[R-390] LINE /AUDIO POTS

Thu Mar 17 18:16:49 EDT 2016

I thought it would be a simple exercise for the reader.  Here is how I would do it:  connect a 100uF/16V in series with the 510Ù and put this between the wiper and ground.  You can connect C604/C607 either directly to the wiper (for a little LF boost) or to the junction between the R and the C (for a little more LF rolloff).  If you use an electrolytic, the polarity is left as an exercise for the student.  This capacitor value gives a LF corner about 10X lower than the one from C607/R620.

---------- Original Message ----------

Date: Thu, 17 Mar 2016 13:53:14 -0400
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: <fengjs at juno.com>,	<r-390 at mailman.qth.net>
Subject: Re: [R-390] LINE /AUDIO POTS
Message-ID: <[email protected]>
Content-Type: text/plain;	charset="iso-8859-1"

I'm sorry but.... not sure of that !
73, VE2JFE

-----Message d'origine-----
De?: R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de
fengjs at juno.com
Envoy??: 17 mars 2016 11:02
??: r-390 at mailman.qth.net
Objet?: Re: [R-390] LINE /AUDIO POTS

It seems to me that a well-placed capacitor could give the pseudo-log taper
without upsetting the bias point.  

---------- Original Message ----------

Date: Thu, 17 Mar 2016 10:01:00 -0400
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'Norman Ryan'" <nnryann at yahoo.com>, "'Steve Toth'"
	<stoth47 at yahoo.com>,	<jupete at internode.on.net>
Cc: <r-390 at mailman.qth.net>
Subject: Re: [R-390] LINE /AUDIO POTS
Message-ID: <[email protected]>
Content-Type: text/plain;	charset="utf-8"

Hi to all,

Despite the trick presented by Steve works practically to "imitate" a log
potentiometer from a linear one, it cannot be used on R-390/390A because the
end to end value of both local and line potentiometers are a part of the
follower tube bias system.
So using a linear pot with the ~500 ohms resistor will change the bias of
the tube with the sliders positions, except when both potentiometers are at
zero, which is not very practical, listening wise...
Bottom line, log law potentiometers are needed there.
The only other "trick" that will work is to use a higher value LOG
potentiometer replacement with a fixed resistor in parallel to make the 2,5K
value end to end.
Ex: you find a 5K LOG pot so you put a 5k (4K99) resistor in parallel to
obtain 2.5K end-to-end and all will work OK.

BTW: who on this thread was complaining about potentiometers found in a
R-390A that measured 5K LOG but were stamped 2500 on the cover ???
I just found two of those "anomalies" in my R-390 parts bin....

73, VE2JFE

-----Message d'origine-----
De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de Norman Ryan
via R-390 Envoy? : 17 mars 2016 03:44 ? : Steve Toth <stoth47 at yahoo.com>;
jupete at internode.on.net; r-390 at mailman.qth.net Objet : Re: [R-390] LINE
/AUDIO POTS

Great suggestion, Steve. So, with a 2.5k linear pot I just connect a 500 ohm
(20% of 2.5k) resistor as you describe -- or should the resistances of pot
and resistor be proportionally higher so they add up to 2.5k?
73Norman

      From: Steve Toth via R-390 <r-390 at mailman.qth.net>
 To: "jupete at internode.on.net" <jupete at internode.on.net>;
"r-390 at mailman.qth.net" <r-390 at mailman.qth.net>
 Sent: Thursday, March 17, 2016 1:29 AM
 Subject: Re: [R-390] LINE /AUDIO POTS

If you need  log pot response from a linear pot one method that works is
connecting a resistor that's 20% of the value of the linear pot between the
ground end solder lug  and the center variable contact solder lug.
- Steve  K7PZN"Always look for a positive solution then Improvise, Adapt,
and Overcome"
"Sometimes it is the people who no one imagines doing anything, who do the
things that no one can imagine".....

      From: Pete Williams <jupete at internode.on.net>
 To: r-390 at mailman.qth.net
 Sent: Wednesday, March 16, 2016 8:53 PM
 Subject: [R-390] LINE /AUDIO POTS

?    G'day  All.....
  Gary Schneider  from  'Playthings Of the  Past... have just  told me that
2.5k pots  only  available in  linear taper.
Cheers
Pete