[R-390] Improve Capt Lee Product Detector using 6BY6

[Larry H]

Hi all,  I've been working on improvements to Capt Lee's Product Detector
mod to the R-390A.  One is using a 6BY6 in place of the 6BE6.  Another is
using a relay to switch the audio for AM or SSB reception (thank you
Jacques Fortin and W7DI (Eugene Hubbell)).  Both are done and functioning
very well and documented on our website (r-390a.net) here: Improving Capt
Lee Product Detector (r-390a.net)
<https://www.r-390a.net/Improving%20Lee%20Prod%20Det%20and%20SSB%20AGC.pdf>

Regards, Larry