I have a CBS 5814a vacuum tube dating from the 1950's. This is supposed to be one of the most sought after 12au7/5814 tubes. The tube works fine and I was told it tested at NOS levels. However, there is a problem. The tube only gets sound in one channel when it is first turned on. You have to wait about 3-4 minutes for it to warm up before it works properly with both channels.
I don't know why it does this. None of my other 12au7 tubes have this problem. I'll give it away for the cost of shipping ($4).
