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vol pots need replacement maybe?


 
6/6/2000 8:12 PM
halouis
email
vol pots need replacement maybe?
'76 tele deluxe (2 vols/2tone). pots all cleaned and lubed.  
 
I still find that as I ease the vol pots down from 10, that it also affects the tone. Full volume give me loudness, and full brightness. As I turn the volume down on either pickup, the tone also begins to flatten.  
 
Is this usual? I don't remember this happening on previous electrics I have owned. SHouldn't it just decrease in volume only??  
 
thanks  
halouis
 
6/6/2000 8:31 PM
Mark Hammer
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"Is this usual? I don't remember this happening on previous electrics I have owned. SHouldn't it just decrease in volume only??"  
 
Yes.  
 
No.  
 
Fender guitars, and indeed ANY guitar with pickups that have single-coil brightness, show a loss of treble as the volume control is turned down. This is a natural product of the circuit formed by the pickup and volume pot.  
 
The traditional response to this is to use a compensated volume control, using a capacitor or cap/resistor combination to bypass the input-to-wiper path in the volume control. The component values for the bypass are selected to provide a low impedance path to the output for very high frequencies. As the volume control is turned up (lower impedance path between input and wiper), the compensation circuit provides no advantage for high frequencies. As the volume control is turned down, the path through the compensating circuit provides a lower impedance path for high frequencies than going through the pot itself (literally, the path of least resistance). This retains highs for a portion of the pot's rotation. In effect, it operates like a volume control that starts working right away for lows and mids, but doesn't start working for highs until you've turned it down a ways.  
 
It may not have happened to you on previous guitars because:  
1) There was little treble to lose in this manner (e.g., humbucker PU's).  
2) There was already a compensating cap in there.  
3) You had lousy speakers in the amp and couldn't hear the treble loss.  
4) The volume pot values were so low that the treble loss occurred at all settings (higher value pots - e.g., 500k - are used to retain more treble, and lower value pots are often used to "tame" excessive treble).  
 
Standard cap values to accomplish this are in the 120pf-500pf range. Some people also put a 120k resistor (or thereabouts) in series with the cap to smooth out the taper of the action over the pot's rotation.
 
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