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Sometimes it is hard to tell which end is up

But in your case, you have the electrolytic capacitor hooked up backwards. Black band goes to ground. Black band with arrows pointing to a terminal indicate negative which goes to ground. Positive goes to the hum pot.

What is going on in your circuit is that the cathode resistor is passing some current from the 45 which causes the resistor to develop voltage across it. Let's call it 40 volts for now depending on your bias. The cap is there just to eliminate the AC signal from the voltage on the cathode to get rid of the feedback. It just has to go in so that the positive goes to the tube and the negative goes to ground. Put a meter across the cathode resistor to see for yourself. The amp will work without this cap, but not sound very good.

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