Two 1N5408 rectifier diodes will replace it. The banded ends of BOTH diodes go to pin 9 on the socket. One unbanded end goes to pin 1, the other to pin 7.
Trouble is, these rectifier diodes are more efficient than the original tube. The original tube would lose around 40 Volts, where the 1N5408 loses less than one Volt. This will raise the DC voltage on the power-supply filter capacitors. If they are old, this stress may cause them to fail soon. Electrolytics will get 'accustomed' to operating at whatever voltage the old 12BW4 tube was delivering. Boosting that voltage, even by 10% or so, can upset the applecart.
New filters will prevent this problem, so long as you use new ones with a high enough voltage rating.
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