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5E3 Volume Pot Taper Discovery


 
3/4/2005 9:11 PM
Thorny
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5E3 Volume Pot Taper Discovery
This might be old news, but the problem isn't with the taper of the pot, but somehow with the loading of the circuit. If you swap the wires connected to the non-grouded terminals the tapers of the pots work great (it is not "cranked" sounding at 3-4 - the volume and distortion are added evenly.  
 
However, they are more interactive too. Shut one off and both are off.  
 
I suspect this is showing my ignorance, and is not new news to everyone (but it sure was to me). Anyone else try this or know this already?
 
3/6/2005 12:44 AM
Amp Guy
"If you swap the wires connected to the non-grouded terminals...."  
 
This sounds appealing. Could you please elaborate, I'm a little confused.  
Perhaps Bruce could comment?  
Thanks.
 
 
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3/6/2005 2:01 PM
Todd
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I am not exactly sure what you are saying, but it sounds like you are basically going against the usual taper of the pots. I believe the volume pots in the standard 5E3 kit are audio taper, not linear. Consequently, if you are reversing the wires on the non-grounded lugs of the volume pots, it sounds like you are creating pseudo-reverse audio pots. It will not be true reverse audio due to the way the logarithmic taper works. Because audio taper is a taper designed to accommodate how the human ear perceives volume changes, using them backwards might create a little more jumpy controls and less sensitivity of the controls at different settings. It might also make volume and distortion adjustments less smooth and more on/off-like at certain settings.  
 
Given how simple it is to rewire the pots, it cannot hurt to try to see if you like it. Keep in mind you will probably have to completely relearn how your amp's controls operate and interact. Using audio taper pots in reverse will make the controls function different compared to almost every other amp on the market. Of course, the interaction of a stock 5E3 is a little different control setup too compared to other amps. Good luck!
 
3/7/2005 3:55 AM
Dave H
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The 5E3’s volume pots are already reversed :) The stock connection is working more like antilog even when log pots are used. Reversing the non grounded wires makes it behave like a log pot should but there are no mixing resistors. Turning either channel to zero grounds the other channel. The stock 5E3 wiring doesn’t use the vol pots as potential dividers in the normal way. The potential divider is the output impedance of the tube driving the pot and the wiper to ground resistance of the pot. The output impedance of the tube is about 40k. Therefore to get a 50% reduction of the signal the vol pot has to be set to 40k. 40k on a 1M 15% taper log pot would be at about 2 on a scale of 12. That’s how the 5E3 is. It doesn’t get much louder when the volume is turned above 2 because it’s already nearly flat out. Reverse the ungrounded wires and it works like a log pot. It will now have to be set to 500k to get 50% attenuation. 500k is at about 8 on a scale of 12 on a 1M 15% log pot so now it’s only as loud on 8 as it used to be on 2. But does it have more headroom? ;)  
 
Dave
 
3/7/2005 8:29 AM
Thorny
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Thank you very much for the explanation.  
 
It does not have more headroom but it is more easily adjustable between 0 and flat out.
 
3/6/2005 10:12 PM
Thorny
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Looking on the back of the 1M volume pot, and labeling the 3 terminals left to right from the back as 1, 2, and 3. I swapped the wires going to 2 and 3.  
 
Stock form:  
1 is grounded.  
2 goes to the .1 uf cap coming from V1  
3 goes to V2 pin 2.  
 
I left 1 grounded.  
I changed 2 to V2 pin 2  
3 to .1 uf cap coming from V1  
 
This seems to make the volume more smoothly adjust, but if either volume is less than 3 it messes with the other one. If you look at most of the other fender amps they are wired like I changed it, not like the deluxe.
 
3/7/2005 3:46 AM
Chris@CMWamps
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circuit idea
Hi Thorny  
 
In a few words: you did change the 5E3 volume-circuit to a tweed Bassman/plexi Marshall style one WITHOUT the 270k or 470k mixing-resistors. Without those mixing-resistors all sound will be gone when one of the volume-controls is set pretty low.  
 
Maybe you can try this but it does need one 1M,log,dual pot and one 0.005 cap:  
 
Check out the 5E6A Bassman and 5F2A princeton schematics as reference.  
Use two 270k or 470k resistors from lugs 2 from the volume-pots going to the 1st V2 grid.  
Remove your stock tone-control.  
Wire the dual-ganged-tonecontrol as following:  
One side for the bright-channel as the 5F2A. For the normal-channel you do the same thing except you don't use the 470p cap on that side.  
 
copyright/yada yada/etc. :)  
 
Chris
 
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