Little Bear P1 Power Supply

Hi Guys, I have a Little Bear P1 valve headphone amp that I have not used for ages. I really used to enjoy the amp when at work, but I have lost the power supply. I just wanted to know if anybody can tell me the pinout for the connector before I try something an blow it up. Is the middle pin - and outside +?

I know it needs a 24volt DC supply. I tried it with a 18 the other day and it made a clicking sound.

Thanks for any advice.

Ross