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Pentode, Tetrode, Triode, Diode... Ode to 'Odes

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Timbre Wolf, Oct 10, 2006.

  1. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Simpleton question about tubes: I've heard all about dual triode preamp tubes, pentode and tetrode power tubes (sometimes run as triode), and I even still deal with diodes (silicon and germanium).

    What are these 'odes that we're counting here? And what are some common types and characteristics associated with the #ode?

    Just wondering...

    - T
     
  2. uberpict

    uberpict Member

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    Node? Must be from Latin and mean something like point or area, just guessing. An ode to the 'ode which gives all our amps their breath of life.
     
  3. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I knew you'd come through with the tech low-down, OTM :AOK

    But what (if any) sonic impact is generally associated with each style. Can you generalize about characteristics that are associated with pentode power, vs. tetrode power, for instance? How 'bout when a pentode is run in triode mode?

    - T
     
  4. uberpict

    uberpict Member

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    Oh God, this is going to get technical beyond comprehension. :D

    Simple rule is pentodes produce more power without breaking up, AFAIK this is why they were created. Of course you could use more triodes to achieve the same power level at the point of clipping but the circuit gets more complex. OTM can explain it far better especially how the screen works and can produce more power in the same socket.
     
  5. teleamp

    teleamp Senior Member

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    A 12**7 is an example of a dual triode preamp tube, simply put it is actually two tubes in one. In a tweed deluxe the mic input goes to one side and the other input goes to the other side, the Matchless Spitfire preamp tube has both triodes paralleled together to act as one tube.

    Pentode/triode power tube operation very simply put, is in pentode you have two energy sources and in triode operation there is only one energy source. Edit Added: Some amp mfgr's use pentode/triode switch as a half power switch.

    Hope this helps.
     
  6. vibroverbus

    vibroverbus Member

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    Hmm... not too sure about 'two energy sources'. There is still only one cathode emitting electrons. I guess you mean the screen & plate? Screen current isn't really directly contributing to the output (unless you had some cathode-follower circuit I guess) Skipping that...

    If I was to super-simplify I'd try this:

    Triodes (3 electrodes) = one pair of electrodes (plate or anode & cathode) conducting, with the 3rd electrode (control grid) controlling the current flow between them

    Tetrodes (4 electrodes) = add an electrode (screen) as a shield for the grid to increase performance

    Pentodes (5 electrodes) = add an electrode (suppressor) to shield the other grids by catching 'stray' electrons from the plate.

    Triode 'mode' is simply a tetrode or pentode with the screen grid disconnected so as to operate more or less as a Triode.

    I'm sure sombody will jump out with comments about why this is inadequate/inaccurate but since I've been trying to explain tube amp operation to a player friend of mine I've been trying to hone the 'simple but not vastly inaccurate' explanation of amplifiers... you have to start somewhere...
     
  7. donnyjaguar

    donnyjaguar Member

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    I think I can summarize the differences betwixt triode, tetrode and pentode tubes - particularly when used in an output stage.

    Triode - not so efficient
    Tetrode - more efficient
    Pentode - even more efficient

    Many tetrode tubes are in reality pentodes with the cathode directly connected to the suppressor grid. Once you go beyond 5 electrodes in a tube I believe the efficiency starts to drop. These type of 6+ electrode tubes are used moreso for mixing applications than power amplification.

    There are generaly 3 types of output sections used in push pull amplifiers. Triode is the most simple, has the least amount of power and gain, but does have an advantage with respect to lowest output impedance. This translates into generally tighter bottom end and highest fidelity. Pentode output stages have far more power and gain, but are more sensitive to changes in load impedance and have more distortion. There exists a third type that falls somewhere between called ultralinear. This basically has all the advantages of triode but with much greater output power. It calls for a specially wound transformer to take advantage of the benefits. There are some musical instrument amplifiers that use ultralinear, but not many. Sun and the Fender 75 come to mind. In the case of Sun, I do believe the transformers were lifted from the David Hafler Dynaco amplifiers. I think it may have been Hafler who popularized, if not invented, the ultralinear output stage. There are some who will argue that ultralinear output sections have the disadvantages of triode and pentode. In my opinion, and others, ultralinear is a fundamentally excellent design and the number of hi-fi tube amps that use it are substantial. That said, I don't think it is the most ideal choice for a guitar amplifier. The number of pentode output stage amplifiers about bears this out.

    DJ
     
  8. uberpict

    uberpict Member

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    I don't think that's quite right about the pentode.

    There's only one cathode or power source but three grids that control the flow of electrons; control grid, screen grid and suppressor grid which makes five nodes or points. Cathode, control grid, screen grid, suppressor grid and lastly the anode or plates. Below is a link to some info.

    http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/audio/part9/page3.html

    Edit: DJ beat me to it! Took too much time looking up the little diagrams, LOL.
     
  9. Swarty

    Swarty Member

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    There are bunch of other ultralinear guitar/bass amps including late '70s early '80s 6L6 Fenders (Twin Reverb, Pro Reverb, Super Reverb, Bassman, etc...), many Ampegs, Marshall Majors, and several boutique amps including some Allesandro and I believe some Dr. Z.
     
