avincenty said:

**I think the way to do it is to use the characteristic curve for the tube and draw the ideal loadline, calculate the plate resistance at the intersection of the loadline and y axis and then find a transformer that is closer to the value x 4.**

For tiode tubes - yup, RP * 4 should get you in the ballpark. Though note that if you know what the "ideal loadline" is, then you have the impedance; the slope of the loadline..

Chosing the best can be a matter of trial and error.

Once you've done it enough, often you can eyeball the curves and get pretty close.

The main tradeoff for triodes is that as you raise the Z, output power will drop, but distortion and output impedance will drop as well. But higher impedance transformers are hard to make with good freq response, so you there's a limit as to how high you want to go.

So, depending on the application and what the key goal is, Zl could be anywhere from nearly 1x Rp to 10x Rp.

In any case, as was mentioned, you need to observe the max power dissipation to set your operating point.

Pentodes work a little differently - the plate resistance is very high. I find the best way to pick the impedance is to take the plate curves and draw in the maximum power dissipation, and then take a ruler and start moving it around to find the best load line. You need to stay under the max power curve, though it's OK to exceed it on peaks. Find a line that gives you the biggest voltage swing. Usually that means intersecting the zero bias line near the "corner".

Pete