Here is a link to the three transistor Neve input stage.

http://www.jlmaudio.com/neve_ba283.htm
The gain of the first stage (TR1) and hence the open loop gain is determined by the ratio of R4 to the parallel combination of R10, R11 and any resistor connected between pins T and V. At the same time, the closed loop gain is determined by the ratio of R9 and the same parallel combination of R10, R11 and any resistor between T and V. To change the gain you add a resistor across pins T and V which both incresaes the open loop gain and the close loop gain.

Here is a link to the Helios three transistor mic pre scehmatic:

https://groupdiy.com/index.php?topic=58393.msg742685#msg742685
The gain of the first stage (VT1) and the open loop gain is set by the ratio of R5 to the series combination of R3 and any resistor connected from C5 to ground. At the same time, the closed loop gain is set by the ratio of R4 to the value of any resistor connected from C5 to ground. To increase the gain you reduce the resistance from C5 to ground which increases both the open loop and closed loop gains.

Hope that helps.

Cheers

Ian