Vintage Ekco RCX 'Ekcosprite' - Volume won't turn down to zero

I was working on restoring an old Ekcosprite recently and came across a puzzling problem where the tubes all tested fine, as did all the components (many were replaced and new)... but I could not shut the voume fully off.  It took some experimenting and head-scratching - and finally a little Googling to figure it out.

Ekcosprite RCX

Its an interesting radio - and while its not strictly a transformerless circuit, it only uses an autotransformer to step the voltage down for the series string of filaments - which is a more elegant solution than the usual big ceramic resistor that a lot of Philips and Mullard sets use.

The volume control is a variable resistor between the grid of the UBC41 (double diode / triode) and ground.  Shorting the grid diectly to ground made no difference and nothing else I tried did either... a real head-scratcher as I've never encountered something like this before... my suspicions moved to the UBC41 itself, even though it was working fine and tested ok.  I didn't have a spare, but my local valve (tube for you American types) guy came to the rescue with a NOS one.

Ekcosprite RCX Schematic

While I was waiting for this to arrive I did a little research and found one post on a forum about a similar problem where replacing the valve fixed the problem... seems like it could be a partial (or full?) short between one of the diodes and the triode (http://www.vintage-radio.net/forum/showthread.php?t=16622).  Whatever the actual cause, the fault was in the valve.  The forum problem was solved by swapping it out with a 10LD3 which is an equivalent - apparently the UBC41 is getting hard to find.  I am pretty sure a UBC81 (easier to get, electrically equivalent but wired differently) would also do the trick.

Info on the UBC41 can be found at the RadioMuseum here: http://www.radiomuseum.org/tubes/tube_ubc41.html

Ekcosprite RCX advertising