Sometimes it is not how good but how bad your equipment reproduces sound. In a previous hackaday post the circuitry of a vintage transistor radio was removed so that a blue tooth audio source could be installed and wired to the speaker. By contrast, this post will show how to use the existing circuitry of a vintage radio for playing your own audio sources while at the same time preserving the radio’s functionality. You will be able to play your music through the radio’s own audio signal chain then toggle back to AM mode and listen to the ball game. Make a statement – adapt and use vintage electronics.
Pre-1950’s recordings sound noisy when played on a high-fidelity system, but not when played through a Pre-War console radio. An old Bing Crosby tune sounds like he is broadcasting directly into your living room with a booming AM voice. You do not hear the higher frequency ‘pops’ and ‘hiss’ that would be reproduced by high-fidelity equipment when playing a vintage recording. This is likely due to the fact that the audio frequency signal chain and speaker of an antique radio are not capable of reproducing higher frequencies. Similarly, Sam Cooke sounds great playing out of an earlier transistor radio. These recordings were meant to be played on radios from the era in which they were recorded.
Choosing an Antique Radio
Vintage radios can be found at garage sales, estate sales, hamfests, antique shops, antique radio swap meets, and Ebay. Millions of radios have been manufactured. People often give them away. For this reason, antique radios are relatively inexpensive and the vast majority are not rare or valuable.
Generally speaking, tube radios must be serviced and may not even work. Transistor radios often work to some level. Try to find a radio that is clean and uses a power supply transformer or batteries.
Click past the break to learn how to restore these radios to working condition
A word of caution with tube radios: your radio must use a power supply transformer or batteries. For your own safety you cannot use a ‘hot chassis’ radio. ‘Hot chassis’ radios use un-polarized power cords, where one wire is connected directly to the metal chassis and the other to the rectifier providing B+ for the radio. Depending on which way the power cord happens to be plugged into the outlet, it is possible for the metal chassis to be wired directly to hot line voltage. Most post-war table-top tube radios are ‘hot chassis’ radios.
Locate where in the circuit the envelope detector ties into the first audio frequency stage. This is typically the first pin to right of wiper pin (bottom view, looking from inside radio to outside) on the volume potentiometer. Wire in a 3-way toggle switch to select between external audio source and radio. Some larger vintage radios have external audio inputs; in this case simply make an adaptor for the external input.
When I think transistor radio, something like the Emerson Pioneer 888 comes to mind. This American-made radio was built in 1957 and uses 8 transistors featuring a push-pull audio output. The front looks like it was part of an AMF bowling alley. For $12 it was not a bad retro purchase.
Rather than fitting a toggle switch to this radio, I modified its ear phone jack to receive an audio input rather than feed an audio output. The neat feature of the ear phone jack on these radios is that it has a built-in switch. When something is plugged in this switch opens up. I used this feature by feeding the low level audio from the AM detector through this switch and out to the volume potentiometer. When you plug in your audio source, the AM signal is opened and your source is then fed to the volume potentiometer. When you remove your audio source it goes back to being an AM radio. See the schematic below for details:
The Collin B. Kennedy model 20 B is an early console radio built in 1929. Its front cover boasts that it is the ‘Royalty of Radios.’ It uses a tuned radio frequency (TRF) architecture that is significantly different from modern receivers.
Conveniently, this radio provides a phonograph input that is not RIAA compensated. Many console radios have this feature. It provides a high impedance connection directly into the radio’s audio amplifier allowing for the connection of an iPod or other audio device. I wired a modern RCA jack to this phono input.
There is a switch on the front panel to select either ‘radio’ or ‘phono.’ By selecting ‘phono,’ the line level audio signal from the phono input connection on the back of the radio is fed into the grid of the detector triode. The output of this triode feeds the audio pre-amplifier which then feeds a single-ended audio power amplifier. I wired my audio device into the phono input by using a simple voltage dividing mixer circuit to combine the right and left channels into a single mono signal.
Antique Radio ‘Boom Box’
Vintage tube radio sound is great at home but is more fun when you can bring it with you. For this I restored an Olympic Model 6-606 battery powered tube radio and modified it so that any audio source can be played through it.
The Olympic 6-606 was built in 1946 and uses either batteries or AC power. Unfortunately, if this radio is plugged in then it functions like a ‘hot chassis’ radio, making it too dangerous to use with an external audio source. For this reason I chose to operate it exclusively on batteries.
Implementing an audio input required the use of resistors in series with the right and left channels feeding an audio step-down transformer. This transformer is similar to those that you might find in old transistor radios. The output of this transformer is fed into a 3-way toggle switch that allows you to select either AM radio or audio input. The first audio frequency stage tube is an 1LH4 which functions as both an envelope detector and as a triode audio frequency gain stage. The audio input/radio toggle switch is a double throw. It interrupts the input to the first audio frequency stage by disconnecting the IF from the envelope detector on one pole of the switch and by selecting between the envelope detector and the audio transformer as an audio source to be fed into the volume potentiometer on the other pole of the switch.
The Olympic 6-606 requires two type-B and two type-A batteries providing 90 V for the plates and 9V for the filaments. Unfortunately, you cannot find these at the local hardware store. For this reason I built a modern battery pack equivalent to the two type-B’s by wiring ten 9V batteries in series. I emulated the two type-A batteries by wiring 6 size C batteries in series. All batteries were mounted on to a sheet of aluminum to provide structure and each output was fused at 0.25 A for safety.
Powering this radio with batteries is not inexpensive. It costs over $30 to fill the battery pack. Fortunately, the battery pack lasts for 15+ hours. With this vintage ‘boom box’ you can bring the sound of tubes anywhere, including parties, the beach or on sailing voyages. In social settings it is a great conversation piece.
Less Sound Quality, More Fun
Old recordings sound best when played through the radios that they were meant to play on. Make vintage radios relevant again! With a simple hack you can play your music or podcasts through a vintage radio’s audio circuitry while also keeping the radio’s functionality.
Acknowledgment: My cousin, Juliet Hurley, for type editing this post.
Gregory L. Charvat is author of Small and Short-Range Radar Systems, visiting research scientist at Camera Culture Group Massachusetts Institute of Technology Media Lab, co-founder of Hyperfine Research Inc. and Butterfly Network Inc., editor of the Gregory L. Charvat Series on Practical Approaches to Electrical Engineering, and guest commentator on CNN, CBS, Sky News, and others. He was a technical staff member at MIT Lincoln Laboratory from September 2007 to November 2011, where his work on through-wall radar won best paper at the 2010 MSS Tri-Services Radar Symposium and is an MIT Office of the Provost 2011 research highlight. He has taught short radar courses at MIT, where his Build a Small Radar course was the top-ranked MIT professional education course in 2011 and has become widely adopted by other universities, laboratories, and private organizations. Starting at an Early Age, Greg developed numerous radar systems, rail SAR imaging sensors, phased array radar systems; holds several patents; and has developed many other sensors and radio and audio equipment. He has authored numerous publications and received a great deal of press for his work. Greg earned a Ph.D in electrical engineering in 2007, MSEE in 2003, and BSEE in 2002 from Michigan State University, and is a senior member of the IEEE, where he served on the steering committee for the 2010, 2013, and 2016 IEEE International Symposium on Phased Array Systems and Technology and chaired the IEEE AP-S Boston Chapter from 2010-2011.