NA6a (power supply for the Kw.E.a and Lw.E.a Radio Receivers)
 

This power supply was built specifically for the Kw.E.a and the Lw.E.a radio receivers. It converts AC utiluty power from 110 Volts to 240 Volts into two DC voltages: 1.90 V - 2.2 A (fillament) and 90 V - 20 to 25 mA (anode). Other radios that can be powered with this unit (with minor but critical modifications) are the Torn. E. b, Torn. Fu. b1(c,f), and the Torn. Fu. d2. Two vacuum tubes are used in this unit: a Stablitron (AZ11) and a Berator (GR150 a/e).

When I received this NA6a, it did not work. Some basic circuit testing revealed several problems:

  • Broken solders on power wires immediately attached to the power switch and the fuse switching system.
  • Dirty power switch connectors.

I detached and cleaned the power switch connectors and re-soldered the power wires. The power supply came to life. The Stablitron started glowing and the power light ignited. The power output was correct. My next step was to see how much AC was percolating through to the output. The RV2P800 is not very sensitive to anode voltage AC seepage, but the fillament voltage is EXTREMELY sensitive to it. 2 mili-Volts of AC will cause the radio to hum unpleasantly, plus it's not good for the tubes.

Testing with a volt-meter and an occilliscope, we determined that the AC seepage was a bit over 2 mili-Volts. This is probably a result of old capacitors. Three out of the four box capacitors have already been replaced with modern ones. The remaining box capacitor needs to be replaced because it helps shape the fillament voltage. These are the best types to work with since they can be emptied of electorlytic material and new capacitors fit easily inside. We installed the modern capacitors inside the box, and sealed it. We then tested the NA6a fillament output and AC was at 0.1 mili-Volt. Very good!!!

The next step was to make sure that our Kw.E.a received the correct fillament and anode voltage. We connected the NA6a and the Kw.E.a with an original cable (which has been circuit tested). The radio worked, but when I tested the voltage fillament and anode voltage, it was too low. I then adjusted the lower variable resistor (W3) until the fillament voltage was perfect.
 
The NA6a.

 

 
The manufacturers data, BAL stamp. and basic critical operations warning: "max. Fillament current 2 Amp. / max. Anode current 30 m. Amp. Operate only at Environmental temperature from -10 to +35 degrees Celcius". The glass window at the top left is for viewing the power light.

 

 

The interface panel of the NA6a. From left to right, top to bottom: utility power receptacle, Kw.E.a/Lw.E.a selector switch, On/Off switch, input power selector switch and fuse, ground conduit. main output connector, and test equipment connector.

 

 

Basic operating instructions printed on a decal on the inside of the interface panel cover.

"Operating instructions !

1. Connect utility power and set the input current. Device is for AC only.

2. Set the utility power switch to the correct utility power voltage. Unscrew the fuse holder and loosen the middle screw, turn the bakelite disk until the correct voltage is shown, tighten the middle screw, then screw in the fuse holder.

3. Switch the receiver selector switch to the desired receiver setting, then connect the power supply to the receiver.

4. Switch the power on, the power light must light."

 

 

When the two red-circled screws are unscrewed on the front of the NA6a, another panel opens revealing the spare fuse compartment. The glued paper has instructions on installing the thermal fuses:

"Installation of the thermal fuses:

Bend part of the thermal fuse upward. Thermal fuse so far into the case of the transformer in washers, until locking contacts the blocking feather/spring behind the beginning of the soldering strip catches."

 

 
The spare fuse container opened, showing the ORIGINAL spare electrical and thermal fuses. The electrical fuses are all functional.

 

 
The left side of the NA6a, showing the input power receptacle, main transformer, a converted box capacitor,

 

 
The reverse of the NA6a, showing all of the box capacitors, and rear wire connecting board.

 

 
This is the NA6a with the power on. Notice the glowing AZ11 (Stabilitron) tube and the power light at the top left of the photo. The red disks at the top of the unit are the original selenium rectifiers, which are in very good condition and operating state for their age.

 

 
Another photo of the NA6a's right side. The power wires at the bottom left of the photo were re-soldered. The tubes at the top right of the photo are adjustable resistors that allowed me to properly set the anode and fillament voltages. The box capacitor at the bottom right of the photo was opened, evacuated of all electrolytic material and new capacitors were placed in it. The contacts were soldered into place and the box capacitor was sealed. Looks very original!

 

 
The NA6a with original cable supplying power to my Kw.E.a.

 

 
Here the NA6a is doing what it was born for: powering my Kw.E.a.

 

 
A closeup of the above photo showing the voltmeter, indicating power to the receiver.

 

 
Closeup of the NA6a showing the power light on.

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