I ping'd Brent Jesse. Here's his response:
I am a bit on thin ice here since these tubes are so rare I have not seen that many of them. The characteristic they all share are the distinctive plates inside the tube. These are the twin black structures you can just see the top of above the black band on the glass. When they are called “round plate” or “oval plate” the name references the two black plates that run up and down between the top and bottom mica. The oval or round name does NOT refer to the mica spacer at the top or bottom. The black plates of these tubes are actually a cylinder or a flattened cylinder and there are two of them spaced apart. The top mica may be round or it may be oval, I know they did make some minor design changes during the production of this tube. The black plates inside are always the same, however. The sound quality of all of these, regardless of mica spacer shape, is superb. They made a “civilian” version of this tube that looked the same, but did not have the CTL or JAN markings on the label.
Keep an eye open for this tube made by Tung Sol for other brands. I have had National Union in stock, and it was the same Tung Sol oval plate with black glass, and the label on the base was even in the silver ink. Sometimes you can find these for a song since they don’t have the Tung Sol name on them!
Best Regards,
Brent Jessee
