Vacuum tube radio kits

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Build the Retro Regen Radio

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In the fast moving world of digital electronics, I find it incredible that the vacuum tube — a piece of early 20th century analog technology — has managed to survive. It should have bitten the dust long ago but that just did not happen, thanks in part to electric guitar enthusiasts and the Soviet and Chinese militaries who kept using them. The former were enamored with the sound of tube amplifiers, and the latter wanted their electronic equipment to survive a nuclear attack. Not only did the lowly vacuum tube make it into the 21st century, it is now on a noteworthy rebound. Today, many vacuum tube types are readily available and at reasonable cost. These consist of "new old stock" left over from 50 plus years ago, and many newly manufactured in modern plants world wide. A past NV article describing how to revive an old tube amplifier inspired me to dust off my memories of past tube projects (some from 40 plus years back) and build a one-tube radio. The result was an exciting and fun project that I wanted to share. The radio is made using readily available parts, it operates on 12 volts making it perfectly safe, and it offers amazing performance for a simple one-tube design.

When I was growing up in the 1950s, my dad dabbled in radio/TV repair. His shop was strewn with all kinds of electronic parts. Add to this a small book of radio projects compiled from 1940’s issues of Popular Science Magazine and I was provided with many hours of experimentation and fun. I particularly remember a shortwave design with the intriguing title “Europe on One Tube.”

I am embarrassed to admit how many hours I spent trying to build radios based on these wonderful articles. Most of the designs used a regenerative circuit invented by Edwin Armstrong in 1912. A few years ago, I stumbled on an unusual regenerative design that operated on 12 volts — not the 100 or more of conventional tube designs. The radio I built turned out to be one of the best ever.

In this article, I will describe how to build and operate the broadcast band version. Should you decide to tackle it, I can promise you many hours of fun both in the building and in the listening for distant radio stations.

Vacuum Tube Background

Vacuum tube technology dates back to the time of Thomas Edison and the light bulb. In 1883, Edison noted that he could get electrons to flow between the hot filament of an experimental light bulb and a positively charged metal plate. The so-called Edison Effect only occurred in the near vacuum of the light bulb.

In 1904, the British scientist, John A. Fleming used the Edison Effect to produce the first practical tube, or “thermionic valve.” Fleming’s diode valve passed electrical current in only one direction, making it useful as a radio frequency detector and a rectifier for converting alternating current to direct current.

American inventor, Lee de Forest added a third element to the vacuum tube design and produced the triode, or “Audion” as he named it. He interposed a grid of wire mesh between the filament and metal plate that provided a way to control the flow of electrons.

The significant feature of his invention was that small changes of voltage on the grid would produce much larger changes of voltage on the plate, resulting in voltage amplification. Thus, a weak audio or radio signal could be amplified, which had many practical applications in telephone and radio communication.

As time went on, other advances were made in tube technology, including the addition of an indirectly heated cathode and other grids. For our purposes, the triode vacuum tube will serve as the heart of the regenerative radio receiver.

Regenerative Receiver Theory

Radio detector circuits take a variety of forms. The simplest is the diode detector mentioned earlier in relation to Fleming. When the triode came along, other detectors were invented including a design called a plate detector. When a radio signal was applied to the control grid of a triode, detected audio could be taken from the plate circuit. The regenerative receiver takes the plate detector one step further and adds a small amount of positive feedback, resulting in “regeneration” that substantially increases circuit gain and selectivity (ability to separate nearby radio stations).

The result is a very simple circuit consisting of only one tube and a handful of components that produce amazing results. Add a couple of stages of audio amplification and you have a radio design that provides hours of fun and listening pleasure!

Circuit Description

The basic circuit consists of a dual triode 12AU7. While this and other similar tubes are meant to operate at plate voltages of 90 volts or more, the 12AU7 performs amazingly well in the current application at only 12 volts. Dangerous voltages normally associated with tube projects are eliminated.

One disadvantage of low plate voltage operation is that it is not possible to develop sufficient audio power to drive a speaker or dynamic earphones. An LM386 IC power amplifier serves this purpose, making the overall design a hybrid mix of vacuum tube and semiconductor technology. The tube circuit consists of two sections: the regenerative detector and a low level audio amplifier. Refer to the schematic in Figure 1.

FIGURE 1. Schematic for the retro regen radio.

The radio frequency (RF) signal from the antenna (binding post J4) is applied to winding L1 of the spider web-wound coil. Winding L1 inductively couples the RF signal to a second winding L2 that — along with variable capacitor C1 — forms a resonant circuit covering the AM broadcast band (550 to 1600 kHz).

Capacitor C2 couples the tuned RF signal to the control grid of the triode V1-A. Resistor R1 provides a DC path to ground and “leaks” electron charge that would otherwise build up on the control grid and prevent the tube from working. A tap on winding L2 provides a small amount of positive feedback that, in turn, creates the regeneration needed to increase the gain and selectivity of the detector circuit.

Circuit gain and regeneration is controlled by varying the plate voltage of V1-A with potentiometer R3 and plate resistor R3. Capacitor C5 bypasses any remaining RF signal on V1-A’s plate to ground, while C3 couples the detected audio frequency (AF) signal to the control grid of V1-B. Resistor R5 provides a grid leak path as described previously, and establishes a small reverse operating bias on the control grid. V1-B acts as a small signal audio amplifier with a gain of five. The amplified signal on the plate is coupled to the volume control R6 by capacitor C4.

From the volume control, the AF signal passes to the audio amplifier module LN-1 that boosts it to speaker volume. Earphone jack J2 is wired so that speaker SPK1 is bypassed if an earphone is plugged in. Power is provided by either a 12 volt battery (binding post J3) or an AC-to-DC power supply (jack J1). Diode D1 prevents current from flowing back into the battery should an AC-to-DC supply be plugged in at the same time as a battery. Resistor R8 and capacitor C8 provide AC hum filtering needed for the AC-to-DC power supply. Resistor R7 and capacitor C6 provide additional AC hum filtering for the more sensitive V1 circuits.

