Replacing half wave tube rectifier with capacitor diodes in series
The Heathkit Warrior HA10 kW amplifier power supply used a transformer with a 3400V center tapped secondary, followed by two 866 mercury half-wave rectifiers connected in a full wave center tapped configuration.  Each rectifier had 1700V on its anode.  

Many owners of this and similar equipment replaced the 866s with silicon diodes when the 1kV diodes became readily available at low cost.  The method used was to string multiple diodes in series, each shunted by a 500k-1M resistor in parallel with a 0.01uF ceramic disc capacitor, usually Z5U dielectric.  

Most likely, what you have are 1kV, 1A diodes, such as 1N4007.  Better replacements are available today, but you would be safe using this same diode in that configuration for the HA10.

Before you replace the diodes, I suggest you replace the filter capacitors and test the choke for shorts.