I’ve got hold of a NOS quartet of UX-120 Radiotron valves made in 1925. These are 95 year old beauties. Unbelievable how well conserved they are and operate.
This valve developed by RCA was the first output valve intended to be used with dry-cells (3 No. 6 cells in series). See the data sheet extract below:
There is no much data available on this valve, if you have some please share and let me know. Here are the curves I traced from a sample which measured 114%:
I developed a Spice model as well for the ones interested in this valve:
You can download the model here: UX120-DHT-model
It should be an interesting candidate for a line stage with its low Mu. With a hybrid mu-follower using the gyrator PCB should work nicely. Could be biased around -6V/6mA which should give a very low anode voltage (aproximately below 70V). This could be implemented with a SiC filament bias array of 6 diodes. The tungsten filament requires only 125mA. Big question here is how microphonic the valve is? Only way is to test it. I will add it on the list 🙂
I received the following feedback from DHT Rob which is worth sharing:
“[…]
The UX120 has phenomenal inner structure. Round anode, round grid and 1 filament wire in the perfect center. Also it probably is tungsten? Due to big getter these aspects most of times are hard to notice (except the tungsten, of course).
It runs at relative low supply voltage and filament has low-wattage.
Sound is amazingly coherent and fluid.
But it IS microphonic.. you will have to take care about that using spring-based mounting or so (the rubber stand-offs are not working here). Also it is susceptable to hum, even from your hands coming close. From my experience placing it in a dug-out from metal helps a lot (punched aluminium will work as well) See attached photo for what I mean, really no more is necessary from an audible approach, measuring might prove different.
It has no real dirve, imho, probably be cause of the low filamentary power..
But.. again, it has special sound! […]” (DHT Rob)