I have had a Marconi DER (Dull Emitter Receiving) valve for quite a few years now. It looks very much like this
http://www.r-type.org/exhib/aab0001.htm except that the bright silver getter covers the top half of the glass envelope and it doesn't have a paper label wrapped around the base. There is a BBC stamp of the 'Postmaster General' type on it which, apparently, dates it to 1922-24
http://g3ynh.info/valves/old/BBC/BBC.html#GPO. The valve's serial number is, I think, K1135. As well as the letters D.E.R. there's also a faint 'Marconi Valve' stamp on the glass.
Today I finally tried it out. At 1.8VDC the filament draws 375mA or so. I bridged the filament with a pair of 47ohm resistors in series and applied DC HT between their junction and the anode, and negative grid bias between the same junction and, yes, the grid. Attached are a couple of plots of anode current in mA, first versus grid voltage with 75V HT between anode and filament and second versus HT voltage with the grid at -1.00V, again relative to the filament. The third picture is a scruffy phone snap of the little fella in my scruffy lashed-up test rig.
With just a milliamp or two of anode current I'm guessing that this valve is a bit tired. I can't seem to track down a data sheet for the DER but I'm sure there are those here who know what the current really should be and so can tell me just
how tired. In any case it's a lovely thing to look at given its utter simplicity and considerable age.
Thanks in advance !
Cheers,
GJ