Construction and Testing

Construction is divided into three stages, namely: constructing the chassis on which the circuit will be built; wiring the electronic circuit; and finally making the spiderweb coil. Some of the construction techniques employed may be new to readers. For instance, the chassis requires basic wood working skills, and the circuit is hand-wired rather than using a printed circuit board. Don’t worry; I will lead you through each and every step.

Constructing the Chassis

First, we will construct the chassis. Traditionally, radio chassis are constructed from aluminum or steel. Metal working has its own set of challenges and requires specialized tools, like expensive chassis punches. I chose instead to use tempered hardwood as a base for mounting components. Interesting, some of the earliest radios were built this way. (Refer to the image at

Start with a 2’ x 4’ piece of 1/8” tempered hardwood. Cut a 7-1/4” strip across the 2’ width, then cut this piece at 7-3/4”, again at 4”, and finally at 3” (see Figure 2). These pieces are, respectively, the base, front, and rear assemblies of the chassis.

FIGURE 2. Hardboard sawing pattern.

To locate holes simply to be drilled, trim quad-ruled graph paper (four squares per inch) to fit each chassis assembly. Spray the reverse side of the graph paper lightly with spray adhesive and place the graph paper so that it lines up exactly with edges of the finished side of the hardboard. Press the graph paper down from the center out to remove an air bubbles and obtain a smooth result.

Use the layout drawings (Figures 3, 4, and 5; files are available in the downloads. Graphics shown here are for reference only.) to mark the drill locations and drill sizes on the graph paper. Note that holes for the audio module LN1, V1, C2, and binding posts J3 and J4 are located using the actual component to ensure correct hole location. Use an awl or ice pick to precisely locate where each hole will be drilled. If you plan to use an electric hand drill, pre-drill 1/16” pilot holes, then use the drill size specified. Clean up the holes by rubbing gently with medium sandpaper.

FIGURE 3. Base chassis drilling template.

FIGURE 4. Front panel drilling template.

FIGURE 5. Rear panel drilling template.

The circular hole for the speaker will require a hole saw. Cut a 2-1/2” diameter hole first, then widen as necessary by sanding the circle edges so that the speaker rim gasket fits snugly into it. With the speaker in place, mark and drill its mounting holes.

Variable capacitor C2 also requires special handling. Place the shaft rim in the 1/2” drilled hole and note the two threaded holes on the front of the capacitor. From inside the capacitor frame, use a sharp pencil to mark the holes. Drill the holes and check that they line up correctly. Jacks J1 and J2 have thread lengths too short to reach through the 1/8” thick hardboard.

A simple solution is to use a flat tip 1/2” drill bit to carefully remove enough material from the back of the rear assembly to allow the threads to protrude sufficiently out the front side.

After all the holes are drilled, peel off the graph paper and clean the surfaces. This is a good time to apply a dark walnut stain if you desire.

Cut two 7-1/4” lengths of square dowel, then use 12 brass wood screws to attach the front and rear assemblies to the base. Use 1/8” pilot holes into the dowel to avoid splitting it (see Figure 6).

FIGURE 6. Side view of assembled chassis.

Mount all the components to the assembled chassis except for audio module LN-1 and the V1 socket. When satisfied with the fully assembled and populated chassis, remove the front and rear assemblies from the base.

Leave the base attached to the square dowels. Chassis construction is now complete.

Wiring the Circuit

The next stage of construction involves wiring the circuit. Mount the socket for V1 upside down on the base, using 3/4” standoffs and 6-32 x 1” machine screws. This will provide a convenient platform on which to pre-wire components associated with V1. If you are not experienced with soldering, search YouTube for “how to solder.” You can also refer to the series, “Basics of Soldering,” which started in the December 2014 issue of SERVO Magazine (

Use Wiring Diagram 1 to wire components connected to V1’s socket. Wire the components in this order: connect the wire from pin 5 to pin 8 first; then connect R5, C3, and finally C5, layering one above the other. The remaining components and wires can be added in any order.

WIRING DIAGRAM 1. V1 socket wiring.

Follow the recommended lead lengths, allowing 1/4” additional to wrap around the connecting terminal for mechanical stability. For instance, if the lead length specified is 3/8”, cut the lead initially to 5/8” (3/8” + 1/4”). Use spaghetti wire insulation on all bare leads longer than 1/4”. Solder all connections.

When done, inspect all soldering joints, then remount V1’s socket right side up.

Before moving to the next stage of wiring, mount the front and rear assemblies to the base. Use Wiring Diagram 2 to connect the wires and components previously prepared on the V1 socket.

WIRING DIAGRAM 2. V1 and front panel wiring.

Now, add C7, R9, and the additional wires including those to terminal strip TS2 and J4. Trim wires and leads to their minimum length and use spaghetti wire insulation on all bare leads longer than 1/4”. When wiring is complete, solder the connections shown filled in with black; leave the gray connections for soldering later.

Use Wiring Diagram 3 to wire the power supply components. Trim wires and leads to their minimum length and use spaghetti wire insulation on all bare leads longer than 1/4”.

WIRING DIAGRAM 3. Power supply wiring.

When wiring is complete, solder the connections shown filled in with black; again, leave the gray connections for soldering later.

The next step is to build LN-1: the audio amplifier module. Instructions are included with the kit. A few changes must be made. Do not install the microphone or the 3.3K ohm resistor (LN-1 R1).

Also, replace the 1K ohm resistor (LN-1 R2) with a 100K ohm, and the 10K ohm resistor (LN-1 R3) with a 680K ohm. The change is necessary to decrease LN-1’s loading effect on V1-B’s output.

