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2/24/18

PULTEC EQP-1A POWER SUPPLY

TUBE POWER SUPPLY

Here's a shot of my prototype Pultec power supply using my 7 pin breadboard adapter. It worked perfectly for the 6X4 dual diode vacuum tube. The rectifier circuit of the Pultec EQP-1A is called a full-wave center-tap configuration.

trenton blizzard pultec power supply

POWER TRANSFORMER

The power transformer is the Hammond 270FX. I chose the 270FX because the 6.3VAC heater winding has a center tap. This center tap is important because the circuit puts about 60VDC on the center tap to help reduce the heater-cathode potential and helps extends tube life.

The 270FX also has a separate 5VAC winding that is typically used for 5V heaters. However, in this application it is only used to power the indicator light. This is a complete waste of a good low voltage tap. I guess you could add USB charging to your Pultec if you desired.

CAPACITOR WORKING VOLTAGE

You need to be careful when prototyping the supply by itself because you run the risk of exceeding the MWV (maximum working voltage) of the electrolytic capacitors.

The brown resistors on the right are my load current to help drop some voltage across the 470 and 1k Ohm series resistors. I'm not pulling the 22mA that would be typical of a Pultec circuit. So, the DC at my first capacitor is almost 400V. That's really close to the 450V maximum rating so I don't operate it for very long.

trenton blizzard pultec power supply voltage

Next in line for this project will be the make-up gain amplifier. However, I'm currently waiting for my output transformer to be built by Cinemag. I'll post after it arrives.

Thanks for reading,
Trenton

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