Just my two cents worth about capacitors (I am not saying that capacitors are the fault):
In general, you can fit polypropylene capacitors, with safety and good performance, to most amplifiers, especially valve amplifiers, and many other circuits.
As a very general rule your capacitor choices boil down to:
(1) Aluminum Electrolytic for polarized high value capacitors, both high current and low current types.
(2) Polypropylene metal film for general use
(3) Ceramic X7R dielectric for decoupling, but not in audio amps or any other low distortion application.
https://en.wikipedia.org/wiki/Ceramic_capacitor
For non-critical applications you can use polyester metal film capacitors in place of polypropylene to save money.
And about resistors:
(1) Metal film are the best for normal resistors, but solid carbon may be required for high frequency work.
(2) Wire-wound for power resistors, unless it is essential for frequency performance to use carbon or ceramic power resistors
Metal oxide resistors are the most reliable, and can be used in place of metal film but not for low distortion applications.
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