie as an example of someone who could do this job well... his comments on this thread, are an example of what I would think could be a great part of a wiki:

    http://www.audiokarma.org/forums/showthread.php?t=443162&highlight=stability

    Bashing through calculations to figure out why an amp is or isn't stable isn't the most fun thing anyone will do- but it's an essential part of making sure a design will work properly. We don't have to get into the hard math- but an explanation of WHY things are done as they are, and why it's important to not 'upset the apple cart'- by being sure to NOT change things arbitrarily in terms of coupling caps, transformers and feedback loop components- is an important point to remember when working with tube amps...


    Regards,
    Gordon.
     
    Last edited: May 16, 2012
  18. BmWr75

    BmWr75 Addicted Member

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    Some more questions for the tube amp experts.

    Many tube amp schematics show that only part of the power transformer output goes through a rectifier to change it from AC to DC power. What parts of a tube amp use AC power and what parts use DC power?

    Is the signal supplied from audio sources to the inputs of a tube amp an AC or DC voltage signal?

    Can the heaters on vacuum tubes use either AC or DC power? If yes, why would one be preferred over the other?

    What do phase inverter tubes do in a tube receiver or tube integrated amp or tube power amp? Or, maybe the questions are, Why does the phase need to be inverted? Or, the phase of what is being inverted?
     
  19. ferninando

    ferninando AK Subscriber Subscriber

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    audio signals are AC.
    DC can be used for heater(filament) voltage to eliminate hum, an AC interference signal.
    No AC power is used after the rectifier tube and filter system has converted it to DC for plate voltages and grid voltages and bias voltages..
    See 'basic electronics, AC theory" for ref.
     
    Last edited: May 16, 2012
  20. whell

    whell AK Subscriber Subscriber

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    This might be a monster project, and may not be easily distilled into a single wiki or thread. Seems to me there in this thread alone there are topics that have been thrown out that might require wikis of their own:

    - electrical theory
    - tube design
    - tube types and applications
    - circuit design
    - amplifier design options and merits
    - tube amp design and variations
     
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