What is the purpose of the 470k (Rp) resistor in the input of a stock (nearly) ST 70? How is the value calculated or derived? As I am using (LAST PAS) a cathode follower out of my preamp do I need this?
The 470K resistor on the input jacks sets the input impedance of a STOCK dynaco ST-70 or any Dynaco ST-70 with a stock replacement driver board. Other types of driver boards (like the VTA driver board) use an "on board" resistor to set the input impedance so these two 470K resistors (one for each channel) are not needed with the VTA driver board.
The general rule is that the input impedance of a tube amp should be at least 10 times that of the output impedance of the preamp being used to drive the amp. The 470K input impedance of a stock ST-70 is sufficiently high to work with any preamp that I know of ..
Your suggesting it is sort of an arbitrary number based on not loading the preamp. I am sure someone has modeled the ST70 input in spice. Whether original 7199s, or, like mine an SV board with 6gh8s. Anybody ready to chime in with WAG of the impedance.