Lowering R31 to 10R helped a bit. Now, there's more than +/- 8V under full load.
Lesson learned - I should not blindly redraw schematics found on the net.
So .. If you plan to build Ampeg VH140 using
schematic from dirtboxlayouts then remove 100R in the power supply section or use much lower value. This is true for 9V supply voltage. I could not (in fact I din't want to) test the circuit with 12V because of the maximum allowed input voltage for the 7660. I only tested 10.5V and there was a slight improvement. But I want the cuircuit to work properly with standard, 9V supply.
In the first place I should describe the problem caused by IC's supply voltage that's too low, here in VH140. Preamp sounds completly fine until gain is @ 14-15 o'clock. Then, when playing palm muted, short notes, there's a brief moment of silence after note dies out and after a half second it comes back to normal "operation". It's like a slow noise gate that works in reverse. When it's not gating you can hear preamp's usual noise with slight background hum, then moment of silence and noise again. With 9V supply it's quite strong effect. With 10.5V only appears at extreme gain setting.
I'm not that good to explain the nature of it but I suppose it's related to lowered headrom @ 4th and 5th opamp, maybe even 3rd one. Also arrangement of the diodes in opamps feedback loop, electrolytics connected between feedback loop and gnd.