PULTEC EQP-1A POWER SUPPLY SCHEMATIC
HIGH VOLTAGE AC
The high voltage transformer I am using is the Hammond 270FX. High voltage AC (alternating current) comes into the supply circuit by connecting the red transformer wires to (J6) and (J8) labeled "RED". Note: Either of the red wires can connect to J6 or J8. But, don't forget to connect the "RED/YELLOW" wire to (J22) labeled RED/YEL or your supply will not work. The red/yellow wire is the center-tap for your high voltage winding and is required with this type of dual rectifier circuit.
TURNING RECTIFIED AC INTO DC
When you send AC current through a diode one half of the sine wave current gets blocked by the diode and hence 'removed' from the total circuit current flow. This process is called rectification.
Although a DC voltage is created with rectified AC, you still need to 'filter' the rectified AC to get it smoother and closer to what a high voltage battery would typically supply. If you try to power your Pultec without filters, the result is a whole bunch of "HUM" in your circuit. For older Pultec EQ's, excess hum could be a good indicator that the electrolytic capacitors have dried up.
6X4 DUAL RECTIFIER
The Pultec EQP-1A uses a 6X4 miniature 7-pin dual rectifier tube connected as a full-wave-center-tap rectifier. 'Dual' means there are two rectifier diodes inside the tube. In order for any full-wave-center-tap rectifier circuit to work, tube or solid state, you must connect the 'center-tap' of the AC winding to your circuit ground. The high voltage winding center tap is the RED/YEL wire on the Hammond 270FX. Note: Your transformer might be different. Always check before connecting!
Here are the pin assignments for the 6X4 tube.
Below is a schematic showing how the high voltage AC is connected to the 6X4 tube. Note: center-tap RED/YEL connected to signal ground.
The rectified AC, from the 6X4 cathode (pin-7), is filtered by 4 simple RC filters made up of (R7-C2), (R6-C1), (R1-C?) and R3-C?). The question marks are because those capacitors are located on the make-up-gain-stage PCB and I don't have it completed at the moment. Also, note the wattage rating of resistors R1, R3, R6 and R7 in this build.
The 6X4 heaters operate on 6.3VAC or DC with a current draw of 600mA. However, for the Pultec EQP-1A you must use AC if you are going by the original design schematic. I use solder pads rather than PCB traces because I like the convenience of printed circuit board assembly, but the connection quality and flexibility of wire.
Green heater wires connect to (J9) and (J10) labeled GRN. Wire is used to connect all tube heaters. The green/yellow transformer wire (heater winding center-tap) connects to a 60VDC node created by a simple voltage divider that feeds from the high voltage DC output. The purpose of the 60V bias is to help reduce the voltage stress from heater to cathode. The 6X4 heater-cathode maximum voltage is 450V.
The power indicator lamp is powered by the 5VAC transformer secondary winding (yellow wires). You connect the yellow wires to (J18) and (J19) labeled YEL and your lamp to (J20) and (J21).
There are plenty of ground connections on the power supply PCB. The RED/YEL high voltage center-tap wire can connect to any of them. However, J22 is centered in the middle on the PCB.
That's about it for now. I'll follow up once I have the real PCB's in my hands.
Thanks for reading,