[R-390] Schematic for 6BY6 PD updated

[Larry Haney]

Hi all,  I updated the schematic for 6BY6 Product Detector in my
document:  Improving
Lee Product Detector and Lankford SSB AGC
<https://www.r-390a.net/Improving%20Lee%20Prod%20Det%20and%20SSB%20AGC.pdf> on
our website.  I changed R599 from 25k to 50k in order to increase the audio
output level of the product detector.  I did this in order to have the SSB
and AM audio levels close to each other.  The 25k was correct on my rx
before I fixed the agc so that the carrier level meter is now much more
linear and correct.  This resistor can be any value between 10k and 1M to
accomplish the desired product detector audio output level.  I think
approximately 50k is a good choice for most everyone.

Regards, Larry