ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Simple Preamp Input Question


 
1/17/2000 11:21 PM
Randal J
email
Simple Preamp Input Question
In the classic Fender two input configuration, one of the inputs goes through a 68k resistor straight into the input of the tube, but the other input adds a 1M resistor in parallel with the same arrangement. Am I correct that the 1M resistor only affects the second input? If so, how does this affect the sound versus the other input? Or does the 1M actually factor into both inputs?  
Thanks in advance....
 
1/18/2000 1:18 AM
Winnie
email

Randal,  
 
If you read the old Fender literature you find that the second input was for a microphone. It's got a little less gain up front and gives more control over the volume.  
 
If I'm not mistaken though that 1 meg resistor goes to ground, not in parallel.  
 
Winnie
 
 
  Friday
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
Have you ever wondered if there is a better way to build a Bassman, Champ, Plexi, an 800, AC-30, Bulldog or Portaflex? Or you wanted to build an SVT with off-the-shelf parts? How about a master-volume amp that doesn’t change tone with the master setting? Everything you need to know is right here, including: proper grounding techniques, wiring methods, and mechanical considerations. Eighteen chapters cover the “iconic” amps everyone knows and loves, with schematics and layouts for each, along with the technical history of the product. Eyelet-board and chassis-mounted tube socket construction is used throughout, for easy servicing and modding. TUT3 is very accessible even if you cannot fully read a schematic and is a "must have" if you are going to build an amp for your self.

Note: The Ampage Archive is an Amazon Associate site. A small commission is paid to the site owner on any qualified purchase made after clicking an associate link such as the one above.
 
1/18/2000 1:24 AM
John Stokes
Winnie is correct. The 1 meg resistor goes to ground in all cases. The #2 input uses the 68k resistors as an attenuator to reduce the gain on that input.
 
1/18/2000 1:43 PM
Randal J
email

You are right, I just meant parallel, with the rest of the circuit, to ground.  
Thanks
 
1/18/2000 2:43 AM
SteveF
email

Hi, Randal. If you look carefully at the  
schematic, you'll see that when a plug is  
inserted into what you are calling the "second"  
input (actually called input 1 on the Fender  
schems.) the two 68k resistors are in parallel  
because of the connection thru the switch  
circuit of the other input. This input is  
therefore a little hotter, having only 34k  
in front of the grid.  
 
The 1meg resistor to ground doesn't have much to  
do with setting the sensitivity of the input  
because it is so large a value, rather,  
it's a necessary component of the gain stage  
itself. If you look downstream in the schematic  
you'll find that at each gain stage there is  
a DC path to ground from its grid. Sometimes it's  
thru a pot, sometimes it's a fixed resistor,  
but it's always there. The circuit won't work  
properly without it. Sometimes the value of this  
resistor is smaller to effect a little interstage  
attenuation, but usually it's fairly large.  
 
Regards, Steve F
 
1/18/2000 1:47 PM
Randal J
email

OK, so the 1M is not a "signal path", or at least not one of any consequence? What flows through it is mostly just bias current?  
Thanks
 
1/18/2000 1:55 PM
Graywater
email

Sorta - the 1M is to reference the grid to ground for DC purposes - the cathode resistor's voltage drop puts the cathode positive to ground, hence the grid is negative with respect to the cathode. There is, for all practical purposes, no grid current flow in this stage.  
 
GW
 
   Page 1 of 3 Next> Last Page>>