[Arran]

The Pen/DD61 was basically a peace offering made to the British Valve Association, by Philco of Great Britain, to incorporate a tube designed by one of the BVA member companies into a British Philco as opposed to using an American tube like all their other models used. If I'm not mistaken the Pen/DD61 is a Marconi Osram design, whom mainly manufacture tubes with 4 volt AC filaments, but for this set they took one of their existing tubes and reworked it with a 6.3 volt heater to match up with the rest of the tubes Philco used in these sets.
 The English like to gripe about these tubes, but that's because they can't wrap their head around the North American RMA system of tubes we use here, substitution is actually very straightforward. What can be done is to dump the Pen/DD altogether, and substitute it with either a #42 or a 6F6G, by either building an adapter using a base from a dead tube, or by drilling out the British style socket and replacing it with an octal or six pin one. Now what about the two diodes in the Pen/DD? Here you can either use a pair of SS detector diodes, such as a 1N34 or equivalent, or you can substitute the 78E tube for a 6B7, which is a second detector/IF amplifier. I tend to think of it in the same way as a set that uses WD11s, yes you could probably find good WD11s, but then you wouldn't dare use them as they are fragile and expensive.
  I ran into a 444 about 15 years ago at a radio club meeting, it was up for sale in a silent auction, it had some issues with the cabinet, some deep scratches or chips or something, I put a bid in  but the seller didn't get what he wanted and ended up taking it home with him. I have seen others turn up on these shores from time to time, either brought over by radio collectors or people who immigrated from over there.
  Knowing what I know now I would probably settle on one with either a bad or missing Pen/DD61 tube to keep the price down, which it tends to since the number of 444s exceeds the number of these tubes available, not unlike an Aeriola Senior, or Radiola III/IIIA. Perfect originality in such a set is not really that important to me since it's not really the greatest performer in the world, it was a mass produced budget set, and probably one of the most common pre war British radios, so how much do you want to put into one to make it play? If they wanted to cut down on the tube count they they should have done what Arvin did and left out the IF amp, or copied the models 80, 81, 84 etc from their American siblings. In my opinion the set to look for in this series of models is called a model A-537, it has five tubes, including a #75 first audio/detector/avc, and all are American types, the extra tube makes a lot of difference in performance, even in old blighty.
Regards
Arran

Here is an older thread on British Philcos, I repeated much of what I posted there.

http://philcoradio.com/phorum/showthread.php?tid=5444