Quad Preamp : verified schematics values

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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pmx1
Bottle Rocket
Posts: 13
Joined: Fri May 19, 2017 5:43 pm

Quad Preamp : verified schematics values

Post by pmx1 » Sun Mar 04, 2018 7:21 pm

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.

pmx1
Bottle Rocket
Posts: 13
Joined: Fri May 19, 2017 5:43 pm

Re: Quad Preamp : verified schematics values

Post by pmx1 » Sun Mar 11, 2018 1:20 pm

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 :

Image

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