Type 30 to 1T4 Vacuum Tube Converter

The Accident

I had built a Doerle Shortwave Receiver that employed two vintage Type 30 vacuum tubes. This receiver, and its pristine Emerson Type 30 vacuum tubes, was the crown jewel of my "home-brew" radio collection. I proudly displayed it, with the rest of my home-brew radio collection, in a shelving unit in my basement. One fateful evening I came downstairs and discovered that the shelf above it gave way, smashing the the two Type 30 vacuum tubes in my Doerle Shortwave. This receiver was of open breadboard design and didn't have an enclosure around it that would protect the vacuum tubes from physical damage.

Type 30 vacuum tubes are relatively expensive, typically between $15 and $20 a piece on most websites, catering to "Tube Amp" enthusiasts. You could also try your luck on an auction site like eBay.  I wanted to make my Doerle Shortwave Receiver operational again, but didn't want to drop between $30 to $40 to make it happen!

Pictured below is my Doerle Shortwave Receiver, before the accident.

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The Idea

Like most antique radio enthusiasts, a have a cache of vacuum tubes and other miscellaneous vintage parts. For some reason I had acquired an unusually large number of 1T4 miniature vacuum tubes. Then an idea formed. Create a tube base adapter that would allow me to substitute a 1T4 for a Type 30 vacuum tube. I liked this idea as it allowed me to get my Doerle Shortwave Receiver up and running at very little cost and without modification. This also allows me to convert back to the Type 30 vacuum tubes should I come across an inexpensive set. 

Challenges

Here are the design challenges and specifications I set for the Type 30 to 1T4 Vacuum Tube Converter project:

1. The Type 30 to 1T4 Vacuum Tube Converter is to be self contained, in a Bakelite Tube Base.
2. Both Type 30 and 1T4 vacuum tubes employ direct filament heated Cathodes, for battery operation. The filament voltage of the Type 30 is 2 Volts where it is only 1.4 Volts for the 1T4.
3. The Type 30 is of Triode design, employing three active elements: Anode, Cathode, and Control grid. This is in contrast to the 1T4 Pentode with employs five active elements: Anode, Cathode, Control Grid, Screen Grid, and Suppressor Grid. I need to determine how to connect the Screen and Suppressor Grids for the 1T4 to operate properly in a circuit intended for the Type 30 vacuum tube.
4. The Type 30 has a 4 pin base while 1T4 has a 7 pin miniature base.

Design

1. I used a 10 Ohm 1/8 Watt resistor (R1) to drop the filament Voltage down from 2 Volts to 1.4 Volts needed for the 1T4 tube.  
2. The screen grid is connected internally to Pin 1 of the 1T4 tube, so no external connection was needed. Pin 1 of the 1T4 will be tied to the filament pin that connects to ground.
3. One external connection was required, violating my "self-contained" specification. I required a connection to B+  for Pin 3 of the 1T4, the screen grid connection. A 50K potentiometer (P1) and test point will be included so as to adjust the screen grid for maximum gain. 
4. A .05uF Screen Bypass Capacitor is connected between the screen grid connection and the filament pin that connects to ground.
5. Wiring connects the Plate, Grid, and Filament connections of the 4 pin tube socket base to the proper pins of the 7 pin miniature tube socket for the 1T4 tube.

Please see my Type 30 to 1T4 Vacuum Tube Converter schematic below, I also included calculations for filament resistor.

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Construction

I bent two solder lugs at right angles, then soldered them to a 7 pin miniature tube socket. This will hold the tube socket to the 4 pin tube base. Sorry about the blurry picture.

I drilled then tapped two holes on the opposite sides of the 4 pin tube base. 4-40 screws will hold the 7 pin miniature tube socket to the 4 pin tube base.

Three small holes were drilled into the side of the 4 pin tube base. These holes will be used for the connections to the 50K potentiometer that will adjust the Screen Grid Voltage.

The 50K potentiometer is held to the side of the 4 pin tube base by bending the three leads once mounted. A dab of Superglue under the potentiometer keeps it firmly in place.

I then connect all the wires, the 10 Ohm resistor, and the .05uF capacitor to the 7 pin miniature tube socket. It is much easier to solder with the tube socket out of the 4 pin tube base.

