Just to pick up on the points above, here are a few examples to consider.
- Your 'speaker system feeds the tweeter via a 3.3uf electrolytic capacitor and the crossover point is at 3kHz. After 30 years the capacitor ESR has increased from 0.5R to 1R and the capacitance now measures 6mfd. In the context of the 8 Ohm tweeter the ESR rise has little effect but the capacitance drift will have moved the crossover point down closer to 1kHz.
Clearly this should be replaced.
- Your amplifier uses an input coupling capacitor rated at 47uf in order to block incoming DC. This feeds an operational amplifier with input impedance at 100k Ohms. Measurement indicates that you need no more than 1uf to give a ruler flat response down to <5Hz. Your 30 year old coupling cap now measures 10uf and its ESR has tripled from the original 0.2 Ohms to 0.6 Ohms. The drift in values makes absolutely no difference to performance, and replacing this capacitor would be pointless. In many amplifiers the coupling and input caps are chosen purely because they are 'large enough' with ample margin and because the manufacturer saves money buying a crate of common values.
- Your valve power amplifier uses valve rectification and employs a cap/choke/cap in the smoothing circuit. These caps are typically around 47-100uf at 450-600v. The first capacitor is having to deal with considerable ripple current close to its limits, while the second receives the filtered signal after the choke. Not only is the first capacitor running near its limits but its running next to a searingly hot rectifier. it will be completely farked in <5 years. Clearly these caps need regular replacement because low ESR is essential for low noise.
- The chip based regulator in your pre amplifier is using a small 10uf electrolytic capacitor on its output to ensure stability. Ripple is low, heat is low, and it runs well within voltage rating. ESR needs to be moderate - too low and the circuit becomes unstable - and therefore even a tripling of the original value ESR has no effect. The capacitance value isn't critical within +/- 50% limits.
If these look fine, and the circuit is quiet - it can clearly be left alone.
Actually using a modern high performance low ESR cap in this situation will often give
worse performance.
Just some examples of why wholesale ripping out of electrolytics is unwise IME.
Not a view I've always held but one developed after spending a good few years ripping things to pieces and testing them
Over the weekend I'll post some actual test results for 30+year old caps. The results will surprise many - they certainly surprised me.
PS: Agree entirely with LPSpinner.