Quad 22 interstage cap?

[neskor]

I'm trying to restore my Quad 22 preamp.
On the internet there are two different schematics.
On first early version there is no input cap, interstage cap is 5000pF and output cap is 0.047uF.
In second (improved) version there is a input cap and interstage cap is now 820pF and output is 0.1uF.
Reason for input cap is known, output is changed probably to improve bass respone, but I don't know why this drastic change from 5000pF to 820pF for interstage cap?

 

[DF96]

In the later version has R26 been increased in value, perhaps to 10M so that the triode now uses grid current bias? That would explain a change in coupling cap, to maintain the same 20Hz rolloff.

 

[neskor]

They didn't change R26 in any revision, it was always 1.5M.

 

[DF96]

I can't believe that they intended a 120Hz LF cutoff. Is this section within a global feedback loop? That might explain it - the change could then reduce subsonic peaking.

 

[advr]

What should be the best values for C13&C16? 820p and 0.1 uf for a bass response?

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[DF96]

This is a non-trivial problem, assuming this is within a feedback loop. Get these wrong and you either lose bass, or get a bass peak due to pushing the loop towards instability.