Add external wires to LN1 as indicated on Wiring Diagram 4, adhering to the color codes. Make all wire lengths 6” initially. Note that the input wires connected to the “MIC” point are tightly twisted for the first 3-1/2”. Adjust R6 (the LN-1 gain control) to maximum, fully counter-clockwise.

WIRING DIAGRAM 4. LN-1 audio module and speaker wiring.

After LN-1 is built, mount it to the base with 1/4” standoffs and 4-40 x 1” machine screws. Use Wiring Diagram 4 to wire audio module LN-1 to V1, the power supply, and the speaker/earphone circuit.

When wiring is complete, solder all connections.

Constructing the Spiderweb Coil

The final stage of construction involves making and installing the spiderweb coil. The fiberboard used as backing for picture framing is an excellent choice from which to make the coil. You may have to search around to find just the right material. The perfect selection will be slightly less than 1/8” thick and similar to (but not as hard as) the tempered hardboard.

Copy the pattern of Figure 7 and glue it to the fiberboard. Use a 3-1/2” circular saw to cut out the coil shape. The finished coil will be nearer the 3-1/4” diameter of the pattern. Use medium grade sandpaper to clean up the edges.

FIGURE 7. Template of spiderweb broadcast band coil.

While holding the coil in a vice, use a hack saw to cut the seven slits. After each slit is sawed, use medium sandpaper to clean it out and round the edges of the cut. This is most easily done by folding the sand paper in half and passing it back and forth within the slit.

Carefully check that the depth of each slit is the same. The circular saw will have made a hole in the center of the coil. Install a 1/4” x 2” long screw in the hole with the head against the smooth side. Secure it with a nut. You will wrap the coil leads around the extended threads of the screw to keep them out of the way while winding the coil.

To make winding L2, wind 5” of #28 enameled wire around the screw, then pass the remaining length through a slit. This will be lead 1. Pull it tight on the other side and pass it down through the next slit. Repeat this until you have made about 65 complete turns. Note that counting the wire turns on either side represents roughly half the total number of turns.

End winding with the wire passing back through the slit where you started. Fold the next 10” of wire in half, wrap it around the screw with lead 1, and pass the remaining wire through the same slit. The folded wire will be the coil tap lead 2. Continue winding in the same direction for about 13 turns, finishing at the same slit as before. Cut the wire to 5” and wrap this lead around the screw. This will be lead 3. Winding L2 is complete.

Leaving 5” leads, follow the same procedure for L1, starting and ending where you finished the L2 winding. Wind about five turns in the same direction. The starting lead is number 4 and the ending lead is number 5. When all windings are complete, dab a little quick set epoxy at the outer edge of the slit to keep lead 5 from unraveling.

Unwind the leads from the screw and gently pull them aside. Drill out the center hole to 3/8”. Cut a 3-1/2” length of 3/8” round dowel. Insert one end in an electric drill and sand the rotating dowel evenly until the nylon collars fit snugly over it. Cut 2-1/2” of the sanded portion and set it aside; refer to Figure 8.

FIGURE 8. Side view of spiderweb coil assembly.

Glue a collar to the underside of the chassis base to hold the dowel firmly in place. Use Wiring Diagram 5 to connect the coil leads to terminal strip TS-1 and binding post J4. Solder all connections.

WIRING DIAGRAM 5. Spiderweb coil wiring.

Insert the dowel through the chassis base into the glued collar. Slip one collar on the dowel, then the coil (lead side down), and a second collar to hold the coil in place as shown in Figure 8.

Use spray-on glue to attach the tuning scale and labels of Figure 9 to the front assembly. Lastly, install the knobs to the front of the radio. This completes construction.

FIGURE 9. Tuning and other front panel labels.

Preliminary Testing

Before applying power the first time, it is a good idea to check for a major short circuit. Before inserting tube V1, turn the radio on with the volume control. Rotate the gain control fully clockwise. Use an ohmmeter to measure resistance between pins 1 and 2 of J1. After a few seconds, it should read 1,000 ohms or higher. A low reading (less than 100 ohms) suggests incorrect wiring and should be investigated.

Next, insert V1 into its socket and repeat the measurement. Now, expect a low resistance of 27-30 ohms. When satisfied with these checks, you are ready for the “smoke test.” Turn the radio on and set the volume control, gain, and tuning dials to mid-position. Apply either a 12 volt battery or AC-to-DC source, and note whether the heaters in V1 glow a dull red.

After warm-up (about 30 seconds or so), you should hear static in the speaker. Rotate the gain clockwise until you hear a squealing sound, indicating that the regenerative detector has passed into full oscillation. Normally, you will operate with the gain set below this point. On especially weak signals or when full selectivity is needed, set the gain just below the point of oscillation.


In urban areas with strong AM stations, the spiderweb loop will be all the antenna needed. Best reception of distant stations will be at night using an outside antenna 25’ to 50’ in length in conjunction with an earth ground. Here again, the Internet will provide lots of advice on installing long wire antennas and earth grounds. Here’s a tip when finding really weak stations.

First, use earphones rather than the speaker to eliminate distracting sounds around you. Next, rotate the gain control until the radio just breaks into oscillation. As you rotate the tuning knob, you will hear whistles that are heterodynes or beat frequencies of the radio’s oscillation and radio stations. Rotate the tuning knob very slowly, and note that the beat frequency starts at a high pitch and decreases as you rotate the tuning knob. When the pitch is very low or disappears completely, you are tuned directly on the station’s frequency. If you tune too far, the pitch will begin rising.

When “on frequency,” reduce the gain until the detector just falls out of oscillation, and you should hear the station. It will likely be weak and fade in and out, so you will have to listen carefully to hear the station identification and get the call sign and location.

