Triode Valves - Fewest Electrodes For Purest Sound.

The Triodes described on this page are PX4, PP3/250, ACO44, 4XP, AC/P1, PX25, PP5/400, PA20, 2XP, ACO42, 2P.

Right from the first radiogram manufactured in 1929, to the last radiogram in 1952, triode valves were utilised by R.G.D.  There were exceptions, televisions did not feature triodes after WWII, and in 1939 the top of the range radiogram utilised tetrodes (though some lower in the range still used triodes).  But generally the triode valve was the output stage of choice.  Why? - the purest sound results from the least interference to the signal being amplified, and the device that performs this function beautifully is the triode valve.

R.G.D. usually incorporated triodes supplied by Mazda, namely AC/P1, PA20, PP3/250, PP5/400.  Other manufacturers produced similar equivalent products that may be interchangeable, for example PP3/250 may be substituted by PX4.  Some triodes that may possibly be found in an R.G.D. receiver are shown below.

Image of early Osram PX4 balloonImage of early PX4 showing logo and type numberImage of early PX4 showing sloping electrode structureImage of early Marconi PX4 balloon

Early Marconi PX4 original 1929 shape PX4 logo clearly visible rectangular box Sloping electrode structure of early PX4 Marconi branded early PX4 about 1929

Image of shouldered PX4 chrome anodeImage of shouldered PX4 dull anode and cooling finsImage of straight sided PX4 dull ladderImage of straight sided PX4 and cooling fins

Shouldered PX4 from 1930s chrome anode Shouldered PX4 & fins to aid anode cooling Straight sided PX4 with dull ladder anode Straight sided PX4 & fins to aid anode cool

Image of PP3/250 balloonImage of PP3/250 1930s shapeImage of ACO44 1930s shapeImage of ACO44 Mullard branded but MOV

Mazda PP3/250 early balloon (PX4 equiv) Mazda PP3/250 later 1930s shape Mullard ACO44 from 1930s (PX4 equiv) Mullard ACO44 but made by MOV

Image of 4XPImage of AC/P1Image of PX25 balloonImage of PX25 straight sided

Cossor 4XP from 1930s (PX4 equiv) Mazda AC/P1 triode from late 1920s PX25 balloon shape high power by MOV Later PX25 shape, coin as size comparison

Image of PA20Image of 2XP with paper labelImage of ACO42Image of 2P

Mazda PA20 balloon  shape 2v heater Cossor 2XP early paper label 2v heater Mullard ACO42 1930s 2v heater Cossor 2P, lower power output 2v heater