My interests in the RS241 idea have been coming back. I've built an amp around this tube a few years back, but it never got past the breadboard. With some spare time on my hand I think it is about time to make a nice project out of this old idea. I've kind of been making preparations for this idea in my Exodus 2 power amplifiers. The UX210 and RS241 can be interchanged pretty easily by switching a cathode resistor and the filament voltage/current (and socket ofcourse). All the supplies are identical, so nothing has to change here.
The RS241 seems to be made for class C if you look at the parameters: mu of 17, low bias, high voltage. Yet looking at the curves will reveal that this tube can be used very well for class A audio. Running it under normal circumstances, the RS241 will be able to deliver around 0.9W into a 12K load (Rp is 4K9) at 400V/30mA. If the driver is up to the task, the RS241 can be run quite a bit into class A2 as well and still stay very linear.
In the load line below you can see that the grid can be run past +10V all the way down to about -30/35V. Biasing the tube around -10V would mean a swing of 23-25V giving a bit over 3.5W output!!! Sort of like a sleeping giant. The low bias means an easy to drive amp, if you only require the first 0.5W. Going positive on the grid will require a good driver (current, impedance, etc.) as there will be noticeable grid current being pulled (3-4mA) by the RS241.
© Jim de Kort