For anyone interested here is the 660 schemo.
It's a bit different from the 670 : there is no m/s matrixing obviously as it is a mono version and there is differences in how the 6386 are biased ( signal/compression amp, check for the negative rail used to bias 6386 cathode (r3 b), it's -16v and is one of the reason of the different sound of 660 ( with higher overall gain of the signal amp...) wrt 670 ) and some slight differences around the timing circuit but it is way easier to read ( and 660 sounded better anyway).
The sidechain amp is the 'middle' section of schematic. Psu is interesting to see too, it's often lacking in 670 schemo: tube shunt regulator implemented. Urei 176 have a 'crude' one implemented too mainly for the 6bc8 stage where compression occur (thanks to an ob2).
It's a bit different from the 670 : there is no m/s matrixing obviously as it is a mono version and there is differences in how the 6386 are biased ( signal/compression amp, check for the negative rail used to bias 6386 cathode (r3 b), it's -16v and is one of the reason of the different sound of 660 ( with higher overall gain of the signal amp...) wrt 670 ) and some slight differences around the timing circuit but it is way easier to read ( and 660 sounded better anyway).
The sidechain amp is the 'middle' section of schematic. Psu is interesting to see too, it's often lacking in 670 schemo: tube shunt regulator implemented. Urei 176 have a 'crude' one implemented too mainly for the 6bc8 stage where compression occur (thanks to an ob2).
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Mainly leakage through the diodes. Vacuum diodes conduct even when the anode voltage is very slightly negative
