With the 4th of July behind us, I should share as a kid that I also found the 4th to be a somewhat sad milestone. Summer vacation from the rigors of attending school was headed on the downward side. In no time it would be time to shop for flannel shirts, heavy socks and winter coats. Gone would be fresh corn on the cob and watermelon. It also meant soon no enjoying sitting on the back porch and watching the fireflies. We are headed on the path to winter. The longest day of the year (for sunlight) has passed too!
Project X Home Power Supply
I managed to acquire the schematic for the Project X home power supply and some information in that design is noteworthy.
A Tribal Knowledge Trick is to use a center tapped secondary winding on the power transformer where the that winding feeds a bridge rectifier and the center tap provides about 1/2 the voltage. In our case if the secondary was 500 volts (250 Volts either side of CT) then the bridge output would be 700 VDC +. Here two VR tubes in series through the dropping resistance provides 258 VDC regulated for the screen supply. [OA2 = 150 Volts and OB2 = 108 Volts: Total 258.]
Also in this supply is a 3PDT relay that is triggered off of Pin 4 on the Power Plug which is connected to the PTT button on the microphone. One set of relay contacts are for turning on an external linear amp. A second set on receive grounds the Speaker Voice Coil but on transmit sets the operating Bias (-60 VDC).
You might ask why the relay on the Bias? With the 4.7K 1W resistor floating (on receive), the bias value is way higher than the Bias level needed for normal operation which effectively cuts off the 8042's. But upon grounding of the resistor the bias level goes to the set value to produce 50 milliamps of idling current.
The last set of relay contacts connects the 3.2 Volts to the Filament Circuit.
The 12.6 VAC is converted to 12.6 VDC to operate the Solid-State and other Electronics in the radio and supply this relay. One additional relay will be included to switch on the main power, so we don't damage the power switch in the radio.
The feature to use instant on final tubes, does place a significant burden on current users some 65 years later to provide the voltages needed to operate the radio. It also makes it difficult to just swap in one of the generic supplies from Heath, Swan or Drake... you'll kill the finals if you do!
Them that know can make things go.
73's
Pete N6QW