Polar and non polar, electrolytic capacitors.

[Canleykid]

I have trawled through past posts trying to find an answer to an anomaly as I see it. From the Mullard 5-20 Circuit I see C10 and C11 are non-polar. From what I have read it would appear that nonpolar (i.e. paper in this case) should not be replaced with modern electrolytic caps. I for the life of me cannot understand what point the original caps achieve by allowing voltage to flow in both directions. As a time constant? the same applies to smoothing caps C12 and C15 (although I presume these could be replaced by electrolytic caps) can anybody shed some light or point me in the direction of where to look for the answers.

[Herald1360]

C10 and C11 are the phase splitter to output valve grids couplers. They need to be non polarised simply because electrolytics would be too leaky insulation wise for this application. In any case, since they're relatively low value, they're easily made without needing to be electrolytic to get the insulating film between the plates microscopically thin to get the required capacitance.

FWIW, voltage doesn't flow anywhere, electrons pushed by the voltage do that (as current). You only (technically) need a non polarised capacitor in a circuit where the ac voltage developed across the capacitor by signal current peaks at a higher level than any dc bias imposed by the circuit- for instance in a loudspeaker crossover network.

The supply filter capacitors could indeed be electrolytics, but the designer probably went for paper in oil as a more reliable high voltage part, 8uF not being too large physically for this sort of capacitor. They would also provide somewhat better high frequency decoupling across the supply than electrolytics.

[DangerMan]

C10 and C11 are the coupling capacitors to the EL34 grids. Any amount of electrical leakage here is unwanted as it will put a positive voltage on to the grids from the anodes of the phase splitter. Electrolytic capacitors will have a normal leakage current that is far greater than the non polarised (paper in oil) types recommended. Modern polypropylene types are ideal here. The insulation rsistance must be very high.

C12 and C13 are HT reservoir and smoothing and as you say, electrolytic types are ok here. The specified capacitors were available at the time and would give a very good life expectancy.
These days, and even back then, it is common to use much larger values for these two caps and dispense with the (expensive) DC choke L1, replacing it with a high wattage low value resistor.
If you look at the circuit for Leak TL25plus you will see what I mean.

[Canleykid]

Thanks for the guidance Dangerman and Herald1360, Your answers have helped a lot re coupling caps. Will rethink appropriate components for modern replacements.