Every receiver I've worked on has one of these capacitors either across the output transformer primary or from output tube plate to ground.  Radios that don't have one of these capacitors are the exception. The "across the primary" connection puts less stress on the capacitor than the other way.  Typically .001 to .006 uF and sometimes in series with a rheostat.  It serves as a tone control.  Without the rheostat, it's a fixed tone control for reducing the tinee (sp?)  high frequency sounds from radios with small speakers or other radios that require less emphasis on treble for better equalization with base.  Mike, W6MAB

On May 2, 2022 at 11:36 PM David Stinson <[email protected]> wrote:

WWII "Liberty Ship" RmCA AR-8506-B receiver
gets back to work.  This one was challenging.
(Got it going, Doug! TNX OM)
Still need to finish alignment.

Video:

https://photos.app.goo.gl/KqWcYNnnFaKxkj5n8

Question:
I probably should know this, but I don't want
to assume.  Can someone please tell me
the purpose of the capacitor across the
Audio Output transformer primary?
C-141 from plate to screen of the 25L6.

https://photos.app.goo.gl/BFyqarNgQ9BuQfbF7

If it's included, or even reduced from .005
to .001, the Audio Output breaks into
"motor boat" oscillation. Lower value gives
slower reflex oscillation, but remove it
completely and it goes away. 
The coupling and bypass caps
have been replaced, as have the power supply
filter caps of course.  Addition of more
bypassing on the B+ buss has no effect.
The bad resistors also replaced,
so the grid isn't floating.

Hints and Kinks about this rig
if anyone is interested.

GL OM ES 73 DE Dave AB5S

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