Tube amp topologies

Discussion in 'Tube Audio' started by Squidward, May 8, 2008.

  1. Squidward

    Squidward Scrappy Mod Moderator

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    I'm writing this for a few friends, it's meant to be an entry level primer on tube amp topologies. I'd like to keep things very general, so that a newbie to the hobby can gain some vocabulary for the various tube types and output topologies. There are exceptions to probably everything I will say, but I wanted to share what I could in as general language as possible. There are other threads for hard-core techies, this one is for the novice.

    Triodes have one and only one "control grid". Triodes can be used in Single Ended Triode (SET) circuits, and in push-pull circuits. If a true triode is used in any circuit as an output tube, it can not utilize "pentode" or "ultra-linear" mode.

    Pentode tubes have three control grids, and the addition of the extra grids allows the pentode to be run in various different output modes: triode, pentode, and ultra-linear. Triode mode mimics a true triode tube, is often of a lower power than the others, but is popular because of it's sonics. Pentode mode usually uses a tube to more of it's maximum output potential, and often gets the highest power ratings out of a tube. It's generally fallen out of favor, but making a comeback in certain DIY circles. Ultra-linear mode is sort of a compromise between the two, with power ratings between triode and pentode. Ultra-linear mode is only possible if the output transformer has a special tap that supports this use.

    There are three basic output topologies for tube amps: Single Ended (SE), Push-Pull (PP), and "Output Transformer-Less" (OTL). OTL amps do not use an output transformer, are fairly exotic due to the number and size of tubes requires, and other factors. They are worthy of mentioning, but beyond the scope of a document aimed at a novice. As both SE and PP amps require an output transformer (OPT), the characteristics of that transformer will dictate many factors about the amp. Generally speaking, an output transformer will be either a single ended OPT, or a push-pull OPT. Either PP or SE OPTs can have an ultra-liner (UL) tap. If your output has it, you can optionally use it. One could argue that the output transformer is the most critical part in a tube amplifier.

    A Single Ended amp uses a single output tube. If the tube is a native triode (300B, 2A3, etc.) this is a SET amp. You can run a pentode in Single Ended Pentode (SEP) mode as well, but it's not a SET amp. Using a pentode, you can have a Single Ended Pentode (SEP), and if your transformer supports it a Single Ended Ultra-Linear (SEUL) mode as well. It is possible to switch between these modes.

    Push Pull uses a pair of tubes to power the speaker. Triode, Pentode, and UL modes are all available here, too, depending on the choice of tube and transformer.

    Parallelism is also possible, usually for more power. A Parallel Single Ended (PSE) amp will use more than one tube run in single ended mode. A Parallel Push Pull (PPP) amp will use two or more pairs of outputs. While it's usually easy to spot a Parallel Push Pull amp because of the number of output tubes (4, 6, 8, etc.), the most common Parallel Single Ended amps can resemble a Push-pull amp, as each have 2 output tubes. Generally speaking, parallel amps are not as common as non-parallel. Vintage parallel push pull amps were often PA type amps. Parallel Single Ended amps seem to be more common in modern times, as people want more power while trying to retain some single ended sonic characteristics.

    Single ended amps are generally prised for their sonics, but put out less power than push-pull amps. They are generally simpler and have less parts than a push-pull amp. They tend to be fussier about hum and noise, so particular attention must be paid to the quality of the power supply, amplifier layout, careful heater wiring, and other factors that induce noise. In some ways, parts selection becomes more critical because there are fewer of them to begin with. This can work to a DIYers economic advantage too, as sometimes it's possible to use a budget for fewer but higher quality parts. While circuit choice is still very important, due to the simplicity of the design, most SE circuits resemble each other to a great degree.

    Push-pull amps generally have more power than SE amps for any given tube. Generally speaking, they tend to be more complex then SE amps, with a higher parts count and more tubes. They have some noise-rejection qualities inherent in their topology, and sometimes this can present less of a problem to get noise free. Choice of circuit can be of particular importance in a push-pull amp, some can really sound better than others. There is also a greater number of circuits to choose from, as different designs handle the more complicated push-pull circuitry in different ways. Parts selection is still important, but perhaps circuit choice is even more critical in a push pull amp.
     
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  2. shelly_d

    shelly_d Not An Audiophool

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    Descriptions look really good to me. I would bring up 2 possible improvements.

    1. If you can - small diagrams of a typical push pull arrangement and typical single ended arrangement

    2. You might want to mention that the pentode can be run in triode mode in a single ended amp. The omission of that might lead a newcomer to think that is not an option.

    Great job all around however
     
  3. AlleyKat

    AlleyKat Super Member

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    Thanks, Ed:thmbsp: Writtten simply enough that even I can begin to understand it. I hope you will continue to enlarge this thread as time permits, I need to brush up on my tube knowledge. Although I built 2 pairs of Dynaco mono tube amps from kits (Model III's and Model IV's), I simply followed the instructions carefully and did my best to solder competently. I had absolutely no idea of what I was doing, or why.

    I really liked both pair, with a particular fondness for the Model III's. Unfortunately the advent of small children in my family ended my tube era.

    You've gotten me deeply into trouble with your kind generosity in loaning us the ST-70 at the Fest, as my wife has decided that the L-710A's will be "her" speakers in her own setup at the new house and that they MUST be powered with tubes. I will rely upon you to continue to expand "Tubes for Dummies" so that I can begin to make an educated decision in our upcoming purchase.
     
  4. Redboy

    Redboy a few good watts Subscriber

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    Sticky this thread! :thmbsp:
     
  5. similost

    similost Rockin and Rollin....

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    VERY good stuff... but you lost me with "control grid"

    I'm trying to understand though. I'm hoping I will need to know all of this stuff very soon.
     
