The PA1 audio output triode is capable of 1.8 Watts at a distortion of 10%. The electrode structure is inclined within the balloon envelope. What appears to be the anode is in fact the anode on the left and the control grid support on the right. We have a later version P.A.1 in the classic or shouldered envelope and the construction can be seen clearly.
The Micromesh/STC Type PA1 (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 PA1 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 PA1 is now a rare valve.
The balloon envelope is 54 mm in diameter and, excluding the B5 base pins, is 92 mm tall.
References: Datasheet & private communication.
Absolute Maximum Operating Conditions¶
Updated March 13, 2015.