Let me echo Uwe's caution: for this conversion to be successful, the tube filaments need to be heat sunk, and the filament curls untangled and repositioned.
...I need a 25uF cap @ 6v parallel with a 3.9k resistor from cathode to ground. 25uF is hard to find and the caps I can find seem to have a very wide tolerance of -10 +75 %. My question is, would a 22uF be suitable in this location?A second question, what is the composition of the brown capacitor in this photo?
Don't worry about the value, 22uF gives a low cut frequency of 2Hz, way out of the audio range.