This month I'd like to present a tube most of you probably never heard of: The 40.
The 40 is a directly heated triode with thoriated tungsten filament developed for voltage amplification stages. It is derived from the UX201A.
It is a high mu triode with an amplification factor of 30, which is unusually high for a directly heated triode. As it is derived from the UX201A it shares the same base and pinout as well as the same filament which needs 5V and consumes 0,25A. The higher mu is achieved by a different grid geometry. Such a high amplification factor does not come for free. The plate resistance is an astonishingly high 150 kOhm vs the 10kOhm of the UX201A. This corresponds to a transconductance of only 200 micromhos. In order to achieve a meaningful amplification the tube requires a very high impedance plate load and can drive next to nothing. In actual use this means that not much amplification will be left. This is probably why this tube never got very popular on and did not see widespread use. However it might be worthwhile to look at it with some more modern approach. A constant current source load should enable to realise the full mu of the tube. Since it runs at very low current of much less than 1mA it could also be loaded with a grid choke. Some of those can carry some DC current. However the winding capacitance of such a choke could already limit the high frequency response. Many years ago I built some test circuits with this tube directly coupled to a UX201A. I remember that I struggled to get a decent high frequency response which started to roll off at 10kHz. The sound I got from the test build showed some nice tone however. I never went beyond some initial attempt to use the 40. Maybe some brave soul will look at it and give it a try? It shows the typical plate curves of a directly heated triode:
Unfortunately I have too many other things on the plate. But if anybody takes up the challenge to use this tube in audio applications, please report back your findings.
I only have ST shape versions of the 40 in my stock but it was also made in globe shape as UX240.
These are some type 40 tubes made by RCA.
Like the 01A almost all of the glass is covered with getter.
Very little of the internal structure is visible through the getter.
A 40 made by Philco:
Different style Philco packaging:
When the filament tis lit up a little bit of the internal structure can be seen:
A beauty, isn't it?