Circuit Modifications and Enhancements

My initial choice of frequency coverage was the AM broadcast band, but the radio can tune the shortwave bands by simply changing the spiderweb coil design. Figures 10A and 10B show a coil designed to cover the 4-14 MHz shortwave bands, including international broadcasts and amateur radio. It requires heavier gauge wire (#16) and larger slits in the spiderweb coil. Because of the very wide tuning range, you may have difficulty tuning signals precisely.

FIGURE 10. Template of spiderweb shortwave coil.

Shortwave receivers often have a second tuning capacitor of a much smaller value in parallel with the existing one. This “band spread” capacitor provides easier and more precise tuning once the general frequency is tuned with the main tuning capacitor.

Rather than adding another variable capacitor, a varactor diode could be used with the band spread tuning accomplished by a potentiometer controlling the reverse voltage of the diode. I have not tried this yet, but see no reason why it wouldn’t work.

I hope you will enjoy building and playing the retro regen radio as much as I have. Though I have not accomplished it yet myself, maybe we will succeed finally in accomplishing the goal of “Europe on one tube!”  NV

Parts List

C1220 pF @ 500 VDC Silver Mica Capacitor 5% Radial LeadAntique Elec4957-220
C230-365 pFVariable CapacitorAntique Elec5317
C3.022 mF @ 50 VDCDisc Ceramic Capacitor 20%Jameco15245
C410 mF @ 50 VDCAxial Capacitor 20%, 85CJameco10882
C5300 pF @ 500 VDCSilver Mica Capacitor 5% Radial LeadAntique Elec4957-300
C650 mF @ 50 VDCAxial Capacitor 20%, 85C 11105
C7.022 mF @ 50 VDCDisc Ceramic Capacitor 20%Jameco15245
C810,000 mF @ 16 VDCAxial Capacitor 20%, 85CJameco93681
D11N4001Diode RectifierRadioShack2761101
J1Power ConnectorConnector, Power, PC712AJameco297553
J2Phone Jack Stereo 2.5 MM, Tip SwitchRadioShack2740246
J3DC (Battery) SupplyBinding Post - Dual Banana Red and BlackJameco125197
J4Antenna TerminalBinding Post - Dual Banana Red and BlackJameco125197
L1Coil - PrimarySee text.  
L2Coil - SecondarySee text.  
LN1AmplifierKit, SUPER SNOOPER - BIG EARJameco151204
R12M ohm1/2 watt Carbon Film ResistorAntique Elec3647-2M
R250K ohm PotentiometerLinear 0.5 watt with Switch (S1)Jameco255549
R3100K ohm1/2 watt Carbon Film ResistorAntique Elec3647-100K
R422K ohm1/2 watt Carbon Film ResistorAntique Elec 3647-22K
R51M ohm1/2 watt Carbon Film ResistorAntique Elec3647-1M
R6100K ohmLinear 0.5 watt with Switch (S1)Jameco263822
R71.2K ohm1/2 watt Carbon Film ResistorAntique Elec3647-2M
R815 ohm1 watt Carbon Film ResistorAntique Elec3649-15
R91K Ohm
100K Ohm
680K Ohm
1/2 watt Carbon Film Resistor
1/2 watt Carbon Film Resistor
1/2 watt Carbon Film Resistor
Antique Elec3647-1K
S1Part of R6SPSTSee R6See R6
SPK1SpeakerSpeaker, Square, Ferrite Magnet, 2.6", 4 ohmJameco99996
V1Vacuum TubeDual Triode 12AU7Antique ElecT-12AU7-JJ
PWR 1AC-DC SupplyUnregulated,12 VDC/750 mAJameco2155006
1Tube Socket for V1 Antique Elec3398
23/4" Nylon Spacer #6 Hole Digi-Key492-1111-ND
2#6-32 1-1/4" Machine Screws   
2#6-32 Machine Nuts   
2#6-32 Machine Lock Washers   
26 Lug Terminal Strip2nd Lug GroundAntique Elec5354
2#6-32 3/8" Machine Screws   
2#6-32 Machine Nuts   
2#6-32 Machine Lock Washers   
41/4" Nylon Spacers #4 hole Digi-Key492-1074-ND
4#4-40 3/4" Machine Screws   
4#4-40 Machine Nuts   
4#4-40 Machine Lock Washers   
12#8 5/8" Brass Wood ScrewsFlat Head  
4#8-32 1/2" Brass Machine ScrewsOval Head  
4#8-32 Brass Machine Nuts   
2#6 1/4" Flat Head Machine ScrewsTrim thread length to avoid contact with moving stator of C2  
1Solder LugAttach to bottom of C2Antique Elec4105
1#6 1/4" Flat Head Machine ScrewsTrim thread length to avoid contact with fixed stator of C2  
11/8" 2'x4' Tempered Hardboard Home Depot7005015
3Nylon Spacer I.D. 3/8" x 3/8" x 1" Home Depot815118
200'#28 Enameled Magnet Wire Antique Elec5824
100'#22 Hook-up WireBlackJameco36792
100'#22 Hook-up WireGreenJameco36822
100'#22 Hook-up WireRedJameco36856
4'Spaghetti Wire Tubing1/16" Black Heat Shrink TubingJameco419127
13/8" Round Dowel Home Depot38-4EDC
13/4" x 3/4" Square Dowel Home Depot34-3HWSQED
11-1/2" Control KnobCommunication TypeRadioShack274-0402
21" Control Knob
Wood Stain to Personal Taste
1/8" Picture Frame Backing
Communication Type







What’s in the zip?
Faceplate Diagrams
Drilling Templates
Sample Sound Wav


Radio Kit - 2 Tube Regenerative Radio

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This radio kit will cover the AM radio band along with the SW radio band. Power for this receiver is 1.5 V DC for the filaments and 45V DC for the plate current. A D cell and five 9V transistor batteries wired in series will work to power this radio. A 2000 ohm headset or an audio amplifier is required. Requires two T-1T4_DF91 tubes (not included).

