05-09-2025, 01:35 PM
Thank you - it's good to know I wasn't dreaming! It was a long time ago, and as most of my design work is audio it's rare to use anything other than a resistive collector load - certainly not tuned. At some point I'll have a dig through my radio collection to see if I have anything that is close to the GEC circuit to investigate this further...
And I can understand why non-polar caps might seem like a bad idea - if we have to rely one oxide layer to not fail with time, then adding a second might not be the best plan
In practice, I think that the reverse-leakage of the wrongly biased oxide layer will allow enough through to help keep the correctly biased layer in good condition, as takes place in a regular polarised cap. Providing the cap sees opposite polarities from time to time, both layers should be maintained.
But modern capacitors do seem to be much better at staying healthy in scenarios where there is no bias. In audio electronics that uses split supplies, which is most these days, most capacitors have zero DC voltage across them, yet can last for decades. Similar caps acting as decouplers in the same equipment don't seem any more long-lived.
Obviously that's all rather anecdotal, but as I mentioned earlier, I'm just describing my experience
Either way, I do like non-polar caps. My opinion of them went up when Cyril Bateman published his articles about capacitor distortion in WW a while back (July 2002). Part 6 is the one that covers non-polar caps. Admittedly, that was focusing on distortion, not reliability. But as I say, in my experience at least, they seem to last just as well as their polarised equivalents. "Equivalents" is the key word there, because the non-polar types are usually 2,000 hour, 85C types. Of course we know that polarised caps are available with longer lifespans and 105C or even 125C operation, so it wouldn't be reasonable to expect the non-polar types to match those. I presume there just isn't the market for long-life, high temperature non-polar caps. Nothing stopping you make your own from two posh ones in series if needed - done that often enough...
Absolutely agree in general about film caps, but worth mentioning that certain plastic dielectrics can be less than reliable - think about how X2 caps gradually lose capacitance because of the self-healing dielectric swelling up. Often stops capacitor-dropper PSUs from working. Those might be considered historic, but we've had some quite modern bits of broadcast gear fail because of this (the capacitor dropper PSU was a start-up circuit for the main unit - long story!). Some LED lamps use capacitor droppers, of course. To be fair, they do lead a tough life in that role...
And I can understand why non-polar caps might seem like a bad idea - if we have to rely one oxide layer to not fail with time, then adding a second might not be the best plan
In practice, I think that the reverse-leakage of the wrongly biased oxide layer will allow enough through to help keep the correctly biased layer in good condition, as takes place in a regular polarised cap. Providing the cap sees opposite polarities from time to time, both layers should be maintained.
But modern capacitors do seem to be much better at staying healthy in scenarios where there is no bias. In audio electronics that uses split supplies, which is most these days, most capacitors have zero DC voltage across them, yet can last for decades. Similar caps acting as decouplers in the same equipment don't seem any more long-lived.
Obviously that's all rather anecdotal, but as I mentioned earlier, I'm just describing my experience
Either way, I do like non-polar caps. My opinion of them went up when Cyril Bateman published his articles about capacitor distortion in WW a while back (July 2002). Part 6 is the one that covers non-polar caps. Admittedly, that was focusing on distortion, not reliability. But as I say, in my experience at least, they seem to last just as well as their polarised equivalents. "Equivalents" is the key word there, because the non-polar types are usually 2,000 hour, 85C types. Of course we know that polarised caps are available with longer lifespans and 105C or even 125C operation, so it wouldn't be reasonable to expect the non-polar types to match those. I presume there just isn't the market for long-life, high temperature non-polar caps. Nothing stopping you make your own from two posh ones in series if needed - done that often enough...
Absolutely agree in general about film caps, but worth mentioning that certain plastic dielectrics can be less than reliable - think about how X2 caps gradually lose capacitance because of the self-healing dielectric swelling up. Often stops capacitor-dropper PSUs from working. Those might be considered historic, but we've had some quite modern bits of broadcast gear fail because of this (the capacitor dropper PSU was a start-up circuit for the main unit - long story!). Some LED lamps use capacitor droppers, of course. To be fair, they do lead a tough life in that role...
