The Cathode Bypass Capacitor of the Power Triode, PX25A (DA30)
The cathode voltage of WE 310A, which is the first stage and the driver stage, is only 2 or 3 VDC, while the cathode voltage setting of direct heated power triode PX25A (DA30) reaches 100 VDC.
In this case, I’ve thought that a margin of about 2 to 3 times the voltage is normally required, so that it was impossible to adopt a Wet Tantalum Capacitor here. In finishing Craig-san's and Ryszard-san's SETs, I aimed to reduce the ESR value as much as possible and flatten its ESR over the entire frequency range by connecting several reliable capacitors with different characteristics in parallel.
As a specific countermeasure to this matter, I first secured more than enough capacity by connecting two types of good quality electrolytic capacitors with slightly different characteristics, such as Mallory and Roe, in parallel.
This parallel connection also efficiently reduces the ESR in the low to mid-low range. For the mid-range to high range, I would like to recommend connecting a West Cap or Arizona hermetic seal oil filled capacitor, of around 0.1 μF to 0.47 μF, which have excellent sound quality and characteristics in this band, in parallel.
From the mid-high range to the highest range, take advantage of the ultra-low ESR that far exceeds the audible range of the Ultimate Ruby Mica. Please refer to the figure of inner view of Ryszard-san’s PX25A mono SET as a sample.
In addition, I've been sticking to wet tantalum capacitor for the cathode bias capacitor in the first stage as I described, but if there's enough space to fit multiple parts, I think it's a good idea to adapt this capacitor configuration.
In self-biased circuits, I’ve often seen a cathode bypass capacitor for power tube in which only one around 100 μF electrolytic capacitor is connected in parallel with a cathode resistor. In this case, I believe the role of this bypass capacitor is completely insufficient, and it is clear that it will not be possible to make the best use of the unique characteristics and advantages of the original power tube.
