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What are these resisters called and...
#1
   
In this schematic looking at R101, R102, R103. what do you call these resisters?, and what effect do they have on the pre-amp? I have read these resisters load the guitar pick-ups and set the impedance of the first pre-amp tube. So I need to know why some amps have R101 at say 33K some, especially those from AX84 do not have a resister there at all. I have seen the R103 resister at 1M up to 4.7M So please explain to me the use of these and how I can best match my Guitars pick ups  to hit my pre-amp as I need, (or want). I use both single coils and Humbuckers. Whats the math or is there any???????
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#2
Getting no response here. So I'll have to at least try to answer my own question.
*1). R101 resister is called RGS or a grid stopper resister and should be mounted directly on the tube socket and not at the in-put jack as is most common. It's main use is to "attenuate only radio frequencies". For a cleaner and slightly darker sound, use a larger value "I.E. 68k", you can use anything from 0 to 100k. For brighter sound and response, don't use one at all. There that answers that question.
*2). R101 and R102 as shown and wired in this way in some Fender and other amps, in-put jacks become paralleled through the switches built into the jacks. The parallel of the two resisters value will be 1/2 of the resistance used, I.E. 68K will become 34K to the tube. So using one jack which may be labeled "Normal" the tube sees the darker 68k resister. Straight shot to the tube. Using the other jack which may be labeled "Hi, or Bright" sees the parallel of the two and the tube sees 34K. There that answers that question. See >https://www.ampbooks.com/mobile/classic-circuits/fender-champ-5e1/< for some answers here.
*3) R103, This resister is called (the/a) grid leak resister. NOTE:  These notes are basic and are just meant to be used on the IN-PUT grid Leak resister. The rating is not important since almost no current flows. The lower wattage 1/8 or 1/4 watt, seams to be better. And to use a modern metal film resistor or non-inductive type. This  resister  is to kill any stray electrons from hitting the grid of the tube. And stray electrons can cause  an oscillation or inductance within this IN-PUT circuit. So mount this resister up on the jack as far away from the tube socket as possible.    A 1meg or higher rating will work. Yea, no math involved, just what your ears hear.
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#3
(03-21-2020, 10:31 PM)mcneb Wrote: In this schematic looking at R101, R102, R103. what do you call these resisters?, and what effect do they have on the pre-amp? I have read these resisters load the guitar pick-ups and set the impedance of the first pre-amp tube. So I need to know why some amps have R101 at say 33K some, especially those from AX84 do not have a resister there at all. I have seen the R103 resister at 1M up to 4.7M So please explain to me the use of these and how I can best match my Guitars pick ups  to hit my pre-amp as I need, (or want). I use both single coils and Humbuckers. Whats the math or is there any???????

First, R101 & R102 are parallel so the resistance is 34-Ohms - that should answer why some amps use 33k, especially if they don't have the low impedance input.  These are creating a low pass filter with the capacitance of the input tube for the purpose of blocking RF frequencies, in particular AM radio.  All resistors have Johnson noise (thermal noise) which can be amplified in the first stage, so the AX84 amps do not include these by default, there is a note on the schematic about this.  Best practice is to start small like 10k (less noise) and increase if you are getting the church station coming through. If you don't need them, you don't need them.  Better to avoid any potential noise at the input. If you do need them, use metal oxide or metal film resistors in those positions.

R103 is there to set the input impedance.  Your guitar pickup is let's say 10k-Ohms and is outputting a small voltage by the string vibration over a magnetic field.  The most efficient way to transfer voltage is from low impedance to a high impedance so R103 is there to setup the high impedance. Keep in mind that the 1M resistor is in parallel with the grid to cathode resistance of the tube, so the input is probably an order of magnitude greater than the source impedance which helps the guitar's signal reach the preamp's input without degradation.  I never use the low impedance input, I believe it was included for use with microphones back in the day and must have some practical uses for guitar too, but not for metal.  If you use the low impedance input then by all means keep it, if not there is no reason to add the extra parts.

Thirdly, piezo pickups have a greater impedance than your standard single coil or humbucker, so 4.7M could be appropriate in that situation.  This wouldn't bother an single coil or Humbucker at all, 1M is just an easy standard and cheap.
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