Beautiful Things


Not far off FWLIW - “strawberry sea anemone” - Actinia fragacea.


very nice lightly fried in butter and spread on a HCB


Two newts swimming in my pond this morning, no doubt feasting on the tadpoles hatching. Thousands of tadpole eggs floating at the moment, the cold spell did for 5 adult frogs so I have been picking them out and re-cycling them in the green bin.


Not Great-crested are they?




Don’t think so, a lot less spotty.


I was given some boxes of old used valves today. Among them were three 813s, some quite nice looking 5U4Gs, a pair of 6A3s (like 2A3s but with 6.3v heaters), a GZ34 and this

So far I’ve only tested the GZ34, which was stone dead (lots of internal shorts, boo hoo) and the above PP3/250 which, up to 250V which was as far as I could go, tested like new (yippee !). It’s basically a PX4. I’ve never owned a PX4.



6A3’s should be nice.


Looks a bit of a beast - what’s it do?


Actually not very much, in a sense. It and its equivalents come from an important point in valve history though. In 1929 M-OV (Marconi-Osram Valve) introduced the PX4 as a development of their existing ‘dull emitter’ range which, in turn, had been developed from the military surplus valves available after the first world war. The PX4 was capable of delivering ‘serious’ audio power - 2+ watts as a single-ended triode and 10 watts or more in a push-pull pair. Around this time talking movies were becoming common and the first radiograms were appearing, so there was a need for significant power. The PX4 became very popular and actually survived in new-design use until after WWII. The PP3/250 was Mazda’s equivalent to the PX4. Mine is a relatively early valve as indicated by its ‘balloon’ shaped envelope.



The PX25 is on my list of amps to hear or try, preferably with NOS valves.



My newt. Sorry about the crappy phone pics, he is about 10cm under the water.


Minute (although he looks about the right size, TBH).



Word is that triode-strapped KT66s have very much the same characteristics as PX25s. I’ve owned a PX25 for a couple of months now too, and I have KT66s, so when I get round to it I plan to check this out. In a parallel universe, where I am more productive than I am in this one, I will one day build a pair of these They were developed through the 1930’s by writers for Wireless World (mostly the editor, W.T.Cocking) and represented pretty much the best that they could do with simple triode push-pull and no negative feedback. Almost immediately after the war Leak’s TL/12 Point One and the Williamson amplifier brought NFB into play and shortly afterwards ultralinear operation with beam tetrodes and pentodes was discovered and the big manufacturers never looked back.



This looks interesting as well, you could be quite busy for a while


Good (at least, objectively good) single-ended performance comes down so much to the quality of the output transformer and the linearity of the output valve.

If I understand it rightly the Loftin-White design (publ 1931 !) helped sort out problems that they had back then stemming mostly from the unavailability of high-gain small-signal valves. Given its very early date it does a surprisingly good job. But over the next decade I think better/simpler solutions became possible. So much of the appreciation of SET designs is subjective though that I’m sure there are still fans.



There was an interesting write up on the Loftin white in (now defunct, sadly) Vacuum Tube Valley Issue 6 a few years ago. Conclusion was that it was designed mostly to be cheap rather than great performing.

VTV archive can be found here:

Some great informaion on the history of tubes, circuits, reviews, schematics, etc.