Using the newer style of tube will result in failure of a 100 ohm resistor and usually a RF choke. In the next issue of the newsletter I will share with you how to change the rather expensive 7360 balanced modulator tube to a 6JH8. It can be done without buying anything but the tube. The next issue will tell you the secret of how to remove the 60 Hz hum in your ear phones when connected to the power supply jack. The insulating material tends to get soaked with oil which can be conductive, and shorts out through the insulation.
Rotary switches are about the most difficult things to repair, if it can even be done at all.
This usually causes the power supply to end up in the scrap heap. Be sure to use a jack that switches the audio to the speaker when the headphones are not plugged in. The subject of hum on Swan rigs was talked about quite a bit on one of the recent Swan nets. I hope it is helpfull, and the following summarizes the article. In the Swan 350 and the 500, the audio line is also used for the AC heater current producing 0.1 volts of AC at the headphones, causing a hum.