  6. Old1625

    Old1625 Super Member

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    Very nicely done!

    But I would change "Pentode tubes have three control grids," as it is the grid closest to the cathode that is the sole control grid, while the next one up is called the "screen" and the one above that called the "supressor".

    The screen, placed on a positive charge and suitibly bypassed to shunt any AC component to ground, serves effectively as an electrostatic shield, which has the favorable effect of reducing the excessive control-grid-to-plate capacitance found in triodes. Thus relieved of this unwanted capacitance leaves the tube unfettered and capable of much greater gain.

    But such a tube is called a "tetrode" until the 3rd or "supressor" grid is added to retard secondary electron emission, where the heated-up plate boils off electrons as does the cathode, reducing the current flow. Given a negative charge with respect to the plate pushes these stray electrons back to the plate.

    I'm sorry to be such a stickler, but perhaps you can take my detailed techie mumbo-jumbo here and put it in your own neat prose... :thmbsp:
     
  7. Old1625

    Old1625 Super Member

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    X2.
     
  8. spartanmanor

    spartanmanor Lunatic Member

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    Same here. You need to dumb it down some more. :D
     
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  9. dnewma04

    dnewma04 The Healer Super Mod Subscriber

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    I'm dumb, I might be qualified to explain the "control grid" to others like me. It's function is to control the flow of electrons between the anode and the cathode in the tube. It's position and makeup determine some tube properties like amplifcation factor and transconductance. I'll let someone else convert transconductance into english, but what is important is that it is inversely proportional to noise. High transconductance tubes have lower noise characteristics. Amplification factor describes the amount of gain a tube has. You will often run across the terms high mu, medium mu, and low mu. These are descriptions of the amplification factors. 6SL7s and 12AX7s are examples of common high mu dual triodes.

    Again, I'm not particularly good with this stuff, so if my description is way off, feel free to correct me.

    Here is a little diagram showing the various parts of a triode:

    [​IMG]

    Here is a diagram of something you might see in the real world and not a schematic:

    [​IMG]


    Pentode diagram commonly used in schematics:

    [​IMG]

    And a tube cutaway type diagram showing the typical construction:

    [​IMG]
     
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  10. similost

    similost Rockin and Rollin....

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    Cool Dave.. thanks.. I understand better now.. guess I should read up more about how tubes are made, and how they work to really understand..
     
  11. Squidward

    Squidward Scrappy Mod Moderator

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    I guess what I hope a newbie takes away from the triode/pentode discussion is that compared to a triode, the pentode is a more complicated device, with advantages and disadvantages. The triode is an exercise in minimalism, it has all the pieces it needs, and no more, to do it's job. Some schools of thought (and hearing) suggest that the triode has a distinct sound, and simpler circuits are possible, so it is the device of choice. Pentodes allow a few more hookup options and flexibility, and to some ears are the superior sonic devices. Speaking generally, amplifiers built around triodes tend to be of lower wattage compared to pentode amps. Triodes are also used often in Single Ended circuits. Pentodes are often used in push-pull topologies, where their additional complexity allows them to achieve higher power through the different modes of operation (UL and Pentode) available to them.
     
  12. RayW

    RayW Parrothead with a badge Super Mod Subscriber

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    A most excellent primer, Ed! Much appreciated. I have a question about the push-pull topology. From the name I would guess that it works similarly to a class B SS amp in that the circuit is split between a "positive" portion and a "negative" portion so that each can do its own job better. Is that correct or am I guessing wrong about the name. (note, I oversimplified but I'm just looking for a general idea about what's actually going on)

    And thanks to Dave for the physics lesson about what happens inside the tube. As I was reading the OP I was thinking I'd have to ask about that but you jumped right on it. :thmbsp:

    And one more about a tube itself...how does the amplification work? The current comes into the cathode, gets passed through the grid where electrons are added and then mroe current comes out the anode?

    Ray
     
  13. jaymanaa

    jaymanaa Lunatic Member

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    The grid doesn't add or take away electrons, it simply impedes the flow of electrons through the vacuum by it being more positive or negative. The tube is a "valve", and the grid is the handle.
     
  14. Army

    Army Addicted Member

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    Tube noob here :wave:

    Keep rolling with it fellas it's starting to make some sense, this will take a while I'm totally tube dumb also :yes:
     
  15. jaymanaa

    jaymanaa Lunatic Member

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    Very good post :thmbsp: and don't forget my favorite, the pentode running as a "virtual" triode in SE.:music: I wish I could have found this thread a few years ago.:yes:
     
  16. doucanoe

    doucanoe Registered Audio Abuser

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    Thanks Ed!

    RC
     
  17. PakProtector

    PakProtector AK member

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    PP can be class A, AB or B. In Class A, the amp behaves like a bridged( not paralleled ) pair of SE amps. Both of the power tubes are conducting for the whole signal waveform. The two phases, or sides are each getting signals that are 180 degrees out of phase with respect to the other. One goes up while the other goes down.

    How we get these two signals is quite another thing; there are many ways of doing it. Any amplifying device can operate in any class; there is no restriction of Class A to SE designs, though SE must be Class A.
    cheers,
    Douglas
     
  18. DaWoofer

    DaWoofer Super Member

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    Isn't the input signal impressed onto the grid?
     
  19. Dave C

    Dave C Super Member

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    What is the function of a "getter" as in Halo getter etc.
     
  20. dnewma04

    dnewma04 The Healer Super Mod Subscriber

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    During the vacuum tube's construction, one of the last steps when sealing up the tube involves removing any remaining gasses. The function of the "getter" is to absorb any residual gasses. Getter is the part of the tube that often looks like it's chrome on the inside of the glass.
     
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