[R-390] LINE /AUDIO POTS

[Jacques Fortin]

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
Envoyé : 17 mars 2016 03:44
À : 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
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