The last two Tube of the Month articles covered variants of the 801A with higher and lower mu. This months tube can be considered as an indirectly heated variant. The 843.
The 843 has very similar parameters to the 801. The amplification factor is 7.7 (versus the mu of 8 of the 801) the plate resistance is 4800 Ohms (4600 for the 801). With a max. plate dissipation of 12W it is actually closer to the 10Y than the 801A.
Being indirectly heated it needs one more pin to bring out the cathode connection. Thus it has a UX5 base. The pinout is pictured on the left. Where it differs from the 801 and 10Y is the heater voltage. It requires 2.5V at 2.5A. Please see the datasheet for all details. With an adaption of the socket and heater voltage this could be an interesting alternative to the 10. While the 10 or 801A require well filtered filament supplies for hum free operation, this tube could be used with AC heaters. It could be interesting as output tube or driver tube and potentially a preamp tube as well. I have not tried the tube myself yet.
As usual, let's have a look at the plate curves to see how linear it is. These are the curves from the datasheet:
The curves in the datasheet are a bit skewed since they also cover the Class A2 operating area with grid current. Linearity of the Class A1 region is difficult to judge. So let's have a look at the curves taken from an actual tube with the curve tracer:
This looks very promising! Definitely a tube which can be considered for audio purposes. It seems that it is not widely used among amplifier builders.
The 843 was initially made in globe shape and later also with the more modern ST (shoulder type) glass:
Let's have a closer look, first the ST type:
As with all ST tubes, mica discs are used to align the internal electrodes.
The next photo shows the indirectly heated cathode.