Long Tail Pair negative feedback question -- 1981 Carvin X-Amp

[LessPaul]

I posted a few weeks ago about some troubles I was having with a heavily modified 1981 Carvin X-amp with a rev B circuit board that did not match schematics. I did my best to identify modifications and reverse them, and I feel confident that the amp is close to how it left the factory. While doing so, I circuit traced the entire amplifier and updated the existing schematics to reflect the Rev B circuit board.

I have one question that outstrips my admittedly limited audio and tube circuit knowledge. The negative feedback resistor went from 22K in the earlier version to only 1.5K in my amp, and I'm left wondering if that too was an owner modification or if it was a sensible factory change. I've documented all noted changes in the attached schematic(s). The values shown on the earlier, revision A schematic are indicated in red. Current values are in black.

Carvin_x100b_1981_Poweramp.pdf
Carvin_X100b_1981_Preamp.pdf

[g1]

The negative feedback resistor is not just the 1.5/22K. It is the ratio of the 10K and 1K5 (or 66K and 22K in previous version). So some difference, but not as major as if it was still (56K + 10K)/1K5.
Also makes a difference which tap the NFB is taken off. Is it still coming off the 8 ohm tap?

[LessPaul]

Thank you. Yes, the NFB is still off of the 8 ohm tap. Would you say it is in the realm of possibility that this is a manufacturer change, possibly in keeping with the slight values changes in the long-tailed-pair?

Although this amp was extensively modified, I’ve not found anything in the power supply or the output section that said “user modification” to me, but noted the rather large value change of the resistor to ground. I appreciate you taking the time to look and explain it.

In days past I could go down the street to the local repair shop and ask. We were lucky in having Enzo as our local guy.

[g1]

Yes I would think it was a manufacturer adjustment of the NFB loop.

[DrGonz78]

Those old Carvin amps always had components added to those boards. Your first thought is oh crap some tweeker got a hold of this thing. Then it becomes clearer it’s a board revision macgyver style. I find those haphazardly installed components can lead to poor solder cracks in one unit I worked on.