ot tubes load impedance qs..

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HI. Is there simple way to get the right load for a pair of tubes, at a selected voltage?. Suppose i want to have a pair of EL34s at 340volts, or maybe 390 instead of the typical 430v. Maybe some chart?... Thanks in advance.
Sorry to ask this in here but ampage is getting strange nowadays. Does anybody know why the mood in the best amp forum??