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68k input resistor?


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2/3/1998 12:51 PM
SProuty
68k input resistor?
Howdy,  
Could someone explain the purpose of the 68k input resistors that can be found on most BF Fenders? How was that value ultimately arrived at as the "standard" and why they weren't required on the stand-alone reverb?  
 
Thanks,  
 
Stephen
 
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2/3/1998 3:23 PM
Bill Bolton

> Could someone explain the purpose  
> of the 68k input resistors that can  
> be found on most BF Fenders?  
 
They are "grid stopper" resistors, intended to stop any tendency of the stage to oscillate due to coupling between the output and input circuits. In normal tube audio amplifier practice they should be close to the tube grid pin.  
 
Fender got away with putting them at the jacks and putting some (jack activated) switching around them to provide different levels of input sensitivity and impedance depending which jacks are used.  
 
BTW, the BF style circuit around the input jacks , with the 68K resistors, can still be found on most Fender amp designs, both tube and solid state, to this day.  
 
> How was that value ultimately arrived  
> at as the "standard" and why they  
> weren't required on the stand-alone  
> reverb?  
 
The exact answers to those questions are probably lost in the midst of time.  
 
68K is within the range of values commonly used for grid stopper purposes in high gain (i.e. preamp) stages.  
 
Cheers,  
 
Bill
 
2/4/1998 6:57 AM
J Fletcher
I've always thought they were also in there to isolate the 2 inputs in the event both were used at the same time.If #1 adjusts his guitar volume control,there's no effect on #2.If the resistors were'nt there,they would interact.
 
2/4/1998 3:49 PM
D.C.H.

A pretty (read VERY) basic follow up question here:  
 
>and putting some (jack activated) switching >around them to provide different levels of input >sensitivity and impedance depending which >jacks are used.  
 
I've built/moded a couple of Fender style amps recently (from old tube hifi, wrecked Fenders, etc) using only one input per channel for a number of reasons, and am still puzzled by this... Of course with only one input I don't want any damping or gain reduction, ie full sensitivity, so should I include the 1m resistor in the jack arrangement or not?  
 
This classic Fender switching arrangement seems straightforward, but I just want to be sure I've got it right.  
 
Thanks,  
 
Dave
 
2/4/1998 5:00 PM
Steve Morrison

Dave,  
 
For full sensitivity with a single input, the 1 meg resistor is the one you want to keep. The 68k's aren't needed, unless you want to use one as a grid stopper as is done on modern Marshalls. For a better understanding of how the switching arrangement works, it might be instructive for you to draw out simple schematics of what the resulting input circuit looks like under these conditions:  
 
1. No plugs inserted  
2. Plug inserted into intput #1  
3. Plug inserted into intput #2  
4. Plugs inserted into both inputs
 
2/5/1998 12:14 PM
D.C.H.
Thanks, Steve.  
 
That's very helpful,  
 
Dave
 
2/6/1998 4:27 AM
Steve A.

Steve, Stephen, et al:  
 
    For those folks out there who don't like math, Dan Torres has a great little article on input jacks that explains how the two Fender jacks work. When plugged into the high-gain #1 input, the two 68k resistors are in parallel which adds up to a 34k "series resistance" between the guitar and the preamp tube, with the tube seeing a load resistance of approximately 1 meg. When plugged into the lower-gain #2 input, the series resistance is 68K, with the load resistance being roughly 68k. With the #2 input, more of the signal is shunted to ground for lower net gain. (With both jacks used, the #2 jack would be slightly hotter than #1, with a 1.128M load resistance vs. a 1.0M load resistance.) Actually, all of the load resistance values listed here ignore the resistance from the guitar itself, which would bring it down to something like 5k to 15k (from the tube's point of view).  
    Unless the low-gain input is needed, you can get a cleaner signal by soldering a 33k resistor to the tube socket and running shielded cable to a single input jack, which would have the 1 meg resistor to ground. (These values are for a stock BF sound; for a hotter input reduce the series resistor to 5k or 10k and increase the load resistor to 2.7M or 5.6M). If you use both jacks, Dan suggests replacing the stock 68k resistors with 10k resistors for a hotter #1 input and input #2 "cool" enough to handle a line level input from a drum machine or whatever. For this example, Dan recommends a 2.7M load resistor to further increase the gain of the #1 input.  
 
Steve Ahola
 

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