04-09-2025, 12:25 PM
I'm fairly happy that the polarity must reverse - not least because if it didn't, they wouldn't have gone to the expense of a non-polarised part 
With no-signal conditions, we know that there is -0.8V at the left of the cap. The other end is at -0.68V. So the positive end is on the right.
With the signal present, the voltage on the left can only be taken "down" the page, i.e. made more positive. As mentioned, I can't predict by how much, but that's the only direction of travel possible.
Of course, VT2's emitter voltage also falls, but that can only fall to 0V worst-case. So if the AGC can pull the left of the cap more positive than 0V, then there's your reversal. I considered doing some sums to see how likely this is, but I keep coming back to my opening statement - it might happen because they fitted the non-polar cap.
Yes, that will likely put the transistor into class C. But it's feeding an IFT, and that's followed by another IFT. So I suspect that will remove a lot of the harmonics, and even if not, how problematic would they be in this set with a very basic audio side? Others will have more insight than me, I suspect, but certainly, I've seen AGC circuits that work this way before.
It's interesting that they fitted another non-polar cap across the LS. You'd think they could have used a regular cap ahead of C22, but perhaps that didn't work for them? Or perhaps there just wasn't the price differential with non-polar caps back then, so there was no reason not to use them? If so, perhaps C11 was non-polar "just in case", which rather undermines my argument!
That's why I said it needs to be tested. If there is a reversal, and the signal-present conditions results in a greater potential than the rarely used no-signal condition (which is only 0.12V apparently), then I'd be comfortable using a polarised part installed appropriately for realistic use. No modern electrolytic will mind 0.12V of reverse bias.
With no-signal conditions, we know that there is -0.8V at the left of the cap. The other end is at -0.68V. So the positive end is on the right.
With the signal present, the voltage on the left can only be taken "down" the page, i.e. made more positive. As mentioned, I can't predict by how much, but that's the only direction of travel possible.
Of course, VT2's emitter voltage also falls, but that can only fall to 0V worst-case. So if the AGC can pull the left of the cap more positive than 0V, then there's your reversal. I considered doing some sums to see how likely this is, but I keep coming back to my opening statement - it might happen because they fitted the non-polar cap.
Yes, that will likely put the transistor into class C. But it's feeding an IFT, and that's followed by another IFT. So I suspect that will remove a lot of the harmonics, and even if not, how problematic would they be in this set with a very basic audio side? Others will have more insight than me, I suspect, but certainly, I've seen AGC circuits that work this way before.
It's interesting that they fitted another non-polar cap across the LS. You'd think they could have used a regular cap ahead of C22, but perhaps that didn't work for them? Or perhaps there just wasn't the price differential with non-polar caps back then, so there was no reason not to use them? If so, perhaps C11 was non-polar "just in case", which rather undermines my argument!
That's why I said it needs to be tested. If there is a reversal, and the signal-present conditions results in a greater potential than the rarely used no-signal condition (which is only 0.12V apparently), then I'd be comfortable using a polarised part installed appropriately for realistic use. No modern electrolytic will mind 0.12V of reverse bias.
