The Micromesh P.A.1 audio output triode from STC is capable of 1.8 Watts at a distortion of 10%. The electrode structure is inclined within the envelope as it was in the earlier balloon envelope version P.A.1.
The Micromesh/STC Type P.A.1 (not to be confused with the earlier and different Mullard PA.1) uses a special construction which achieves an extraordinarily high value of gm (up to 12 mA/V) in a very compact valve. The P.A.1 and its professional 6V brother the 4033A were wonder valves when introduced in the mid-1930s but they were difficult to mass-produce.
STC were able to make enough for their own needs (the 4033A was used in telephone repeaters) but the British Government had to blacklist them (ie banned them from being designed into military or essential civilian equipment) because of nervousness about obtaining further supplies in the quantities that might be needed in a war situation.
Type P.A.1 is now a rare valve.
The inclined anode with side micas to hold it rigid.
This view appears to show the anode but in fact the anode is on the left and it forms an open cup into which the grid, on the right, is housed. The small diameter wire grid is fixed along its length to the long plate, thus keeping the dimensions stable and the grid cooled. This very small spacing between grid and cathode gives the high mutual conductance.
Here the top of the grid winding can be seen within the anode. The Type designation is etched into the glass.
The classic envelope is 46 mm in diameter and, excluding the B5 base pins, is 101 mm tall.