History of PX 25

The original PX-25 tube design and manufacture took place in the mid-20's, though GEC and others like M.O.V., Osram and Marconi produced them until the end of WWII, with limited production units appearing as late as 1965. This tube was officially discontinued after the war. The primary reason was the new breed of higher power tetrodes (nearest equivalent being the KT-66) that sparked a lot of interest because of their extra grid. These tetrodes were developed initially to replace the triodes for higher power delivery. Only later did astute listeners realize that these new tubes gave up detail, resolution and ultimate fidelity when compared to the purer triodes.

KR Enterprise, now known as KR Audio, decided to re-manufacture this tube with our input. The KR implementation is a bit different than the original. Its plate doesn't take as much voltage. The original could handle close to 450v whereas the KR tube is comfortable at around 370v (maximum). We run this tube at a bit less than maximum (around 360v) where the distortion level are optimized. The linearity of this tube measures better than most other "single-ended" type triodes and better than any new production tube that we have yet measured. Its frequency response is excellent even a bit below 10Hz, but even more surprisingly capable of HF extension better than 100kHz at less than 1% in our circuit.

This tube has a very similar midrange sound to the 2A3 but much more extended frequency extremes and more power. It is also a high Mu tube, which allows the design of an amplifier with greater damping factor (better control over speaker drivers). Compared to a 300B, it possesses a lower grid-to-anode capacitance (around 8pf rather than 13pf) and a higher anode impedance (1.2k compared versus 800 ohms). This allowed us to design an output transformer with extended bandwidth that transcends the capabilities of a 300B or 2A3. The result is a micro-watt amplifier capable of much more impressive performance than its puny 6 wpc would suggest on paper, provided that it is used with the proper speaker system. The total project and combined development of this design took about 17 months, including time spent with the research of the actual tubes from Riccardo Kron.