Tube DIY Asylum

• jus' remember - NoWay 16:59:08 02/22/07 (6)
  In Reply to: Jim, Douglas..used a 13k load and it now sounds super posted by Bas Horneman on February 22, 2007 at 16:20:10:

  A true pentode takes special attention for its load line placement; the g2 voltage is one of the critical inputs.
  
  

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  • Re: jus' remember - Jim Womble 04:05:55 02/23/07 (5)
    In Reply to: jus' remember posted by NoWay on February 22, 2007 at 16:59:08:

    Yes, you are correct. In this application I wanted to make sure the plate voltage at full drive would never be lower than the screen voltage. I checked my notes last night, I actually have 250 on the plate, 140 on the screen, giving 110 volts of swing room, far more than the 300b requires.
    
    

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    • Re: jus' remember - Dave Cigna 14:01:11 02/23/07 (4)
      In Reply to: Re: jus' remember posted by Jim Womble on February 23, 2007 at 04:05:55:

      "In this application I wanted to make sure the plate voltage at full drive would never be lower than the screen voltage."

      why?
      
      

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      • Re: jus' remember - Jim Womble 09:53:27 02/27/07 (0)
        In Reply to: Re: jus' remember posted by Dave Cigna on February 23, 2007 at 14:01:11:

        When I first started researching the pentode/300B scheme, I read some of Thorsten's background material, including an article from the French publication that originally proposed using the EL84. TL credits the design as being a Verdier design. Somewhere in all that stuff I had remembered the comments that the tube was much more linear if the screen voltage was lower than the plate at full drive. The original model 91 had somewhere around 100 volts on the screen, while the anode was much higher.
        
        

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      • because..... - Peter Pan 19:49:46 02/25/07 (2)
        In Reply to: Re: jus' remember posted by Dave Cigna on February 23, 2007 at 14:01:11:

        > > "In this application I wanted to make sure the plate voltage
        > > at full drive would never be lower than the screen voltage."
        >
        > why?

        I would have thought a look at the curves, including screengrid current vs. anode voltage would provide a BFBFOTO....

        Think happy thoughts, use fairy dust - Peter Pan

        BFBFOTO = Big Footballing Blinding Flash Of The Obvious

        
        
        

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        • must be I'm just dumb - Dave Cigna 02:01:06 02/26/07 (1)
          In Reply to: because..... posted by Peter Pan on February 25, 2007 at 19:49:46:

          

          Or maybe it's the lack of fairy dust.

          I don't see anything footballing obvious happening around Va = Vg2 (250V in this graph.)
          
          

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          • You can't tell from that alone - Russ57 12:15:25 02/26/07 (0)
            In Reply to: must be I'm just dumb posted by Dave Cigna on February 26, 2007 at 02:01:06:

            If however you look at the way in which the curves differ when going from G2 voltage of 200 volts to 300 volts the picture begins to get a bit clearer. Even moreso if you do a transfer function. You won't have near the useable range but what you do get is more linear. So "if" you don't need the gain it is possible to improve some with this technique. It might be easier to see if you look at small signal pentodes.

            Russ
            
            

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