Tube DIY Asylum

Posted by Maxamillion (A) on July 01, 2009 at 13:34:33

Caucasian Blackplate got me thinking about using a red LED to bias my 2C22 tubes in a preamp, and that led me to think - what's so special about an LED? Wouldn't any diode with the right forward voltage work as well?

What about one of the new Cree Silicon Carbide Schottky diodes or standard Si Schottkys? The SiC have a forward voltage of 1.8V, in the same range as the red LEDs, and they're only $0.93 at Digikey. Plus they should be low in noise being a majority carrier device, right?

Edits: 07/01/09

• 6AL5 - mikeyb 12:54:07 07/02/09 (1)
  In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:34:33

  You might consider a 6AL5 dual diode. Keep the sand out ...
  

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  • RE: 6AL5 - Maxamillion 13:04:44 07/02/09 (0)
    In Reply to: RE: 6AL5 posted by mikeyb on July 02, 2009 at 12:54:07

    Not really enough room under the lid for a tube, plus the added heat would be a negative in the enclosed chassis.
    
    I do like the idea of it though...
    

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• Cree SiC Schottkys might work - Maxamillion 09:56:25 07/02/09 (2)
  In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:34:33

  
  
  

  
  Here is the forward voltage vs forward current plot for the Cree SiC 1A rated Schottky diodes. x-axis = volts, y-axis = amps

  Dynamic resistance seems to be in the range of ~0.8ohm from 10mA to 100mA, and the curve is very ateep and linear. This is lower dynamic resistance than any LEDs I have seen to date. Looks like these might be worth experimenting with as cathode bypass diodes. They have ~1V forward voltage at 100mA, as opposed to pure Si diodes, which have a much lower forward voltage.

  Thoughts?
  

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  Edits: 07/02/09   07/02/09   07/02/09   07/02/09   07/02/09   07/02/09

  • Closeup of voltage/current chart - Maxamillion 11:44:38 07/02/09 (1)
    In Reply to: RE: Cree SiC Schottkys might work posted by Maxamillion on July 02, 2009 at 09:56:25

    
    
    

    
    Here is a blowup of the lower region of the forward voltage vs forward current curve for the Cree 1A SiC Schottkys
    

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    • RE: Closeup of voltage/current chart - Maxamillion 14:07:33 07/02/09 (0)
      In Reply to: RE: Closeup of voltage/current chart posted by Maxamillion on July 02, 2009 at 11:44:38

      Just ordered 20 of the OPE5594A IR diodes from Allied (back ordered though!). Nice and linear in the 10-20mA range I'll be working in.
      
      When they get in I'll compare them to the Cree SiC I already have and the stock RC circuit - should be interesting. Will be a couple weeks until I can get it done, though.
      

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• RE: Diode Biasing using other than LEDs - twystd 21:48:24 07/01/09 (1)
  In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:34:33

  If we really think of the impedance while still maintaining a certain bias, then I think battery bias must come into consideration. I've always liked the sound of battery bias, but really never understood why.

  I think that it may be that you can retain a certain negative bias, while presenting a very low AC (signal) impedance to ground, without the coloration of a bypass cap. The internal resistance of a NiMH battery is extremely low, and 2 of them in series will get you about 2.4V of negative bias, which seems to be close to your target. Here is a Wiki link that gives a chart to the various batteries. Seems as if you could achieve a -2.4V (two 1.2V cells in series) bias with an internal resistance of less than 14 milliohms.

  twystd

  • http://en.wikipedia.org/wiki/Internal_resistance   (Open in New Window)

  
  

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  Edits: 07/01/09

  • RE: Diode Biasing using other than LEDs - Michael Koster 07:56:58 07/02/09 (0)
    In Reply to: RE: Diode Biasing using other than LEDs posted by twystd on July 01, 2009 at 21:48:24

    "Seems as if you could achieve a -2.4V (two 1.2V cells in series) bias with an internal resistance of less than 14 milliohms."

    Maybe...

    That 7 milli-ohm figure is a marketing number for electric vehicle
    batteries. The other figure given in that chart (650 milli-ohms) is
    very close to what I actually measure on 2 new AA NiMH batterise in
    series in a battery bias circuit (1 ohm total). If you use these in
    the cathode branch with 20 mA or so through them you will stabilize
    at 2.8-2.9 volts in circuit.

    These are all measured numbers
    

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    Edits: 07/02/09   07/02/09
• RE: Diode Biasing using other than LEDs - Maxamillion 13:42:12 07/01/09 (3)
  In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:34:33

  Of course that 1.8V forward voltage is at 1 amp, it's more like 0.9 - 1.0V in the region we are talking about, but still, would a few of these strung together be usable for cathode biasing?

  What about standard Si Schottky diodes - just curious.
  

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  Edits: 07/01/09   07/02/09

  • RE: Diode Biasing using other than LEDs - Michael Koster 09:49:06 07/02/09 (0)
    In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:42:12

    I would definitely give the SiC diodes a try. The impact of any differences
    in noise will depend on the circuit and application.
    
    I was only familiar with the low voltage drop (0.2V) Si Schottky diodes.
    I'd like to work out a few nice low impedance ( < < 10 ohms) devices myself.
    For something like a 5842 or 6C45Pi output stage even 10 ohms in the cathode
    adds a lot to the effective plate resistance.
    
    Michael
    

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  • RE: Diode Biasing using other than LEDs - Michael Koster 15:58:26 07/01/09 (1)
    In Reply to: RE: Diode Biasing using other than LEDs posted by Maxamillion on July 01, 2009 at 13:42:12

    
    
    

    
    Here's something to try if you want about 2.5 volts - a small MOSFET, like this 2N7000, with gate connected to drain. Dynamic resistance is around 10 ohms (similar to an LED)and voltage drop is Vth.
    

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    Edits: 07/01/09   07/01/09

    • RE: Diode Biasing using other than LEDs - tubegarden@aol.com 20:30:10 07/01/09 (0)
      In Reply to: RE: Diode Biasing using other than LEDs posted by Michael Koster on July 01, 2009 at 15:58:26

      You may use tube diodes. After you finish with SS marvels, try some old tubes :)
      
      That sequence brought me great joy :)
      
      Happy Ears!
      
      Al
      
      

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