Now I see what’s going on. First, the 6SL7 will be damaged with 400v because its max plate voltage is 300 volts.
This is a simple common cathode amplifier. The 200K plate resistor and 400V B+ at 120 volts yields a plate current of about 1mA per side on the 6SL7 so a 125K dropping resistor is needed for 370 to 120 volts (2mA times 125k = 250V). I can’t tell the color codes of the resistor because the red circle covers the bands. That resistor is shunted to ground by the smaller electrolytic, creating an RC filter which reduces power supply noise. A better way of dropping voltage than a zener.
Using a 120 volt zener will work, but it’s not ideal. Zeners hunt for their voltage and that creates noise on the plate, which greatly increases distortion. The solution is to bypass the zener with a 1,000uF 400V capacitor. You have the ground bus right there covered by the bottom of the red circle if you want to go that route.
The best solution is to use two dropping resistors of 62K each, each with a 47uF shunt capacitor to ground, that is two RC filters and better noise reduction. But the space for that is very tight.
Get rid of the 1000V zener, it’s nothing but trouble. And please, ALWAYS use one hand when probing around tube amplifiers (put your other hand in your pocket) and never let any part of your body touch the amplifier chassis because those voltages are lethal and linger long after the power cord is unplugged.