Posted by John Swenson (A) on January 17, 2007 at 15:34:16

I'm planning on resurecting the BDT preamp design, but I need a good output transformer. I was planning on the B7 5k/600 but VS says this is no good for PP use (parafeed or otherwise) so what do I use? The BDT tube has two plates and is inherantly differential so I really need a PP transformer. Mike, do you have anything that will work with this?

Other companies make some PP specific line out transformers, do I have to use one of these? I love the MQ iron I have and would like to stick with it if I can.

Thanks,

John S.

• Does it have to be PP? - VoltSecond 19:19:39 01/17/07 (4)
  In Reply to: More questions on PP pre out posted by John Swenson on January 17, 2007 at 15:34:16:

  

  I paralleled two plates in a single envelope in one of my Headphone amps. This is the amp that sounded the best, but it didn't have enough gain to be run from a portable CD player (A 115V player could drive it just fine.)

  The parts on the grids are a cross feed circuit you don't want in a preamp.

  R5 and R7 worked the magic in paralleling the two tubes. R5 added to R7 should approximately equal the bias resistor R4
  
  Play safe and play longer! Don't be an "OUCH!" casualty.
  Unplug it, discharge it and measure it (twice) before you touch it.

  . . .Oh!. . .Remember: Modifying things voids their warranty.
  
  

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  • Voltseconds, can you explain the function of - Tanker 15:44:28 01/23/07 (3)
    In Reply to: Does it have to be PP? posted by VoltSecond on January 17, 2007 at 19:19:39:

    R5 and R7 in your configuration? Thanks
    
    

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    • oops, I wasn't scrolling down - VoltSecond 17:55:18 01/23/07 (2)
      In Reply to: Voltseconds, can you explain the function of posted by Tanker on January 23, 2007 at 15:44:28:

      R5 and R7 couple the ac current from driving the primary into the cathode of the tubes. This makes the voltage on the cathode of the tube change less and thus the tube will have a Rp closer to having a voltage source on the cathodes.

      R5 and R7 also causes any cap on the cathode to behave like a larger value.

      Note: When paralleling tubes, the plates can be tied at dc if the cathodes are not directly dc coupled. The cathodes can be tied at dc if the plates are not directly dc coupled. This was explained at VSAC 1999?

      
      
      Play safe and play longer! Don't be an "OUCH!" casualty.
      Unplug it, discharge it and measure it (twice) before you touch it.

      . . .Oh!. . .Remember: Modifying things voids their warranty.
      
      

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      • Voltseconds, got one more question - Tanker 16:02:38 01/24/07 (1)
        In Reply to: oops, I wasn't scrolling down posted by VoltSecond on January 23, 2007 at 17:55:18:

        How do you calculate the bias current in this case? Also I am not really sure I understand what you meant by "cathode can be at DC if plate is not DC couple or plate can be at DC if cathode is not DC couple". Can you explain a little more in detail? Thank you.
        
        

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        • Re: Voltseconds, got one more question - VoltSecond 20:57:29 01/24/07 (0)
          In Reply to: Voltseconds, got one more question posted by Tanker on January 24, 2007 at 16:02:38:

          1. Assume zero current in R5 and R7 and calculate normally.

          2. If you solder the plate of the two tubes together and at the same time solder the cathodes together, one tube can hog the bias current. By having seperate bias set resistors in either the plate or cathode, this hogging is reduced to where we don't care about it.

          Slight differences in AC operation aren't that big of a deal in most cases.
          
          Play safe and play longer! Don't be an "OUCH!" casualty.
          Unplug it, discharge it and measure it (twice) before you touch it.

          . . .Oh!. . .Remember: Modifying things voids their warranty.
          
          

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• Re: More questions on PP pre out - mqracing 18:46:59 01/17/07 (0)
  In Reply to: More questions on PP pre out posted by John Swenson on January 17, 2007 at 15:34:16:

  Hi John:

  Hey thanks for thinking of us for this project. Unfortunately, I am going to have to pass due to other production commitments we have and we really need to focus on our two preamp designs and get them done.

  best of with your new project,

  Mike
  
  

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