RoHS Compliant

Filament Voltage (Uf)1.5 VDC
Headset / Amplifier Resistance2 kΩ
Plate Voltage (Ua)45 VDC
Packaging Dimensions8.6 in. × 7.8 in. × 3 in.
Weight (Packaging)0.91 lbs.

Questions and Answers

Click each question to see its answers.

1 answers If I wish to run this with an Audio Amp and a 8 ohm speaker do you have an amplifier that would work for such a situation?

Asked by Anonymous on August 5th, 2016.

Unfortunately, we would not know what audio amp you would need to power an 8ohm speaker. Outside of selling the kit, we do not offer anything in the way of technical support.

Answer this question

1 answers Hi Could this receiver be easily modified to cover the 75m AM HAM band and be a companion receiver for the Pine Board Project kits also offered here?

Asked by Anonymous on February 8th, 2018.

Matt H

February 8th, 2018

Unfortunately we do not know if this would work. You might want to contact Bob at for more information.

Answer this question

1 answers It was recommended to use K-934 AM Wave-Trap and K-935 Antenna Tuner with this project....but I can't find any such product here! Also, the instructions that came with my kit are printed in high-contrast black and white and I cannot read the coil illustrations at all. Can I get a new set of instructions mailed to me or send a pdf via email? I can verify I bought this product already.

Asked by Anonymous on March 29th, 2018.

K-934 and K-935 are items we do not carry. If you are having trouble reading your instructions please email customer service at: [email protected]

Answer this question

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Introduction: Make a One Tube Radio

*** This is my entry in the Vintage Contest. When I read of the contest sponsored by Crosley, the first thing I thought of was tube radios. I've wanted to try building a tube radio for a while, so here's an instructable on how I did it. If you like it, please don't forget to vote for me in the Vintage Contest. ***

Do you ever wonder what it was like in the early days of radio? Have you ever wanted to build your own radio to listen to those invisible waves that propagate through our skies? Ever wonder how electronics filled with little glass lightbulb things managed to work decades before the invention of the transistor? Do you miss the days when phones were stupid, but people were smart?

In this Instructable I'll attempt to guide you through the steps of building a one tube radio for picking up AM broadcasts. This will include parts sourcing, prototyping and finishing steps.

Step 1: Why Build a Radio When I Can Buy One?

Unfortunately, it can be much cheaper and easier to buy a radio these days than build one. Good portable radios can be had for under the cost of parts used in this instructable and will almost certainly have superior performance, as well as be capable of receiving the FM band as well. This instructable is not about making a radio better than one you can buy, nor is it about making a radio for less than you can purchase one for.

I built this radio because I think that it's kind of amazing how a little evacuated glass tube with some bits of metal inside can turn radio waves into sound, and also because we don't seem to do much practical stuff in school, mostly just calculus... Through the course of building it I learned a little about how radios work, and got to try out a new prototyping method as well. Overall, I'm not disappointed about taking the time and spending the money to build this little receiver.

Step 2: The Circuit

Unfortunately I don't have any experience with CAD software for drawing schematics, so I've instead provided a scan from my notebook drawings of the circuit as I constructed it.

The radio is a regenerative receiver using a combination triode-pentode tube (which is like having two tubes in one, so I guess this is actually a two tube radio?). The triode section is used in an Armstrong regenerative detector and the pentode section is used as an audio amplifier. There is an output transformer (more on this later) so normal headphones or an external amplifier can be connected. If you have high impedance headphones, you can use them without the transformer, but I found my pair to be very uncomfortable.

Power is supplied from two batteries, an "A" supply to power the filaments (~ 6 volts) and a "B" supply to provide the plate current. I'll go into more detail on these parts in the following steps.

The original circuit is from

which I built with parts I had on hand. I made a "spider web" coil form rather than the PVC tube used in the radiomuseum article. This instructable includes information on creating coil forms and winding them.

Step 3: A Little Bit of Theory

When I started building this radio I knew nearly nothing about radio, but through the course of this project I learned quite a bit about it and about tubes in general. I'd like to give a good explanation of the principles of operation and theory, but I feel that I still am not quite qualified to go too in depth into the subject. I will, however, provide a little bit of theory as I understand it. If I am wrong about any of this, please let me know so that I can correct it.

Nearly all that I have learned regarding this topic has been from Mr. Max Robinson's site, Fun With Tubes, and the radio construction pages of Mr. John Fuhring. Both of these websites contain a great wealth of information and I would highly recommend them to anyone interested in radio and vacuum tube technology.

First a little about tubes. In a diode tube, there are two main components; an anode and a cathode (also a heater, if the tube cathode is indirectly heated). When the cathode is hot, the electrons have enough energy to travel through the vacuum from cathode to anode (recall that conventional current flows from positive to negative, but electrons flow from negative to positive), similar to a semiconductor diode. When the cathode is cold, the electrons have less energy and current flow is little to none. In a triode tube, an additional component is added; a screen of wire mesh known as the "control grid," or simply, the "grid." Similar to the gate of a semiconductor FET, a small change in voltage on the grid can cause a larger change in current across the cathode and anode (the British often called tubes thermionic valves, as it's easy to imagine the tube as being a valve, with the grid being a handle which controls the flow of liquid [current] through the valve [tube]). This allows for signals to be amplified - a breakthrough at the time of the triode tube's invention. The tube used in this instructable contains both a triode and a pentode section. A pentode has three grids, an anode, and a cathode, adding up to five components. The addition of two extra grids give the pentode some slightly different properties, which are a bit beyond the scope of this instructable.

