08-18-2013, 09:16 PM
I think that I have mentioned this in previous threads but getting the negative connections mixed up on the filter caps in a pre war radio is one of the most common errors people make. The point to remember is that the center tap of the high voltage winding is always the most negative point in the circuit, if it happens to be connected directly to the chassis then that means the chassis is also the most negative point, if not then assume that the chassis is more positive then the center tap of the power transformer.
They used to use this a lot in pre war sets, and sometimes the odd early post war set, as cleaver way of creating a C- voltage, sometimes called a fixed bias supply. However they do not so much create a C- voltage as make some element of a tube, usually the cathode, more positive with respect to the control grid, creating a small potential between the two. This originated with the earlier AC sets where they used directly heated cathode tubes like #45s but they carried on the practice long after there was no need to do so. Later on in the 1930s some sets dispensed with this idea and went with a cathode bias, they made the cathode more positive then the chassis by using a resistor and capacitor connected between the cathode and the chassis. This is what most normal sets used, then you have some whacky circuits employing things like bias cells, Canadian built Rogers and Marconi sets loved using those, probably one of the most worthless ideas they ever came up with.
Regards
Arran
They used to use this a lot in pre war sets, and sometimes the odd early post war set, as cleaver way of creating a C- voltage, sometimes called a fixed bias supply. However they do not so much create a C- voltage as make some element of a tube, usually the cathode, more positive with respect to the control grid, creating a small potential between the two. This originated with the earlier AC sets where they used directly heated cathode tubes like #45s but they carried on the practice long after there was no need to do so. Later on in the 1930s some sets dispensed with this idea and went with a cathode bias, they made the cathode more positive then the chassis by using a resistor and capacitor connected between the cathode and the chassis. This is what most normal sets used, then you have some whacky circuits employing things like bias cells, Canadian built Rogers and Marconi sets loved using those, probably one of the most worthless ideas they ever came up with.
Regards
Arran
