Posted by Jon Lane (D) on January 03, 2009 at 11:00:01

Hi All,

I'd like to use the VT-101 (807) in my K&K ST70 which would require reconfiguring heater supplies to 12.6v. It appears there's plenty of current on-board (only 4x 0.7A drawn for the VT-101 quad.)

I feel I should know this but I'm rusty: Referring to the K&K ST 70 schematic, Can I re-rig just the power tube's heaters as follows?

1. Connect one end of all VT-101 heaters to the "outside" of one 6.3v secondary;
2. Connect the other ends of the VT-101's heaters to the "outside" of the other 6.3v secondary, thereby deriving 12.6v for the output tubes;
3. Connect the "insides" of those secondaries together, lift and float the original pair of heater secondary center taps, and connect this new derived center tap to the ground arrangement provided;
4. Leave the driver tubes wired as they are.

The end result is I get 12.6v for the outputs, create a new, single center tap and reestablish it at original heater system potential via C6 and R2, and now have the driver tube's heaters connected end-to-end.

Something doesn't feel quite right with the driver tubes being tied at the heaters -- electrically each one is connected across half the "new" 12.6v heater secondary and neither would be technically centered around ground potential any longer. What am I missing?

• Correction - Jon Lane 19:00:03 01/04/09 (0)
  In Reply to: RE: Reconfig ST 70 output tube heaters? posted by Jon Lane on January 03, 2009 at 11:00:01

  Kevin's right about the 807 not being what I was misremembering to be a light-duty 837/VT-101.
  
  The (12v/7-pin+cap) 837/VT-101 looks like a heavier (6v/5-pin+cap) 807/VT-100 but it's the opposite: The 807, despite the smaller plate, has the higher Rp...and is suitable for audio, which kinda helps.
  
  Confusing things more, I've got some 1625/VT-136, which are a 12v 807, also with a 7-pin base+cap.
  
  All in all, in this application the 807/VT-100 is the better choice.
  
  

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• RE: Reconfig ST 70 output tube heaters? - KevinC 13:20:16 01/03/09 (1)
  In Reply to: RE: Reconfig ST 70 output tube heaters? posted by Jon Lane on January 03, 2009 at 11:00:01

  So long as you get the two 6.3v secondaries phased right so that they add to make 12.6 volts, then there should be no problem with what you propose. In any case, I suppose you meant 837 instead of 807? Maybe that's what you felt was missing? ;=))
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • RE: Reconfig ST 70 output tube heaters? - Jon Lane 13:59:50 01/03/09 (0)
    In Reply to: RE: Reconfig ST 70 output tube heaters? posted by KevinC on January 03, 2009 at 13:20:16

    Thanks, Kevin.
    
    And yes, I meant 837. ;o)
    
    

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