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Gain Stages of V1

Discussion in 'Amps and Cabs [BG]' started by carbonfold, Jul 28, 2012.


  1. carbonfold

    carbonfold

    Oct 13, 2006
    Dallas, Texas
    Question.

    Say we are building a preamp on V1 with a 9 pin 12AX7 with two inputs. What are the audible differences between running only one side of a 12AX7 vs both sides in series vs both sides in parallel vs running them independently into V2?

    Hopefully I make sense. My curiosity asks because I haven't really read anything yet that "somewhat" lines it out.
     
  2. I personally have not experimented doing this, but apparently Matchless used this concept.

    The best advice I can give...get "Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. He has a chapter on paralleling.
     
  3. the differences are pretty huge. You're talking about some pretty different circuits with varying amounts of gain there
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    One of the advantages of a parallel gain stage is that it is quieter than a series configuration. This is why parallel stages are used in hifi amps.

    A parallel configuration does a few things. The parallel plates act as parallel resistors which means that the output impedance is lowered. This can be useful for driving a tone stage. The lower resistance means less noise so this configuration is good for a V1 position. Two parallel triodes also means increased current and power handling capacity over a single triode. Current capacity is doubled so it acts as a better driver. Power handling capacity is not necessarily doubled, it depends on the tube. The gain is also increased in the parallel configuration. In parallel the input capacitance increases. This can help fatten up the tone a bit.

    There are tradeoffs in a series vs parallel topology. How it sounds very much depends on the rest of the stages that follow. The circuit dictates if it is useful. Quieter, higher current capacity, higher gain, fatter tone. All this, except the higher current capacity, can be achieved with two series stages.
     
  5. carbonfold

    carbonfold

    Oct 13, 2006
    Dallas, Texas
    Would there be any benefits to just using one side and not using the other side? It just be less gain I assume?
     
  6. I'm not sure I understand your question about just using one side.

    A dual triode like the 12Ax7 can be considered to be two independent tubes, in one convenient glass envelope. One side can certainly be used and the other side left idle.

    The amount of gain you wind up (after using some combination of one or both sides, whether is series or parallel) depends on the design of the circuit. You could have V1a as a conventional common-cathode gain stage. Stack V1b in series with V1a, with the same topology, for more gain. OR...V1b could be designed as a cathode follower (still in series with V1a) and reduce gain.
     
  7. carbonfold

    carbonfold

    Oct 13, 2006
    Dallas, Texas
    Yep, was talking one side of the 12AX7 something such as:

    input-->V1A (V1B not even used and left to idle with no signal)-->Volume-->V2A



     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There are many different options, here are two

    input -> V1A -> Volume -> V1B -> Tone stage -> V2A ...

    or

    input -> V1A -> Tone stage -> V1B -> volume

    Inserting a tone stage results in a loss so it is followed by a gain stage to make up the loss and sometimes condition the signal. The first one hits the tone stage with a higher gain signal. More distortion at higher levels. The second option is more conservative with more headroom. I comes down to those tradeoffs that I mentioned. Every design is a compromise to some extent.

    Find an amp that you like and copy the design and layout. Someone else figured out what works and how to do it so take advantage of that.
     
  9. Again, grab a copy of the book I mentioned--"Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. Here's his web site with lots of free info. http://www.freewebs.com/valvewizard/
     
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    +1

    That's a good starter book, it is fairly easy to read without a lot of formulas. Even better, Chapter 1 is available as a free download. Read it and you'll have the resources to design V1.
     
  11. jastacey

    jastacey Supporting Member

    Feb 8, 2009
    Houston,Tx
    Some of the old Traynor YBA-3 amps used only one side of a 12AX7 in the V1 gain stage, all the ones that I have seen/worked, on have been the non-master volume type
     

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  12. carbonfold

    carbonfold

    Oct 13, 2006
    Dallas, Texas
    Also the Sovtek Migs.


     

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