Power supply sag and load lines

[dtbradio]

Hi All!

I've been looking around Google, etc, trying to find any general rule of thumb with regards to power supply sag when calculating load lines. The measured B+ when the tubes are cold always drops when the tubes begin conducting, barring a regulated HT supply. Maybe I'm over-complicating things, but it would seem that each power supply change (resistor value, cap value, etc), will yield a different plate voltage, giving different bias level needs, etc. Is there a general rule of thumb for this? Thanks for any and all insight!

 

[AMR / iFi audio]

Hi All!

I've been looking around Google, etc, trying to find any general rule of thumb with regards to power supply sag when calculating load lines. The measured B+ when the tubes are cold always drops when the tubes begin conducting, barring a regulated HT supply. Maybe I'm over-complicating things, but it would seem that each power supply change (resistor value, cap value, etc), will yield a different plate voltage, giving different bias level needs, etc. Is there a general rule of thumb for this? Thanks for any and all insight!

Some variation in the loadline is to be expected, even solely based on the fact that one model of tubes has variation from part to part.

Changes in series resistances in the power supply will change the operating point.
Changes in capacitance won't.

The rule of thumb is: You calculate the amplifier circuit with specific power supply parameters. Then you design a power supply that matches that
The stiffer (less load dependent) the power supply, the better.