@Deb, the output tube - 2A3 or 300B - has no effect on the driver operating point because the grid draws no current. So changing the output will not have any effect on the driver and no adjustment is needed.
@Grainger, once the tube is conducting the shunt regulator kicks in and the power voltage for the driver is stable. But more relevant is that the driver is loaded with a current source, which (once the soft start has kicked in, about 30 seconds) sets the current, no matter what the supply voltage might be. In our experience with 5670 tubes, they stabilize quickly and further heating (as the envelope and internal parts temperature stabilizes, maybe 30 minutes?) has no detectable effect on the bias needed to obtain a stable plate voltage. But it's a reasonable precaution to re-check the bias after 30 to 60 minutes in case your particular tube in not as stable as the half-dozen we tried. Especially if you have changes out the tube for another type...
Again in our experience the 5670 does not drift, so the 100-hour check showed no change. But since many tubes these days are showing up with incompletely formed cathodes, I recommend the check anyhow.
