The PL84 pentode is very well known out there. I’m not going to write about it as there is plenty of information about the use of this pentode in push-pull amps, etc as well as comparisons with the EL84, EL86 and Russian equivalents such as 6P14P, 6P15P and 6P43P. I recommend you to check Klau’s work here.
I have a nice set of Telefunken PL84 which I may be using in triode mode as part of a Spud project I’ve been working on the design for some time. I’m interested in triode-strapped curves but also will be looking at tracing the Schade-feedback curves when I get the time to do so.
I quickly found a sample valve that was close to the pentode specs at 90%. Good enough for my purpose so I proceed to trace the triode curves quickly in uTracer.
I should have traced the curves beyond 200V, but the 6 curves traced are good enough to generate an accurate triode model. A different story is developing a pentode/tetrode model as you may be aware of.
The triode model can be easily generated:
The spice Pl84-triode-spice model is very accurate. The only disadvantage of this valve is the maximum 250V anode voltage which limits the available output power in a push-pull design compared to other similar valves like 6P14P, etc.
With a 5K Zaa transformer you can get 7W in class A1 with a distortion a bit higher compared to 6P15P but not that much:
An extra 1.5W can be obtained if we use a 3K transformer instead:
