One rule to use when working on any high voltage electronic circuit:
Keep one hand behind your back when poking around. Fatal electrical jolts pass through the heart. This is usually by the current passing in one arm and out the other. When the path to ground through your body is through your legs you are less likely to go into fibrillation or cardiac arrest from an electrical shock.
Electric heart defibrillators work by passing a current through your heart from a capacitor charged up to a high voltage. Your tube radio works on high voltage electric fields provided by capacitors charged up to high voltage. If in doubt, short out all capacitors using a resistor connected to alligator clips with one hand before working on the circuit. An alternate way is to connect an old analog voltmeter across the capacitor and wait until the meter reads zero before proceeding. The input impedance to the meter is low enough to make the cap discharge.
