I just got this preamp and it has tubes that I am not familiar with. One tube is a WY3P another is a WY4P and the other strange one is a 5Z4P. Are there tubes that can be switched with any of these? Does "P" designate that is a Chinese tube? Makes sense due to the fact it is Chinese. But the interesting thing is the WY3P tube is like a night light. It glows so bright. I was wondering what are the brightest glowing tubes (in normal operation) out there?