Tube radio iPod amplifier - Page 1
Forum section: electronics / projects

author: TheCommonCathode
author_rank: Contributor
timestamp: January 07, 2014, 08:58:45 pm
content: I picked up an old tube, AM radio, and wanted to run my iPod through the tubs and get that nice sound out of it. I saw two terminals on the AM tuner, but don't know what they're there for. Wondering if you guys could shed some light on this idea.Here are some pictures of the inside of the radio.If you guys need more pictures, I'd me more than happy to supply you with them.Sent from my iPad usingTapatalk

author: SeanB
author_rank: Super Contributor
timestamp: January 07, 2014, 09:08:32 pm
content: First that radio is in all likelyhood connected to one leg of the mains, so the chassis is at mains potential. you could likely connect to the audio amplifier at the volume control using a small audio coupling transformer to give isolation, and add a switch to select the audio input or the radio detector.Still will not have any good reproduction, the audio amplifier is not going to be very good frequency response wise, and will roll off at both the bottom and top of the audio spectrum. But that will work, just use almost any AF coupling transformer, or a small output transformer.

author: dannyf
author_rank: Super Contributor
timestamp: January 07, 2014, 09:29:03 pm
content: QuoteAppear to be AF portion of the circuit - if so, there must be a switch somewhere to turn to that two wires. Using those two terminals would be the best solution for sound quality.Otherwise, if the radio works, you can build a simple am transmitter and use the rf part of your radio to receive the audio from your ipod.

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 07, 2014, 09:51:38 pm
content: Yes. definitely a "hot chassis" radio, also known as the "All American 5" setup. The circuit common is tied directly to one side of the AC powerline. This thing may even predate the use of polarized plugs, so there is a chance that circuit common is at 120VAC with respect to earth ground!If you want to add an external audio input, the easiest way to do it would be a small audio transformer (with suitable insulation ratings). Try injecting the audio signal (secondary of the new transformer) at the output of the detector stage, or at the top end of the volume control. feed your iPod in to the ISOLATED primary of the transformer. MAKE SURE that there is no direct connection between the iPod connections and any part of the radio circuitry.Why you would want to do this is beyond me. AA5 radios weren't exactly "hi fidelity" devices when they were new, and age probably hasn't helped any. You will get some tinny, low-bandwidth monaural sound, similar to what that radio got from AM broadcasts. Not worth the effort IMHO.If you want to play with tube audio, you would be much better served building a simple tube amplifier from scratch, using a cheap 6AQ5 or similar tube. You can design it to avoid the hot chassis hazards, and use better speakers and output transformers than those found in an AA5 radio, to get decent sound quality. You could even build 2 channels, and get stereo sound....

author: vindoline
author_rank: Unknown
timestamp: January 07, 2014, 10:21:26 pm
content: I'll second what N2IXK said about the hot chassis. These can be very dangerous radios to use (much less modify!). If you aren't careful, you can easily end up connecting the "ground" side of your iPod directly to the 120V mainsAlso, this probably isn't the best forum to ask your tube radio questions in. Modern engineers with their fancy-schmancy SMD components and microcontrollers don't have much time for tubes! Here is one link that can get you going:http://antiqueradio.org/iPodAdaptor.htmif your radio has an external phono input. If not, you need to add a switch that toggles between the radio feed and the iPod feed. At a minimum, you will need to obtain the schematic for the radio and learn some of the basics of radio restoration. This is not a bad place to starthttp://www.antiqueradio.org/recap.htmThere are many, many reasons that someone might want to use an old radio for this. They can have a great "cool" factor. Many of the styles and designs are outstanding. It's fun. It's how I got back into the electronics hobby after 30 years. You can learn a lot about electronics and radio design.Have fun, but be safe.John

author: TheCommonCathode
author_rank: Contributor
timestamp: January 07, 2014, 11:01:31 pm
content: Building my own sounds interesting. I've got two boxes full of tubes. I probably have some 200+ tubes. ~how difficult would it be build one? (I'm a fair beginner with kind of stuff. I have a general knowledge of electronics, but nothing TOO too advanced.)Sent from my iPad usingTapatalk

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 07, 2014, 11:35:00 pm
content: A tube audio amplifier isn't exactly "advanced" electronics. Googling "tube iPod amplifier" turns up several designs already tested and proven, including a few which are available as kits. There is a vast amount of literature on tube amplifier design out there. If you get hold of an old RCA tube manual, they always had a few "reference designs" in the back that might provide a good starting point.The biggest thing to bear in mind with tubes is the hazard of high voltages. Tubes typically need a few hundred volts to operate, and that is more than enough to knock you on your butt or even kill you if you get careless. Be aware of where you are putting your fingers, disconnect power entirely (and discharge filter caps) before making circuit changes, and make sure your meter/scope/etc/ can handle the voltages present before connecting.

author: TheCommonCathode
author_rank: Contributor
timestamp: January 07, 2014, 11:56:19 pm
content: The more I think about it, the more I want to try an use the radio. But without making an AM transmitter, since all the amplification stuff is there. So going on that, what would I need to make music come out?Sent from my iPad usingTapatalk

