[LenH]

As I am nearing completion of restoring a 37-62 radio, I see that the 10 ufd electrolytic capacitor is wired with the positive (+) connection grounded. I would like to hear why that was done for this Philco design. Does it have anything to do with the connection to the 6F6 output tube grid and its bias?

[morzh]

It is in many designs.

The reason is because it is between the Rectifier's negative and the GND, and the rectifier's negative is ALWAYS THE MOST NEGATIVE POINT in any schematic, so everything else is positive relative to it, and so is the Chassis.

[LenH]

Thanks for the reply. That helped and is what I assumed, but I am really curious about what this capacitor is doing in the rectifier circuit. Perhaps I need to brush up on rectifier circuit design and filtering (?).

[codefox1]

My response was redundant.

[BrendaAnnD]

It's "C" supply filtering. (B-) for bias.

[morzh]

LenH

It has little to do with rectifier design. They simply need a negative bias and so the rectifier negative through resiztor creates negative voltage towRds ground and this cap simply filters it. In some schematics it is left unfiltered.

[exray]

Lesson. Ground, negative and chassis are not always the same and the terms are not necessarily interchangeable.

[morzh]

"Ground" which is in systems without separate chassis Ground is the same as the "chassis" is defined as "0V common reference point". It is not negative or positive. It is the common return or common reference.

For example in a typical by-polar Op Amp schematic there will be two power supplies, Positive and Negative, such as +/-15V. GND is simply the common reference there.

[Arran]

"Does it have anything to do with the connection to the 6F6 output tube grid and its bias?"

Yes it does, the most negative point in an AC radio is the center tap of the power transformer. So the way they developed the bias voltage was by connecting a large wire would resistor with taps, or series of resistors, to the center tap to make everything else more positive with respect to the center tap including the chassis, the chassis would usually be at the end of the resistor network.
This is a fixed bias supply and was very common in pre war AC radios. So to an electrolytic cap the chassis would be the + side and the center tap would be the - if one was installed there. Later on they used a paralleled resistor and capacitor between the cathode of the power output or other tubes and the chassis to make the cathode more positive with respect to the grid.
Regards
Arran