I drilled another hole in the 4 pin tube base then I threaded a test lead with alligator clip through it. This will be used to connect to B+, needed for the screen grid.

The other end of the test lead is connected to an end terminal of the 50K potentiometer.

A black wire is soldered between the other end terminal of the 50K potentiometer and one of the filament pins of the 4 pin tube base. A dab of solder at the end of the hollow tube base pin holds the wire securely in place.

The 4 pin Bakelite tube sockets I had on hand have a built in pin on the side, I will use this as a test point. I soldered a white wire to it. 

The white wire from the test point is soldered to the wiper of the 50K potentiometer. A second white wire is soldered to the potentiometer and will eventually connect to pin 3 of the 7 pin miniature tube socket. ​

Time to wire the 7 pin miniature tube socket to the 4 pin tube base. 

Two 4-40 screws, on opposite sides, hold the 7 pin miniature tube socket to the 4 pin tube base. 

Here is a side view of one assembled Type 30 to 1T4 Vacuum Tube Converter. Two converters are required for my project.

Here is a top view of an assembled Type 30 to 1T4 Vacuum Tube Converter.

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Continuity and Resistance Checks

I always like to take continuity and resistance checks with my Multimeter before putting a piece of newly built equipment into production. I want to make sure that I wired it correctly. I built two Type 30 to 1T4 Vacuum Tube Converters, you can see the second one in the background.

Testing

Once the Type 30 to 1T4 Vacuum Tube Converters passed continuity and resistance checks, it was time to put them into a live circuit. You can see them mounted in the white porcelain tube sockets in the picture below. As you can see, the yellow leads from each Type 30 to 1T4 Vacuum Tube Converters are connected by alligator clips to the B+ (90V and 45V) terminals of my Doerle Shortwave Receiver. 

The next step was to install the 1T4 vacuum tubes into the Type 30 to 1T4 Vacuum Tube Converters.

Time to power up my Doerle Shortwave Receiver, equipped with the Type 30 to 1T4 Vacuum Tube Converters and 1T4 vacuum tubes installed, to see if they work. The wood cabinet sitting below my Doerle Shortwave Receiver is a "Battery Box".  The Battery Box contains Two D batteries, wired in series and tapped, used to provide 1.5 and 3 Volt A+ Filament. In addition, it contains Ten 9 Volt batteries, wired in series and tapped, to provide 22.5, 45, and 90 Volt B+  Anode. The 9 Volt batteries in the Battery Box are getting weak so I had to a connect extra ones externally to achieve the 90 and 45 Volts B+ required for the Doerle Shortwave Receiver.

Success! I could immediately hear atmospheric "hiss", once my Doerle Shortwave Receiver, equipped with Type 30 to 1T4 Vacuum Tube Converters, was powered on and a suitable antenna and ground were connected. I could pickup several AM an CW (Morse Code) transmissions after I fiddled with the Tuner and Regenerative  controls. I use a "home-brew" amplifier to amplify the audio output instead of high impedance headphones. My amplifier is housed in the black project case in the the picture below.

Screen Grid Voltage Adjustment

If you recall, each Type 30 to 1T4 Vacuum Tube Converter had a 50K potentiometer installed so that the Screen Grid Voltage could be adjusted for maximum amplification. In order to adjust the Screen Grid Voltage, I tuned my Doerle Shortwave Receiver to a fairly week AM broadcast transmission then adjusted the potentiometer on each converter for maximum station loudness. Measuring the Screen Grid Voltage to ground with my Multimeter, I found the following to be the optimal Voltages:

Type 30 to 1T4 Vacuum Tube Converter used in the RF (Radio Frequency) Section: 37.5 Volts
​Type 30 to 1T4 Vacuum Tube Converter used in the (Audio Frequency) Section: 45.6 Volts

Conclusion

My fabricated Type 30 to 1T4 Vacuum Tube Converters were a complete success, making my Doerle Shortwave Receiver once again operational. I had all the parts on hand so there was no expense. The best part is that at any time I can remove the Type 30 to 1T4 Vacuum Tube Converters an replace with an actual Type 30 vacuum tubes.