[Michael Wiebusch] found the leftovers of a wrecked vintage tube radio in a pile of electronics junk. Unfortunately, he could not recover any vacuum tubes in it. And to his dismay, it didn’t even have the output transformer, which he figured would have been useful in a guitar amplifier project. The output transformer is not easy to come by nowadays, so he was hoping to at least score that item for his future build. All he could dig out from his dumpster find was a pair of speakers and he ended up building nice Output-Transformer-Less Tube Guitar Amplifier around them.
Valve output stages are generally high-impedance which means they cannot be directly interfaced to low impedance speakers. An impedance matching output transformer is thus used to interface the two. Back in the day when valves were still the mainstay of audio electronics, many cheap amplifier designs would skimp on the output transformer to save cost, and instead use high impedance speakers connected directly to the amplifier output.
[Michael] found a nice reference design of an OTL amplifier for a 620 ohm single speaker. He decided to use the same design but because these speakers were about 300 ohm each, he would have to wire his two speakers in series. At this point, he decided to make his build useful as a proper guitar amplifier by adding a preamplifier stage replicated from another design that he came across. A regular halogen lamp 12V transformer takes care of the heater power supply for all the tubes, and a second, smaller 12V transformer is wired backwards to provide the 300V needed for the plate supply.
The final result is pretty satisfactory, considering that it all started with just a pair of junked speakers. Check out the result in the video after the break.
[J.B. Langston] has some vintage late-40’s/early-50’s tube radios that he wanted to repair – a Motorola All-American 5 AM radio, an Air Castle AM/FM radio and a Sears Silvertone AM/FM radio. He goes over, one by one, the three vintage radios, the problems they had, and how he got them back into working order. No finding a replacement microchip here, this was all about replacing capacitors and finding vacuum tubes!
In contrast to most modern builds we see on Hackaday, vintage radios are fairly simple – mainly turret-board builds with a transformer, resistors, capacitors, coil and tubes. The main issues in any vintage electronic repair is checking the capacitors because old wax paper and electrolytic capacitors can degrade and will need replacing. When repairing the All-American 5, [J.B. Langston] had an issue with the transformer, and he goes over how he fixed what’s called silver mica disease in it. While many parts were replaced with modern equivalents, only a selenium solid-state rectifier in one of them was replaced by a different part – a silicon diode and a high-wattage series resistor.
Looking at the inside of some of these radios, it’s surprising that they could be restored at all – 65-odd years of rust, dust, dirt and grime will take their toll – but [J.B. Langston] was able to fix all three radios and clean their Bakelite cases so they look and work like new. He goes over what he discovered, how he fixed the problems and the links to where he got help when needed. We’ve seen some great vintage radio projects over the years, including adding RDS (Radio Data Systems) to a vintage radio, converting a vintage radio with modern technology and even some other radio restoration projects.
About a decade ago I started a strange little journey in my free time that cut a path across electronics manufacturing from over the last century. One morning I decided to find out how the little glowing glass bottles we sometimes call electron tubes worked. Not knowing any better I simply picked up an old copy of the Thomas Register. For those of you generally under 40 that was our version of Google, and resembled a set of 10 yellow pages.
I started calling companies listed under “Electron Tube Manufacturers” until I got a voice on the other end. Most of the numbers would ring to the familiar “this number is no longer in service” message, but in one lucky case I found I was talking to a Mrs. Roni Elsbury, nee Ulmer of M.U. Inc. Her company is one of the only remaining firms still engaged in the production of traditional style vacuum tubes in the U.S. Ever since then I have enjoyed occasional journeys down to her facility to assist her in maintenance of the equipment, work on tooling, and help to solve little engineering challenges that keep this very artisanal process alive. It did not take too many of these trips to realize that this could be distilled down to some very basic tools and processes that could be reproduced in your average garage and that positive, all be it rudimentary results could be had with information widely available on the Internet.
If you ever wanted to build your own tube amplifier but you were intimidated by working with high voltages, [Marcel]’s low-voltage tube amp design might spark your interest. The design operates with a B+ (plate) voltage of only 40v, making it less intimidating and dangerous than many other amps that operate over 300V. It’s also incredibly easy to build—the whole design uses only 11 components.
The amplifier is designed around the ECL82 tube, which includes both a triode and a pentode in one package. The ECL82 is practically an amplifier in a tube: it was designed for low-cost electronics like record players that needed to be as simple as possible. The triode in the ECL82 is used as a pre-amplifier for the incoming signal. The pentode is controlled with the pre-amplified signal and acts as a power amplifier.
[Marcel]’s amplifier also uses a PY88 tube rectifier instead of semiconductor diodes, making it an entirely silicon-free design. Although [Marcel] hasn’t posted up detailed build instructions yet, his simple schematic should be all you need to get started. If you want some more background information about tube amps but you don’t know where to start, check out our post on basic tube amp design from earlier this year.
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