I'm looking for another 3cpx5000 tube to top off a one and a half amp 5000 V PS I just built. There were some on eBay a while back that were guaranteed for only $125. Should have gotten a couple for that price. ANOTHER QUESTION: Is a sk-840 socket ( 5cx1500 ) modifiable to adapt to the 4cx1500 tube??? I have 3 of the sk-840 sockets and chimneys. Larry N5BIP