AB763 Single channel question

[Joel]

I’m looking at Hoffman’s single channel AB763 schematic and I’ve got a question regarding the 47K resistor (R30) to ground that follows V3-B.  What is its function?  Can it be omitted?  What effect would omitting have?

I’m planning to put a Master volume control at this point in the circuit.  The plan is to omit the 47K and use a standard 1M pot wired as volume control. 

I’m hoping that some more knowledgeable people will chime in and enlighten me.

Cheers.

[Joel]

After some research I think I've answered my own question.

A 47K load on a 12AX7 triode stage is quite low.  It will load the stage down.  The AC load line will be rotated away from the DC load line, reducing voltage gain and increasing 2nd order harmonic distortion.  The reduction in gain approaches 50% - this is not ideal.  More 2nd order harmonic distortion isn't bad IMHO, and some will be cancelled in the output stage anyway (the PI will add a bit too, so the 2nd order distortion is still up).

Replacing the 47K resistor woth a 1M pot will restore the triode stage gain back closer to our DC load line type operation, and reduce 2nd order harmonic distortion back to "normal" levels for a tube amp.  More gain here will drive the PI harder and provide more PI/output distortion when the amp is pushed.

Maybe I'm wrong, but I'll progress with replacing the 47K with a 1M Master Volume pot.

[sluckey]

That 47K resistor simulates the missing 50K tremolo pot in the original AB763. Hoffman uses a different tremolo circuit so he added that resistor to prevent the amp from being too gainy. Your 1M will work but I would suggest using a 100K MV. Here's what I did...

     http://sluckeyamps.com/tdr/tdr.pdf

[Joel]

Thanks for the reply Sluckey.  And thanks for the pdf file.  That will come in handy.   Cheers.