[R-390] 26Z5W - 12BW4 replacements

[David Wise]

Another possible replacement is the 12CT3 or 25CT3.
Like the 6V4 it's a little taller than the 12BW4
(2-7/8" vs 2-3/8") but should fit.

To use a pair of 25CT3s, move 1 to 2 and {3,8} to 9.
That's it.  To use a pair of 12CT3s, do the above
and rewire 4/5 to put the heaters in series.
Pins 1, 3, 7, and 8 are labeled "IC" (Internal
Connection) and can't be used as tie points.
If you're willing to inspect your 12CT3s to confirm
that 1 and 7 are actually NC or plate, jumper 1-2-6-7,
move {3,8} to 9, and put the heaters in series.
This arrangement will accept 12BW4s and _those_ 12CT3s.
Obviously this merits a big CAUTION in the radio's
maintenance book.

The xCT3 may have an availability advantage, as
RCA Receiving Tube Manual RC-29 (1973) lists them
as "active" while 12BW4 is "replacement only".

If you do any of these mods, PLEASE put a big warning on
the PS not to install 26Z5s.  Otherwise the next
poor schmuck will Do The Right Thing and... BOOM.

Regards,
Dave Wise

> From: Biddle, Richard [mailto:s-biddle@ti.com]
> 
> There is a good article on using the $4 12BW4 in place of the 
> 26Z5W on 
> 
> http://www.mines.uidaho.edu/~glowbugs/r390_psmod.htm