Pye P114BQ

[Gary5117]

Hi everyone.

I'm trying to understand radio schematics and I wondered if anyone could help with the description of this PYE radio cct in which it says the volume control acts as a diode load. What is a diode load, why do we need one? Your help is much appreciated.

[PJL]

Without a 'load', the AC signal rectified by a perfect diode would pass just half cycles of the AC carrier signal but in this circuit and without the 'load' it would discharge C18 leaving it to rest at a -ve DC corresponding to the AC carrier voltage peak. The volume control acts as a load on C18 and discharges the capacitor but slowly enough to filter out the unwanted carrier frequency but not fast enough to remove the audio.

[Gary5117]

Ah yes, that makes sense I put a capacitive dropper in a champion radio and couldn't understand why the rectified B+ wasn't getting up to the required voltage. It turned out that the capacitor was only working in part because it was also feeding the valve heaters which I guess were acting as a capacitive load....but not enough.
That is so helpful
Thank you
Gary

[Cobaltblue]

If you have not already seen it there is some additional information on this subject here:

https://www.vintage-radio.com/repair...rf-stages.html

Including a nice drawing that shows what PJL describes.

A lot of good reading here:

https://www.vintage-radio.com/repair...ion/index.html

Cheers

Mike T

[Gary5117]

In this cct is it also doing the same for C19?, and does the 10Mohm Resistor R8 do the same for C20?

Thanks

[Gary5117]

Thanks Mike T I will have a look.