As to circuits: if you have room, install a 12AV6.
If you don't have room, or don't want construction hassles, hold your nose
and use a 40 volt power supply op-amp and two 18 volt Zener diodes in series
to drop B+ and give a reference 18 volt above ground. A fixed gain
will be fine, say 5-10 depending on the cartridge output.
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