[charokeeroad]

Here is a copy of an email from Tom Albrecht who helped me resolve the 2.3v CRT vs the 6v CRT delema. Often it's not possible to tell the difference between the two even when the CRTs are marked with one or the other designations. Only a current test can tell them apart. I found a few sets clearly marked with "21SFA" inside the plastic housing which is not a correct description of any CRT the closest is SF21A which is a 6.3v CRT. After testing the current in these CRT they were found to be good 6v CRTs.

"From: Tom and Catherine Albrecht
To: Cherokeeroad
Sent: Monday, February 11, 2013 10:33 PM
Subject: Re: 21SFA

John,

I studied the schematic and confirmed that the original SF-21A CRT has a 600 mA filament current (same as all standard 6 volt CRTs). That would seem to suggest that both of the tubes you are looking at are 6 volt tubes. At some point if you disassemble the plastic CRT housing, I bet you'll find a number on the tube for a standard 6 volt tube.

If you find a CRT that draws 600 mA at 2.34 volts, then that is an original CRT with 2.34 V filament. At 6 volts it would draw substantially more. A 6 volt tube will draw less than 600 mA at 2.34 volts.
By the way, while they are warming up, they draw quite a bit more. They settle into their nominal value once the filament is hot.

Tom"

Hope this helps.