Why High Plate Voltage in 2A3 AMP?

Plate voltage isnt' what will kill the tube per se, it's more about dissipation (watts). B+ shouldn't really be a comparison factor if shopping for amps, it's but one part of a large picture. the difference between 320 and 390 is not that large really.

Benefits: sometimes cathodes (filament voltage) are floated higher than typical ground potential, for reasons of noise rejection. This is done with a voltage divider. That might impact where the B+ needs to be in relation, depending on how the power supply is set up. many variables, hard to generalize.

-Ed

Another way to do bias is with batteries (e.g. rechargable lead-acid usually). The positive side goes to the filament/cathode, neg to ground, and the current passing through it to ground keeps the batteries recharged. Sometimes people throw an LED in there too to adjust current draw.

Scratch-building is the way to go, btw. ;-) Stop over to TubeDIY on AA sometime for some real fun discussion on that.

-Ed