  10. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I think this has helped me to best understand the sonic characteristics between these power tube types. Thanks DJ!! (I appreciate the other replies as well)



    Now I wonder about the complicated topic of running a pentode power tube as a triode. Does such a pentode then have a more linear output?

    And what about tetrodes: can they be run as triodes? And what is the sonic signature of a kinkless tetrode (as opposed to a kinky tetrode ;) )?

    - T
     
  11. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Here's a list of preamp and power tubes, sorted by their design types:

    Triode Preamp
    • 12AX7/ECC83/7025
    • 5751
    • 12AT7/ECC81/6201
    • 12AU7/ECC82/5814
    • 12AY7/6072
    • 12AV7/5965
    • E180CC/7062
    • 12BH7
    • 12BZ7
    • 12DW7/7247
    • 6SN7/5962/VT-231
    • 6SL7/ECC35/5691
    • 6DJ8/E88CC/6922
    • 6EU7
    • 7199 (one triode plus one pentode)
    Triode Power
    • 2A3
    • 300B
    Tetrode Power
    • 6L6 types (including 6L6G, 6L6GC, 5881, 5932)
    • 6V6 types (including 6V6G, 6V6GT, 7408, VT-107)
    • KT-61
    • KT-66
    • KT-77
    • KT-88
    Pentode Preamp
    • EF86/6297
    • 12BY7
    • 7199 (one pentode plus one triode)
    Pentode Power
    • EL84/6BQ5/7189
    • EL34/6CA7
    • EL33
    • EL37
    • EL156
    • 6550
    • 7027
    • 7581
    • 7591
    • 7868
    • 6K6GT
     
  12. teleamp

    teleamp Senior Member

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    What I meant by two energy sources (I was trying to explain in simplest terms) was the plate and screen grid connections, In pentode the plate gets fed from the OT and the screen grid gets fed from another node of the power supply. In triode operation the plate still gets its current via the OT and the screen grid gets current from the plate as well.

    If the screen current isn't really directly contributing to the output, why does an amp make more power in pentode than triode? I am still learning the theory and am curious.


    Thanks
     
  13. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    :jo Sorry to say, you lost me with that chart, OTM. And I've been really trying to study it. Is it supposed to represent a tetrode output tube being run optimally for linear operation? Or is it about a triode? Too many variables a boggologlin' my mind :crazy

    Maybe you can provide a little interpretation/translation for us mortals? Thanks!

    - T
     
  14. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Sorry, no lightbulbs are clicking. I know you're trying, and so am I, but we're missing each other.

    Its a two-axis graphic representation, but there are no units of measure. I don't know what the vertical axis is supposed to represent (four simultaneous factors?), and the horizontal axis is a ratio of two other variables. How does one change the ratio of screen load to plate load impedence anyway? Does one remain the same while the other is increased or diminished? What are high- and low-level distortion? I tried reading the text in the link, but its just not giving me that Edison effect.

    I think this is an attempt to graphically express a concept better handled with differential calculus (perhaps?), but it doesn't translate to this guitar player - I don't have a background in electrical engineering.

    How do these variables relate to something I can affect by adjusting my amp? Can I adjust something on an amp to try to experience tetrode-ultralinear-triode effects? Or is this just a design theory?

    Sorry, I'm a bonehead who needs a more visceral explanation. :( Thanks for trying - wanna take another angle on it? :eek:

    - T
     
  15. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    ...
    I still don't know what high level distortion and low level distortion mean. Can you define those terms? The article didn't elucidate that to my satisfaction.

    Thanks.

    - Thom
     
  16. donnyjaguar

    donnyjaguar Member

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    Nice graph OTM. I should probably clarify that when I'm making reference to a Pentode output stage I'm meaning Tetrode. Given that I'm an EL34 junkie, I'm thinking all electrodes are broken out to separate pins. In what I call a Pentode output stage I'm connecting the cathode directly to the suppressor grid hence there are only 4 connectors a la tetrode.
     
  17. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Uh-oh, now I'm really confused.

    I originally had what I thought was a simple question. I wondered if, perhaps, certain sonic characteristics might be associated with triode output vs. tetrode output vs. pentode output, for instance. Or, in preamps, triode vs. pentode (ex: 12AX7 vs. EF86).

    > Anyone want to make some simple, sweeping generalizations (that, of course, won't hold up in specific circumstances, due to amp circuit design differences)?

    - T
     
  18. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    The article alludes to "the tetrode sound" - that's exactly what I would like to hear described.


    I'm not really familiar with ultra-linear guitar amp examples previously named - I'll have to seek out sound from them. But there are a couple of triode output amps that I've found:
    - Marc Auble's Low Power guitar amps use 12AU7 or 5687 output (some appealing sound clips at Indoor Storm's Auble page)
    - Chris Siegmund's Diamond amp uses 300B output (I'd love to play/hear that!)
     
  19. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    DING DING DING!! Now you're talking my language -thanks for dumbing it down for me :D

    - T
     
  20. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    (OT)

    A master drummer from Ghana gave me valuable insight into West African culture: they've only got one word for music/dance - they're not separate concepts, because they never had one without the other.

    Back to the topic... it doesn't hurt to try descriptions. Thanks for the excerpt, I missed that before.

    - T
     

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