Load question (or "what is R2 in the Duncan Amps PSU Tool...??") (N00B Alert)

Hi,

This is a total newb question. But, if I'm using the Duncan amps PSU tool, do I use the plate resistor for the load, the plate resister added to the internal plate resistance of the tube?

And if there is more than one tube, do I use the plate resistance (question 1)  in parallel?

> do I use the plate resistor for the load

If the tube were a dead-short, this would be valid.

If the tube were a dead-short, you could replace it with a much cheaper wire and get the same result (no signal). That aint right.

> the plate resister added to the internal plate resistance of the tube?

No. This is closer, but the data-sheet Rp is not given for the same conditions you will work at, varies a lot, and is small-signal which gives a modest error on triodes but is way-wrong for pentodes.

You want a rough analysis of your tube stage. Will the plate sit all the way at B+? No. Will it be stick to ground? No. The most general assumption is that it will be "halfway". i.e., if B+ is 400V, the plate will be around 200V. And if plate resistor is 100K, and has 200V across it, then clearly the current is 200V/100K= 2mA. And if resistor+tube drop 400V and the same 2mA flows in both, then the whole stage is 400V 2mA and therefore "acts like" a 200K resistor.

And.... in self-bias stages, this approximate equivalent resistance will stay nearly-the-same over a _wide_ range of B+. With same bias and load resistors, at 200V total the plate will sit somewhat higher than "halfway" and the equivalent may be nearer 240K.

Looking at many-many designs, plate is usually in the range from 40% to 70% of B+. The canonical Fender 12AX7+1K5+100K stage sets its plate near 69% of supply voltage. The stage acts like 320K. Lower-Mu tubes are sometimes biased with plate below half. Effective resistance is 1.5X to 2X the load resistor.

And all tube-work is approximate.

Take some rough assumption and run with it.

When you don't know better, double the load resistor.

> And if there is more than one tube

Take each stage and parallel them. One Fender-stage, 320K. One fat triode cooking 22K, say 40K. 320K||40K= 35K. That's "R2" in PSD.

Note the "R2" designation changes as you add filter stages.

Note also that the 320K || 40K calculation is not very different from assuming just the 40K. When one stage is much bigger than the others, it is often sufficient to estimate that and then round-down to cover the small crumbs.

I'd back up a bit... If you are ready to design the power supply you must already know the idle or quiescent current being drawn from the supply by your amplifier circuit. With PSUD you use Ohm's law to calculate the equivalent load resistance, or use the constant current load PSUD offers instead of the load resistance.

You also know the maximum or peak current drawn, for example if you're designing a power amp stage it could draw more current at full signal condition. If there is a varying current draw, you can use the PSUD step load function to check the dynamic response to load changes.