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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Poor Design for a Preamp posted by FlaCharlie on February 16, 2025 at 11:42:34:
The output impedance of a triode gain stage is the plate resistance in parallel with value of the plate resistor (the resistor between the power supply and the plate of the tube).
A triode needs to see a plate load resistor value of 3 to 5 times the tube plate resistance. (more if you have the supply voltage that will allow it or better yet a CCS)
A gain stage wants to see a load impedance 10 times the output impedance of the stage to prevent loading and rotating the load line for the triode towards the vertical causing a loss of gain and an increase of distortion.
When you drive a pot the total impedance of the pot is the load that is being driven by the preamp's output impedance (assuming there is no added resistance to ground following the wiper).
The output impedance of the pot (this is the value that is driving what ever follows the top) is dependent on the position of rotation.
If we assume that the impedance of that is driving the pot is zero (and of course it's not ), the output impedance of the pot at the -6db position is 1/4 of the value of the pot. So a 100k pot has an output impedance of 25k when the wiper is at the position with 50k above and 50k below it.
It would not take very much shunt capacitance to start disturbing the highs with a drive impedance of 25k.
So pots are a balancing act. Too low of a value and the source (the preamp in this example) has a hard time driving it, too high of a value and the pot has a had time driving what it is trying to drive.
All of the numbers that describe all of this are easily found on the internet.
Tre'
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- RE: Poor Design for a Preamp - Tre' 13:18:33 02/16/25 (0)
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