Aha! Thanks! I hadn't seen the PS schematic before.
Hi Steve,
Looks like the guys have answered your questions well.
I agree that when going
above about 1600-1800 VDC HV, a Variac and separate screen supply is the best way to go with 813s due to the moderate waste of power and heat when tapping the HV supply as Don pointed out. Also,
the Variac gives you exquiste control of screen current on the fly as you change HV settings, plate loading and other things that influence screen current. No need to regulate the RF final screen voltage since the screen current is at steady state (~80mA) when dead carrier or under full modulation.
However, if the modulators are run with screen voltage as tetrodes (instead of triodes as shown) then we definitely need a regulated modulator screen supply.
An RF final screen resistor limits you to one basic optimized setting. A screen resistor HV tap works great in smaller rigs and also eliminates the need for the self-modulation screen choke, so it has its advantages too. A matter of preference in many cases...
Consider blowing some pressurized air through the 813 bases and upwards. Especially for the modulators running class AB2, this can increase the tube dissipation rating from 125 to maybe 160 watts. I see a difference in color from dull red to gray when I do. I have also used lantern chimneys, available on the web to do the job. Blown 813s (2X2) is quite the rig and will do 1KW carrier @ 140% modulation at 3KV when pushed to the insane limit. Running at 600W carrier is a very FB setting.
T