intro / AB763 PI design question

[mikepukmel]

Mike,
To limit blocking distortion you want output tube grid stops to be as high as possible without losing to much treble from the high frequency roll off that is introduced by the grid stop + miller capacitance at the output tube grid.

With 6V6 you should be able to use 47K without any worries, you might even try 100K. I would put in 100K and if you lose too much top end then just solder another 100K across them to reduce them to 50K.

That addresses blocking distortion in the power amp. Use similar in the preamp - as high as you can without losing too much top end. A high gain 12AX7 triode stage (preamp) actually has a much or more miller capacitance at the grid as an output tube in pentode mode does - somewhere around 100 to 200pF, so similar values should work.

In "nerd speak" blocking distortion occurs whenever the discharging time of the coupling caps is longer than the charge time - so bias shifts occur.
Increasing grid stops increases the charge time AND limits the charging current.

Cheers,
Ian

Some reading:
What Is "Blocking" Distortion?

Thanks Ian!! I have stops on the preamp both sides. I had emailed Aiken with a question about the phase splitter, said he didn't know why people don't use them there, but said, as you mention that the output tubes are the first place to bump the stop. I have work to do!!!

[pdf64]

...To limit blocking distortion you want output tube grid stops to be as high as possible without losing to much treble from the high frequency roll off that is introduced by the grid stop + miller capacitance at the output tube grid.

With 6V6 you should be able to use 47K without any worries, you might even try 100K. I would put in 100K and if you lose too much top end then just solder another 100K across them to reduce them to 50K...

Bear in mind the 100k limiting value for g1 circuit resistance in fixed bias http://www.mif.pg.gda.pl/homepages/f...163/6/6V6S.pdf
Your grid leaks would have to be a rather low value to meet this constraint