OK, so, you need to set up away to get the current or voltage measuring resistors on the cathode. It is hard to see where the cathode is grounded but you need to get 4 matched 10 or 1 ohm resistors and put them between the ground and the cathode. If you mount 4 new 2 place solder lugs where the original 1 place lug is now you can add the 10 0hm resistor to the new lug and run the ground to the lug. If you mount 4 probe test points and run a wire from the cathode to the test point you can check the cathode bias from the top of the chassis. Just a suggestion if it did not occur to you. If the original resistors are all off specs and you need to replace them then doing this mod will be easier.