[Boatanchors] Patterson Power Supply Help Needed

[Drew P.]

 Joe Connor wrote:

"I want to substitute a PM speaker because I don't have much choice. The schematic lists the field coil at 1.5K ohms. My plan is to change the filter caps and substitute a 3K/10W resistor for the field coil just to see if the receiver works. Does the sound reasonable? The two original filter caps are 8 mfd. Should I up the second one to 22 mfd for better filtering now that there is no field coil or am I better off adding a second 3K/10W resistor and a third 10 mfd filter cap?"

A 1.5K resistor of appropriate power rating could be used, but the output filter capacitor might need to be increased to compensate for the inferior filtering as compared to a choke or speaker field coil.  Increasing the output filter capacitor would not increase rectifier and power transformer peak current. Do not increase the input filter capacitor by much, as such would increase peak current.

A better series filter element would be a nice iron core choke. One salvaged from a tube era TV set or radio would work.  Modern substitutes are available; Mouser carries some made by Hammond. 

An audio or power transformer used as a choke would, as pointed out, have trouble with low inductance due to core saturation.  A transformer could be modified to saturate at higher DC current by disassembling the core laminations and reassembling so that all of the E's are in one stack and all of the I's are in another.  When reassembling the core into the winding, space the E stack away from the I stack with a piece of paper to provide an air gap. Less gap gives higher inductance and lower core saturation current; More gap gives lower inductance and higher core saturation current. This would be easiest with a bobbin-wound transformer.

Drew