[Amps] 4CX300a Seal Leaks

[John Finner]

I read a post by Carl, KM1H on August 10, 2009 that read:

There are several ceramic tubes that are notorious for seal leakage. The  4CX300A is a classic.

This post is directed to Carl. The list might also be interested. I would be interested in a brief explanation of the problem.

I have 7 NOS JAN 4CX300A's an am building a 3 tube passive grid driven amp. The date codes are 7204, 7432 and 7634 if this makes a difference. Are the JAN Eimac tubes less problematic? 

The amp is in a simibreadboard state with 4CX250B's. Since I now have 3 sockets for the 4CX300A's, I will switch tubes and raise the Ep to 2500 V. I do not want to waste my time if I am going to have a problem with this change.

John WA4EPI