Tubes Asylum

RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp

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Tre'

In Reply to: RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp posted by Caucasian Blackplate on April 20, 2019 at 09:11:38

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"Still Working the Problem"

https://www.psaudio.com/wp-content/uploads/2016/05/BHK-Pream (Open in New Window)

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RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp - Mister Pig 14:22:38 04/21/19 (3)
Hey I have seen that! It came in the box with the pre amp I bought.
What info in that manual did you want me to reference? I cant tell by your post.
Cheers
Mister Pig

Edits: 04/21/19
- RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp - Tre' 18:32:12 04/21/19 (2)
  "The BHK Signature Preamplifier features two 12AU7 dual triode vacuum tubes in the input stage that act as a zero feedback balanced input amplification pair. A unique self biasing constant current source and user selectable switch combination allows those who like to roll their own tube selections to place any acceptable 6, 7 or 12 series vacuum tubes inside the preamp. The twin hand-selected vacuum tubes that ship with the BHK are the preferred models we recommend and use in PS Audio's Music Room One reference system."
  
  "The BHK preamplifier is supplied with 12AU7 dual triode vacuum tubes. These have a heater (filament) voltage of 12 volts (that's what the "12" in 12AU7 indicates). Replace these tubes with a similar design that runs on +12 volts for its filament voltage.It is possible to use a lower voltage tube, such as a 6 or 7 series dual triode. To use a +6 or 7 volt tube, you must change the filament voltage. This can be accomplished by moving two jumpers, abeled JP11A and JP12A, inside the BHK Preamplifier. You can also change the bias current running through the tube if you wish. The bias we use from the factory is 4 milliamperes. Using the second internal jumper set, labeled JP7 and JP8, this can be increased to 5 milliamperes by using the supplied connector to jumper JP7 and JP8 (they are normally not jumpered for 4 ma). If you don't understand these setting and terms, best to probably leave it at the factory setting. Otherwise, if you really need to know, contact our service department."
  
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  - RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp - Tre' 18:42:04 04/21/19 (1)
    First I must say I have no idea what function the tube is serving in the preamp. Gain stage? Cathode follower?
    
    That being said, my only comment would be 4ma (or 5ma) is not a good idle current operating point for either the 12au7 or the 6dj8.
    
    I call BS. Feel free to prove me wrong. My mind is always open to good engineering explanations. But please, no voodoo talk.
    
    Tre'
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    - RE: Need To Develop A Short List Of Tubes For a BHK Pre Amp - Mister Pig 06:24:30 04/22/19 (0)
      I just have a minute. But in the BHK pre amp the tube is used as an input gain stage, but the heavy lifting of gain is done by Mosfet outputs. The gain of the tubes are variable and part of the volume control. I am just heading out the door, but lifted this paragraph from the Stereophile review that is descriptive of this element of the pre amp.
      
      Then there are those "wonderful tubes"-BHK's phrase: two 12AU7 dual-triodes, which provide voltage gain and are involved in the preamp's innovative two-stage volume control. The BHK Signature's fine increments of volume adjustment (0.5dB) are achieved by altering the tube-stage gain-and every 10-12dB a relay clicks in, activating a step in a discrete stepped attenuator.
      The BHK preamp is fully balanced from input to output. Its output stage riffs on the traditional push-pull design, using all-N-channel power MOSFETs in place of the more common complementary arrangement (footnote 1). The N-only topology is something King has been doing for a while in his amps and preamps. "The preamp's tube stage as well as its MOSFET N-channel output buffer are independent circuits and without global feedback," McGowan says. "They achieve their low distortion and characteristics through careful design and high voltages."
      This link takes you to the measurements section for the pre amp.
      https://www.stereophile.com/content/ps-audio-bhk-signature-preamplifier-measurements
      Cheers
      Mister Pig
      
      Edits: 04/22/19 04/22/19 04/22/19

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