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A triode driver example

Posted by Tre' (P) on June 25, 2012 at 15:30:26

In Reply to: It dawns on me that the grid resistor does lower the source impedance but... posted by Tre' on June 25, 2012 at 14:55:16:

I use a 5687 as a driver for my 6b4g SET.

The 5687 has a plate resistance of about 2500 ohms.

I use a CCS instead of a plate resistor so the output impedance (at the plate) is 2500 ohms.

I use the mu output of the CCS. It's output impedance is about 500 ohms.

This is without considering the grid resistor of the 6b4g.

If I use a 10k ohm grid resistor that will change the drive impedance since it is in parallel with the output impedance of the CCS.

10k//500ohms=476 ohms

If I use a 470k ohm grid resistor the drive impedance will be 499 ohms.

If I take the output off the plate and use a 10k grid resistor the output impedance would be 2.5k//10k=2k and the load line for the 5687 would be more vertical and I would lose gain and the harmonic distortion would increase.

a 470k grid resistor give us 2486 ohms and the load line would stay pretty horizontal.

With any one of those, the Miller is fully driven and the -3db point is more than a decade out of the audio band.

Tre'

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

Edits: 06/25/12

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