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 Tubelab Discussion and support of Tubelab products, prototypes and experiments

DougL
diyAudio Member

Join Date: Feb 2004
Location: Wheaton IL.
Quote:
 Just for kicks, could one put a CCS in the R14 position? Using a simple one like the one used as a plate load for the 5842, assuming you had the parts laying about?
Quote:
 In principle, the source follower is most linear with a very large source resistor (R14), or a current source in that position.
Yes, you could in theory substitute a current source.
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus

Last edited by DougL; 18th February 2010 at 12:54 AM. Reason: Clarity

 18th February 2010, 02:59 AM #12 jrenkin   diyAudio Member   Join Date: Nov 2007 Location: Denver, Colorado "in theory" makes me nervous. Maybe i will build with the resistor, measure the current then try swapping in a CCS for fun and listening.
Sheldon
diyAudio Member

Join Date: Dec 2003
Location: San Diego
Quote:
 Originally Posted by jrenkin "in theory" makes me nervous. Maybe i will build with the resistor, measure the current then try swapping in a CCS for fun and listening.
Don't worry about the theory part. I don't think Doug was questioning the validity. It will work as long as your current source can handle the voltage. From a precision driven engineering perspective, it's the preferred approach. Will it sound better to you? Not necessarily. You'll have to try. No need to measure the current. You can estimate it from the expected voltage over R14 (grid voltage - bias supply voltage). For -40V on the grid, you'd see about 5mA. That's a good starting point.

Sheldon

 18th February 2010, 09:34 PM #14 DougL   diyAudio Member   Join Date: Feb 2004 Location: Wheaton IL. I was questioning if the current source would be an improvement in that application. Since the 20K resistor is much larger than the 2 ohm output Z, I would guess that the difference would be very small. That is in stark contrast to the CSS plate load, which helps a great deal. I would trust that George tried it, and he judged it was not worth the extra complexity. __________________ Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well) Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
Sheldon
diyAudio Member

Join Date: Dec 2003
Location: San Diego
Quote:
 Originally Posted by DougL I was questioning if the current source would be an improvement in that application. Since the 20K resistor is much larger than the 2 ohm output Z, I would guess that the difference would be very small. That is in stark contrast to the CSS plate load, which helps a great deal. I would trust that George tried it, and he judged it was not worth the extra complexity.
I agree, and don't think it would be an audible improvement. But part of the fun of DIY is to try stuff as a learning experience.

There is one possible modest advantage of the current source in this application, and that is that the quiescent current through the FET remains the same, regardless of the output tube. With a resistor, the current can vary significantly between tube types. Does it matter? Unlikely, but for those obsessive types...

Sheldon

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