In the attached schematic, I have drawn a regenerative receiver, based upon the original designs of Mr. Edwin Armstrong, who created this design prior to the First World War. If you're familiar with crystal radios you'll
recall that the signal picked up by the main coil (adjustable through use of the tuning capacitor) is fed through a diode detector and then fed into a pair of headphones (or crystal earpiece). In this case, the signal is instead applied to the grid of the tube, through a resistor and capacitor (sometimes called the grid leak resistor). The small signal appearing on the grid causes larger fluctuations in plate current, which can be heard as sound through the headphones. However, what makes Armstrong`s circuit special is the addition of a second tickler coil winding, which allows some of the amplified signal to be fed back into the main detector coil.This signal is again amplified by the tube and the observed signal is even louder. However, the regeneration must be controlled, or else the set will begin to oscillate, creating an awful high pitched screaming sound, and also possibly radiating radio frequency interference which will disturb your neighbors if they listen to AM radio anymore. In the drawn circuit, the regeneration is controlled mechanically, by moving the tickler coil with respect to the main coil. I had hoped to build a radio using this method, but it proved difficult to prevent the tickler from shifting during use, so I decided on a more typical potentiometer regeneration control instead. In the circuit on the previous page, the triode section of the tube is used as a regenerative detector, but the signal is fed into the pentode section, which acts as an amplifier.

Step 4: Parts List

Here's a list of the parts used to build this radio. While most components are common, some are hard to find these days (the tube, variable capacitor and high impedance headphones/output transformer), so I'll include more information about those in later steps.


1 x 6JC8 triode-pentode tube (details in step 5)

1 x Noval tube socket (or whichever type matches the tube you use)

1 x Tuning capacitor ( ~ 360 pf details in step 6)

1 x Output transformer or high impedance headphones (details in step 7)

1 x 3.5mm audio jack (if using output transformer)

1 x 100 Kilo Ohm potentiometer


2 x 2.2 Mega Ohm

1 x 120 Kilo Ohm

1 x 10 Kilo Ohm


2 x 100 picofarad (marked "101")

1 x 0.01 microfarad ("103")

1 x 0.1 microfarad ("104")

Thin wire for winding coils (I used the insides of a cat 3 cable)

~ 50 ft antenna wire (shorter works, but longer is better)

Alligator clip leads

1 x 9V battery connector (mine was salvaged from a dead battery)


Wood for the "breadboard" base (I used a scrap piece of alder)

Spray lacquer or other wood finish (optional)

Brass escutcheon pins (copper nails would also work if you can find them)

Piece of sheet metal for the faceplate (I used a piece of aluminum)

Cardstock or similar material for making dial

Sewing needle and disposable ballpoint pen for dial

Piece of dial cord (I used a strand from the inside of a piece of paracord)

Wood screws

2 x knobs

Scrap sheet metal for mounting components

Step 5: Finding Parts: a Tube

With transistors being so prevalent these days, it can be hard to find vacuum tubes anymore. In this radio I used the 6JC8 tube, because I could purchase it locally. The original circuit which uses a PCF801 is based off another circuit which used a 3A5 tube. I purchased my tube from PacificTV for $3

Antique Electronic Supply is a good source for tube radio parts, and a quick search for "triode - pentode" came up with 3 pages of tubes to choose from. They don't have the PCF801 or 6JC8 in stock, but I think that the 6U8 should work if the two cathodes are connected together.

When selecting a tube it's important to keep in mind the required filament/heater voltage and current, as you'll have to make up a battery to provide the required voltage. Most American tubes start with a number indicating the voltage (6.3 volts for the 6JC8 and 6U8). 6V and 12V at around 0.3 - 0.6 amperes are common, but other voltages are also available. Notably there are tubes such as the 1V6 which are designed to run on batteries and require much less filament current and a lower voltage (the 1V6 uses only 40 mA at 1.25V compared to the 450mA at 6.3V of the 6JC8).

You'll also need a tube socket to match your tube, which can be harder to find than the tubes themselves sometimes. I purchased mine from PacificTV, but Antique Electronic Supply also stocks them.

Step 6: Finding Parts: a Tuning Capacitor (or How to Dissect Clock Radio)

Another uncommon part is the variable capacitor (older sources sometimes refer to this part as the "tuning condenser"). Crystal radio builders may be familiar with the aluminum "air variable," type shown in the above image, but these are getting to be rare and expensive these days, so in this Instructable I use the more common miniature plastic "polyvaricon," (Polyester Varible Condenser) which are still manufactured and can be purchased from several online component suppliers, including Scott's Electronic Parts among others.

However, the most readily available, and often cheapest, source for the enterprising experimenter is the ubiquitous alarm-clock radio. I know, I know; it may seem crazy to tear apart a good radio for parts to build another (most likely inferior in performance) radio, but I couldn't find a cheaper solution anywhere else. Broken alarm clocks can be had for free, and I purchased the one in the images from a charity thrift store on half-off electronics day. Where else can you get the capacitor, mounting hardware and a few parts for other projects for only $1.12?

Step 7: Finding Parts: Output

If you have a radio you'll have to have a way to listen to it, of course. You can use a pair of high impedance headphones, like the pair shown above, but these are not produced anymore, and can be expensive. I purchased the pair in the picture for $12, which almost doubled the cost of this project for me.

The alternative is the use an output transformer which matches the high impedance output to the lower impedance of modern headphones. However, an audio transformer could cost as much or more than the high impedance headphones in some cases! But thanks to Charles Wenzel's incredible pages on building transistor radios, I discovered that common household step-down transformers (commonly found in "wall wart" plugs) can be used for the output. Simply attach the primary (120V) winding to the tube output and the secondary winding to the headphones. You'll have to experiment with a few to see which one works best. In fact, if you've taken apart a clock radio for the capacitor, you can also probably use its transformer as well; now that's an economical source of parts, eh?

