[Boatanchors] 75A-3 Filter Question

[Glen Zook via Boatanchors]

The R-390A filter should work fine in the 75A-3.  I have one in my 75A-4.  That required using a 9-pin miniature plug.


 
Glen, K9STH


Website:  http://k9sth.com
On Sunday, May 11, 2014 3:42 PM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
 
Good luck finding the 6 kc bathtub filter.  I've only seen one in my
life and the seller wanted $450 for it.  The 390 filter will work I'm
told but I have never tried it.  For casual AM ragchewing you don't
need the steep mechanical filter skirts.   There have been 2 or 3
articles in ER over the years on how to homebrew a filter for the A-3
using LC passive components or a ceramic filter.  You can cut out a
perf board rectancle and mount parts on it.  The hardest part is
finding stiff wire of the proper gauge to make pins that seat securely
in the filter slot but it can be done.  mount the pins and solder to
them then cover them with epoxy so they stay in place.  The actual
filter slot pinout is different between the two slots so you have to
make sure you have the pins for the signal path going in the right
holes.  I can't remember for sure but the signal side going in, may
have B+ on it  so you have to have a ceramic blocking capacitor
between it and your filter circuit.  The original B type mech. filters
don't care and they can go in any old way but a homebrew filter has to
match the slot pinout.    See "Putting the 75A-3 on AM" ER number 64.

A wider filter plus pulling the audio output tube and bringing the
audio on the AF gain pot out to an external p.p. tube amp and a big
speaker and you have a nice set for AM ragchewing.

73,

Rob
K5UJ



On Sun, May 11, 2014 at 1:56 PM, bill kipping <ke7kk at earthlink.net> wrote:
> Greetings all, My late collins 75A-3 could use an AM bandwith mech. filter (F455C60). I do have an NOS Collins F455R80 (8kc) for an R390A. Has anyone had success using the 390A filters in the A3 requiring the "C" type factory filters.  Thanks 73 Bill KE7KK
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