Quad Preamp : verified schematics values

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pmx1

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Some info, I didn't find elsewhere :

One part of the currently available Quad Preamps schematics is missing. This is the all important "post lead" frequency shaping (and the corresponding LDR switches).
AFAIK, this important network of 2 resistors and two capacitors accounts for a large part in of the "stripes" variation.

I've traced the missing components on my unit (PCB rev. "QUAD 1G", 1/10/90):
  • On channel 1, (from last tube output to ground) : 1nF to GND, 47nF link capacitor, 475k//250pF then 100k//500p.
  • On channel 2, (from last tube output to ground) : NO capacitor to ground (or parallel to plate resistor), 47nF inter-stage capacitor, then 475k // 180pF followed by 475k//680p to ground
.

Based on the information (schematics) available, on my Quad Preamp, the lead channels (from input to output) looks like :
  • Channel 1 Lead Mode is close to a MkIIC+ (AFAIK on the MkIIC+ the top resistor is 220k instead of 475k, but schematics may vary ?)
  • Channel 2 Lead Mode is close to a MkIII Blue Stripe (based on the available schematics, the top capacitor is supposed to be 120pF instead of 180pF)

Hope this helps.
 
While analyzing the bass response of my Quad Preamp, I found a few differences with the schematics available here :
http://tubefreak.com/fschema.htm

Here are the corrections :

2gv1093.png
 

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