Hi Gadget,
The schematic is online at
http://techpreservation.dyndns.org/s...ics/Zenith.htm -- it is chassis # 8C02 in the drop down (if you have the djvu viewer-- if you don't I can scan my paper copy and send that).
The resistor is the 680 ohm heading into the 6BJ6 (into the screen grid I think, as eufaula1 suggests).
I was also wondering if the silicon diode is providing too much juice. I put a 50 ohm, 10 watt dropping resistor before the diode, but maybe this isn't enough? I had some people say 50-100 ohms would suffice, and others say no resistor at all was needed.