01-06-2021, 09:24 AM
Looking at this, the stage immediately following the Baxandall tone control looks marginally st6able to me.
There's a LF roll-off caused by the 0.1μF and 1MΩ to pin 7 of the 12AX7, and there's a further roll-off caused by the 0.22μF capacitor from pin 8 of the 12AU7 back to the Baxandall network.
At low frequencies, these will each add phase shift approaching 90° , to the 180° inversion of the 12AX7 voltage amplifier... Total 360° shift, and with a few more degrees from (say) the roll-off caused by the 100μF cathode bypass capacitor, you've got yourself a circuit which will softly motorboat.
There's a LF roll-off caused by the 0.1μF and 1MΩ to pin 7 of the 12AX7, and there's a further roll-off caused by the 0.22μF capacitor from pin 8 of the 12AU7 back to the Baxandall network.
At low frequencies, these will each add phase shift approaching 90° , to the 180° inversion of the 12AX7 voltage amplifier... Total 360° shift, and with a few more degrees from (say) the roll-off caused by the 100μF cathode bypass capacitor, you've got yourself a circuit which will softly motorboat.