Thomas Mayer presented in his blog a great article about this DHT. A very interesting valve for preamps given its unusual high-mu (circa 30). It could be used in filament bias despite its high filament current needs (see the curves). Ideally choke loaded or IT transformer coupled, I suspect it could perform well with a mu-follower gyrator to provide a low output impedance given its high Ra (for those who are not uncomfortable with some sand assistance 🙂 )
Here are the curves:
I found Ra to be around 20-25KΩ, instead of the higher value highlighted by Thomas.
Here is the SPICE model:Triode 841-Composite
Which produces a nice set of curves extending to the positive grid current region suggesting an interesting use in A2:
Where Oh where are these 841/VT51 tubes to be found, seems impossible to find
I just got a sample one for tracing 🙁