Step 8: Breadboarding: Building the Base

Wow, over a half dozen steps before we even get into the build? Sorry for boring you up 'till now, let's get into the actual construction.

The circuit is built upon a wooden base with components soldered to little brass nails. This is the classic "breadboarding" technique used in the early days of radio (before those white plastic breadboards, or course; experimenters used to swipe actual bread-cutting boards from the kitchen to build circuits on).

I had a scrap of alder which was too long and too narrow, so I cut a couple pieces, planed and glued them together before cutting to the final dimensions. I gave the board a couple coats of spray lacquer before moving on to the next step. My final dimensions were approximately 15cm x 10cm (about 6" x 4"), which is actually a bit small; you'll see in later steps that the components get a little crowded near the end. I'd suggest a slightly bigger board to allow for easier component placement.

Step 9: Winding the Coils

The coils used in this radio are wound on "spider web" coils made from the hard board material found inside of binder covers. This was surprisingly difficult to work, and I found that it was easiest to hack out the rough shapes with a coping saw, then refine the circles with some stout scissors. The large disc is 6.5cm (about 2.5") in radius and the small disc has a radius of 4.5 cm (about 1 3/4").

Once cut, the coils are very easy to wind, providing that you remember that you must make coil forms with odd numbers of slots - my coil forms have 11 slots each.

To wind the coils, simply start at one slot and go around the circle, tucking the wire over one section, under the next, over again, and so on. I wound 45 turns on the large disc for the main coil, followed by another 6 turns (of a different piece of wire) for the tickler coil. The small disc has 15 turns and is used for antenna coupling (it is the small coil connected between the antenna and ground).

The coils are mounted on a scrap piece of wood which I stained with "mahogany water varnish" to contrast the pale alder breadboard.

Step 10: A Power Supply

What's an electronics project without power? This radio uses two battery supplies, the "A" supply for the tube heater/filament and a "B" supply to provide plate voltage. The "A" supply needs to match your tube heater voltage (in this case 6.3V) while the "B" supply is much higher, at 45-90V (or higher, in AC powered sets).

The "A" supply can be just about any ~6V source, such as 4 D-cell batteries in series, just remember that the tube will draw about 400mA (unless you are using a low current type) so small batteries (like AA) will be drained very quickly in this application. I use a couple of Li-ion 18650 cells with a large ceramic resistor in series (2.2 Ohm, 7 watt). A rechargeable lead-acid "gel cell" would also be suitable, and a 6V wall adapter should work as well, though I have not tried this.

The "B" supply is not much more difficult to construct. You'll need a half dozen of the cheapest 9V batteries you can find (the low cost "heavy duty" type will work here as current draw is very low, expensive alkaline types are not required). I bought three packs of "Sunbeam" batteries from the local dollar store for $1.25 each. Soldered in series they give ~ 54V. A connector can be made from the terminals of a dead 9V battery.

Step 11: Breadboarding: Mounting the Tube

Now that you've built the breadboard, I suggest that you mount the tube first, before any other components. First bend out the solder lugs of your socket (if possible), then mark their positions on the board. Next drive brass pins into your marks. The tube socket can then be soldered into place. This connection method is quite secure and allows you to change the tube if required.

I prefer to add another row of pins in a circle around the tube socket lugs, which can be soldered to instead of soldering directly to the lugs themselves. This is very useful as it allows you to experiment with different types of tubes. You'll see in the next step why this was especially important for this build...

Step 12: Breadboarding: Building the Circuit

Once the tube is mounted, mark the tube pin configuration onto the breadboard keep in mind that the datasheets list the pinout as a bottom-view of the tube. I neglected to remember this, and as a result wired the tube pins backwards. Luckily I had a second row of pins around the tube socket lugs, so I just cut the bare wires and rewired the tube socket, saving a bunch of time.

Unfortunately I didn't get many pictures of the breadboard wiring, but it's fairly easy; just add more pins as needed and connect the components, taking care to reference the schematic as you go.

Step 13: Breadboarding: Testing

Once you have the circuit wired you can begin testing it. Connect your high impedance headphones or transformer and pair of ordinary headphones. Attach as long of an antenna wire as possible (I use 50 feet of wire thumb-tacked to my ceiling) and connect the ground to some grounded metal object (if possible, this is not absolutely required). Connect the batteries and wait a few seconds while the tubes heat up. You should be able to hear a hissing sound when the regeneration potentiometer is turned up, which may turn to an unpleasant squeal when turned too high.

With the regeneration turned to a point where the hissing is loudest, but not quite a squeal, you should be able to hear the distinct "radio tuning sound" (the stereotypical radio sound used in movies) - a series of whistles and varying tones as you turn the dial. If you "tune in" on one of these whistles and turn the regeneration down slightly, the whistling should turn into the sound of a radio station.

I was able to hear the local AM station clearly, and proceeded on to building the faceplate. If your radio is not working at this point, then please see the troubleshooting page (step 19)

Step 14: Building the Faceplate

Once you have the breadboard circuit built to your satisfaction, it's time to build a faceplate to hold the knobs and give a more finished appearance. My construction techniques again borrow heavily from the webpages of Charles Wenzel, who uses similar construction for a some of his transistor radios.

I cut a piece of aluminum sheet to the width of my breadboard with a pair of tin snips (final dimensions were roughly 15cm x 10cm, or 6" x 4"). Next I used some sanding sponges to sand down the surface, then followed the sponges with steel wool. Finally the surface got a good rubbing with some automotive aluminum polish for a shiny finish. I wasn't able to get a perfect mirror finish using this method, but the images show that the surface is quite shiny. If I were to do this again I'd purchase some fine grit automotive sandpaper or extra fine steel wool and use those before the final polish. Alternatively, paint would probably have been an easier solution.

