I am repairing an old tube-powered audio signal generator (HeathKit AG-9) and want to replace the old wax/paper capacitors that control the frequency of the oscillator circuit. The unit uses a bridged T filter to set the oscillation frequency:
simulate this circuit – Schematic created using CircuitLab
And here is a photo of the caps in question:
Looking at mouser, I see Polyester, Polypropylene, and PET caps in suitable capacities (47pF-500nF) and voltage ratings (100V). Which of those would be suitable for this application? I am concerned about temp stability primarily, but also need a relatively low tolerance part because the exact capacitance is critical to the functioning of the circuit.
Best Answer
The old Wax Paper capacitors For those who have never seen one, you can get an idea of their size by the size of the resistors. The large brown cylinders are the capacitors :
The tolerance on the capacitors wasn't so good. And they were huge physically. Not the best tolerance on temperature either.
You can safely choose among any capacitor that has same voltage rating, capacity, tolerance, and temperature dependence, when it comes to replacing old Wax Paper capacitors.
(Unless you are a super audiophile) winky winky.
EDIT 1 : Robert Endl pointed out in a comment that perhaps the capacitor might be Mica. In the photograph mica capacitors can also be seen. They would be the blocks that appear with color dots on them (similar to resistor color code). If indeed the OP's capacitor turned out to be a mica capacitor, they are often now days called "silver mica" capacitors. If the capacitor is a mica, then there are silver mica capacitors available.
EDIT 2 : With new photo from OP : The two red capacitors on the left do appear to be Mica capacitors. Just to understand Mica capacitors, they are very high Q ( high quality) capacitors that have very low loss. Very temperature stable too.