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)