If you do choose to polish the faceplate, I'd suggest covering the whole thing with painter's tape to prevent scratches in the subsequent steps.

Step 15: Mounting Components

Now this is where you have to get a little creative. Of course the way you mount components will differ depending on what components you have on hand. Here's how I secured my components. The images have a couple of additional notes.

The potentiometer and tuning capacitor bushing-thing are mounted through holes in the faceplate and are fitted with black knobs. I attached the coils using four brass pins.

Some very small holes were drilled to prevent the wood from splitting.

The output transformer was simply mounted with two wood screws.

I used a common PCB mount audio jack, but the panel mount variety would have been easier to use. I fixed it in place with hot glue then drove in a couple of pins to stop it from shifting backwards when a headphone plug is inserted.

The indicator LED was mounted in a common chromed bezel. I chose green because it reminded me of the green "tuning eye" indicators which some tube radios have.

Step 16: Making the Dial

The radio dial pointer was made from a common (at least in Canada) Bic brand pen. The smaller front piece (I call this the "section" in the pictures) holds the needle, which is made of, well, a (sewing) needle. The section couples with a piece of the pen barrel which is glued to the tuning capacitor with hot melt glue. I think that the pictures and notes can do a better job of explaining this than my text can.

The dial is fashioned from some cardstock and red paper. I had thought that this would allow the user to note down the position of stations on the dial, but in the end I couldn't bring myself to write upon the radio dial...

Step 17: Putting It All Together

At this point I must apologize for my lack of pictures for this important step. With the impeding contest deadline, and more importantly, rapidly approaching exams, I neglected to take many pictures for this step. If you have any questions, please let me know so I can include any details I have missed.

Once you have the parts mounted, you can screw on the faceplate and attach your knobs. Mine are simple black knobs affixed with setscrews. I just used clip leads for antenna and ground connections, but you could always use terminal strips for a more finished look.

Step 18: Finished!

Well, thanks everyone who got read this far, and sorry for being so lengthy in some of my writing.As I've stated before, I'm no expert of radios, and this is the first that I've actually built (other than a couple crystal radios, and some transistor ones that didn't really work). If you have any suggestions for me, please feel free to contact me and let me know how I can improve this design, or my methods in general. Also, as this is my first instructable, I'm interested in any feedback, so please give me some advice on how to make my next instructable better!

If you do make a radio like this, I'd love to see some pictures of yours as well. If you're considering this project, I'd suggest collecting up all the parts first, then putting the radio together a little at a time. If you ever get frustrated, just take a break and come back to it later. In the few projects I've built I can say that rushing through constructing electronics often leads to damaged components and that lovely burnt resistor smell...

If you liked this instructable, please don't forget to vote for me in the vintage contest.

The next step will have a little about the performance of this radio as well as some basic troubleshooting procedures.

Step 19: Performance and Troubleshooting

Overall, I was happy that my radio worked, but not all too thrilled with it's less-than-stellar performance. However, it did do a better job than any crystal radios that I've built so far, so that could just be due to my poor quality antenna (~ 50 feet of wire attached to my ceiling). I was able to receive the local AM station clearly and with good volume, but any more distant stations produced squealing, but all but disappeared when I turned the regeneration down. If you have any advice on how to improve this set, please let me know.


This list comes from Chapter 9 of Alfred Morgan's "The Boy's First Book of Radio and Electronics" (with some modifications to the original text to better fit this instructable)

1) Test that your headphones are working - If you are using modern headphones, simply test them with an MP3 player, or the computer you're using to read this instructable with. If you have high impedance type headphones, put them on and touch the wires to the terminals of your "A" battery supply (6 Volts). You should hear a click each time you touch the wires to the terminals.

2) If the tube heater does not illuminate, check that your "A" supply is good. If the batteries are good, then check that your heater wiring is correct, as well as that you have copied the tube pinout properly. I copied mine backwards first and had to do a bit of rewiring.

3) With the heater/filament warm, disconnect and reconnect the "B" supply. If you can hear sounds in the headphones when you connect and disconnect the headphones, then your wiring is likely correct. If the radio does not regenerate (squeals and whistles are not heard on any setting of the regeneration control) try reversing the tickler windings. If your tickler coil is attached backwards, the radio will not regenerate.

4) If regeneration is inconsistent across the band, you may have to add a turn or two to the tickler winding.

5) If you can't cover the whole band with the tuning capacitor, you could try adding or removing a turn or two on the main winding. The downside to using spider coil forms is that the tickler must be unwound to modify the main coil. A coil tapped every few turns (like a crystal radio) would be interesting to try.

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One Tube FM Super Regen Receiver - 12BH7A 12V DC Radio

This gorgeous old-style radio is actually a DIY kit, made from cardboard. The faux-wood case hides a hybrid of the modern and the ancient. The radio stage uses vacuum tubes to receive and produce the sound, whereupon it is sent to an IC, or Integrated Circuit.

The Franzis Tube Radio kit comes with all the parts, knobs and dials you'll need to build it (although the PDF instructions are only in Dutch or German), and the product page says that this is a world tuner, suitable for cruising the long-bouncing airwaves at night, ham-radio-style.

Even if you don't want it, take a look at the instructions (the PDFs can be had from the product page). You'll be treated to an incredibly in-depth manual full of black and white photos and even circuit diagrams. If nothing else, it would make a great gift for any tinkering nerd in your life.

The Franzis Tube Radio kit costs €50, or $66, plus shipping from Germany. I like it so much I just ordered one. Happy Christmas to me!

Franzis Tube Radio kit [Conrad Electronics via Retro Thing]

See Also:


Radio kits tube vacuum


#355 Let's try to build a Vacuum Tube Radio


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