Tubes Asylum

RE: Questions about tube bias

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jyourison

In Reply to: RE: Questions about tube bias posted by Tre' on July 14, 2011 at 10:57:25

Tre', sorry for the inconcise questions. I'll try to do better next time and not use the post to process my thoughts. Your response was as helpful as it could be given the imprecise nature of my queries. Thanks for trying to engage me in the middle of all that.

Your explanation of CCS operation as causing the bias to be what it needs to be is very helpful.

--Jeff

Follow Ups:

RE: Questions about tube bias - Tre' 22:02:00 07/14/11 (4)
"Your explanation of CCS operation as causing the bias to be what it needs to be is very helpful."
And when the CCS is in the plate circuit, the plate voltage will be what it needs to be, under the bias condition is finds itself (bias voltage at the cathode (this depends of the value of the cathode resistor) for that tube to draw the current that the CCS is set at.
For instance, a 6sn7 with a CCS plate load set at 6.5ma, with the B+ supply voltage anything 30 volts or so over 200vdc.
(the 30volts keeps the CCS running. Can be more but never less. There's more to it than that but that's for another time)
If you have 6.5ma flowing through a 923 ohms cathode resistor you will have 6 volts bias and the plate voltage will be about 200vdc.
If you have 6.5ma flowing through a 615 ohms cathode resistor you will have 4 volts bias and the plate voltage will be about 160vdc.
If you have 6.5ma. flowing through a 307 ohms cathode resistor you will have 2 volts bias and the plate voltage will be about 120vdc.
Without a CCS (plate resistor instead) everything changes when you change the cathode resistor.
Everything changes if you change the supply voltage.
Everything changes if you change the value of the plate resistor.
They all interact.
I love CCS plate loads, they make things easy and give the tube a more horizontal load line to work into. That lowers harmonic distortion.

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 07/14/11 07/14/11
- RE: Questions about tube bias - jyourison 08:44:06 07/15/11 (3)
  You've convinced me to try a plate load CCS, Tre'. I know I've seen examples, and I've already built a cathode CCS for a phase splitter. It shouldn't be too hard to try a plate version in a design. I may even try a tube CCS sometime just for kicks (in the cathode for starters) -- I got a bunch of 6CB6s from Jim McShane at a low price to play with.
  - RE: Questions about tube bias - Tre' 11:58:22 07/15/11 (2)
    "I got a bunch of 6CB6s from Jim McShane at a low price to play with. "
    Those will work and sound good but the transconductance is much lower than a depletion mode MosFet.
    It's so cheap and easy to build a CCS using two DN2540s in cascode with gate stop resistors and a current set resistor.
    They will out perform the pentode.
    Tre'
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    
    Edits: 07/15/11
    - RE: Questions about tube bias - jyourison 12:12:38 07/15/11 (1)
      Tre', I have used those same depletion MOSFETS to good effect and I agree with you. Thanks for the diagram.
      
      My idea for the pentode CCS using the 6CB6 is absolutely irrational in terms of practical design and quality. It's nuttin' but sheer hobby, the journey of learning by doing, and pure "just 'cuz it would be cool to try."
      
      --Jeff
      - RE: Questions about tube bias - Tre' 12:43:47 07/15/11 (0)
        
        In Reply to: RE: Questions about tube bias posted by jyourison on July 15, 2011 at 12:12:38
        
        My friend Bill built using pentodes and they worked and sounded fine.
        
        When measured they were not nearly as "constant" current vs. the MosFets.
        
        The "measurement" method was to simply remove the cathode bypass cap and look at the amplitude (using a scope) of the AC across the cathode resistor when driving the tube with a known fixed signal from a signal generator and comparing each CCS.
        
        Understand that there is no such thing as a truly "constant" current source just some that are more so and some that are less so.
        
        Have fun.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"

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