[R-390] A bug and its fixing under way

[Jon Schlegel]

Hi all,

At one time I measured one of the stock 12 mH inductors and found 
that it was near self resonance 455 kHz.  That plus the rather high 
published plate resistance of the 6BA6 type tube of 1 Meg (I think) 
shows they want the input Z of the mech filter to be the only 
significant load on the plate of that stage.  Consequently the mech 
filter input* will be looking into a very high source Z.

If you find an inductor that is self resonant higher than 455 kHz, 
you could likely just resonate it to 455 with some extra shunt 
C.  The higher the Q of the inductor the better.  Can't remember what 
the resonant Z of the stock 12 mH inductor was other than it was "up there."

* I had measured the in/out Z on the filters I had on hand to be 
around 20k to 50k ohms.

Regards,
Jon - WA3MVM



At 09:52 PM 8/3/2009 -0500, Jim Haynes wrote:
>On Mon, 3 Aug 2009, Roy Morgan wrote:
> >
> > Are they twelve millihenries?  (I've not replaced any, yet, and don't have
> > the parts list at hand.)
> >
>Yes, a part I found at Mouser, which is nothing like the original
>epoxy-cased toroid but is 12 mH.  Will have to figure some way to mount
>it, since the original toroid serves as tie points for the connections.