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 08, 2014, 12:08:29 am
content: As I mentioned above, you need a small audio transformer, with sufficient insulation to withstand 120Vac between the windings. a 1:1 turns ratio will likely work OK.Connect the secondary side across the 2 ends of the volume control pot in the radio. Connect the iPod output to the transformer primary.You should probably disconnect the existing wiring from the radio detector stage to the volume control, to prevent sound from the radio from being mixed with your iPod audio.This, of course assumes that the radio itself is still operational. If not, you will need to get it working first. At least the audio stages need to be operational. It is quite likely that the radio needs a few capacitors replaced to restore it to proper operation. The AA5 was a pretty generic design, with only minor variations between manufacturers. A typical schematic available here:http://www.wa2ise.com/#aa5Use the top version. The lower version is the older version using octal tubes, where yours uses 7-pin miniatures. The audio circuitry consists of the triode section of the 12AV6, and the 50C5 output tube.BTW, the "2 terminals" on the "AM tuner" (actually a variable capacitor) are not actually terminals at all. They are trimmer adjustments used for aligning the front end of the radio. If you turn them, the radio will need to be realigned if you want to use it to listen to AM again....

author: dannyf
author_rank: Super Contributor
timestamp: January 08, 2014, 01:07:27 am
content: You can do all that.Or you can pick a crystal oscillator + a small resistor + a battery + a capacitor and begin listening your music on the radio's am dial.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: January 08, 2014, 01:18:07 am
content: You should really get a 1:1 isolation power transformer and remove the line to chassis connection and "deathcap" for that before trying ANYTHING with it. They can be made safe quite easily, but are a deathtrap without making these changes.

author: Stumpy12
author_rank: Newbie
timestamp: April 15, 2014, 08:56:21 pm
content: I don't know if you got around to doing this yet, but I have done this modification a few times. Apart from everything mentioned so far (it wouldn't be a tube amp form without the required "hot chassis" warnings!), there are a couple of other things which may help.1) Impedance matching: the output of the iPod and the input into the first audio stage of the radio have very different impedance. The audio transformer, in addition to the previously-mentioned isolation, will help correct this. A 1:1 will work, a 10:1 normally used for audio output will work better, especially with the gain. The low side goes to the iPod, the high impedance side goes to the radio. Given that the radio probably doesn't have great frequency response (more from the speaker than the amp circuit itself), you don't need an expensive transformer.2) The iPod doesn't have a huge voltage output--approximately 2 volts. You may still may not get the amount of gain you were looking for, and it will probably play at a lower volume than the radio. You could put in a cathode-bypass capacitor on the output tube (cue rebuttals about cathode bypass from the forum....), but the tradeoff will be higher (though minimal) distortion.3) IF you have a schematic and feel comfortable doing this, I would recommend replacing the detector portion of the detector/1st stage audio tube with a solid state diode. This will prevent any stray oscillations or signals from the IF stage from distorting your iPod signal. This tube would then be dedicated to being the first stage of the audio amp circuit. (cue angry retorts to the idea of putting solid state components in a tube radio from the forum....). I have found this to make the greatest difference to overall sound quality. I have done this on some old console models, and it sounds beautiful.Don't get frustrated; it won't produce the best amp in the world, but all of this can be a good learning experience. (cue warnings about electrocution from the forum...).

author: nadin2
author_rank: Newbie
timestamp: October 22, 2014, 05:41:31 pm
content: I have a FADA 1000 that I want to connect an iPod to. I do not believe the chassis is hot. I need to confirm where on the 12SQ7 I should connect the output of the iPod.

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 22, 2014, 11:44:37 pm
content: The FADA 1000 is a hot chassis radio (the 12SQ7 is a dead giveaway, being part of the octal AA5 lineup), and a rare, collectible one as well. 2 good reasons NOT to try modifying it.http://www.antiqueradio.org/fada1000.htmThat said, if you wanted to inject an audio signal, it would go into the top end of the volume control, or into the grid of the 12SQ7 triode section (pin 2, IIRC). Use an isolation transformer in either the power line or the audio line to reduce the shock hazard.http://en.wikipedia.org/wiki/All_American_Five

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 23, 2014, 12:26:59 am
content: Man my parents have one of those big furniture turntable/radio things, it's awesome. It even has a graphic equalizer in it!Of course the record player is junk, never put a record on a turntable like this one, it has like 5 pounds of tracking force, you can play your record exactly once!But the radio is awesome, and it has line inputs!

author: calexanian
author_rank: Super Contributor
timestamp: October 23, 2014, 04:54:24 am
content: Quote from: Alex Eisenhut on October 23, 2014, 12:26:59 amThat may not be a record player, but a record cutter.. Or the tone arm bearing is locked up somehow.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 23, 2014, 05:47:53 am
content: I have a classic 1957 Thunderbird, but I really want to make a lawn tractor. Can I just rip the trunk lid off and cobble a wooden box in there to haul manure in?I confess to having not the slightest clue why anyone would want to use an old, clunky, dangerous (possibly lethal) radio with notoriously poor audio performance and connect their MP3 player to it. Especially if the old firebottle gadget was some sort of collector's item that would be ruined by such defacement.If someone wanted to have a lousy speaker to connect to their MP3 player, there are plenty of toy plastic things at Radio Shack, or even a convenience store that would probably perform just as badly as an old AA5 radio. And without the real risk of electrocuting the user and then burning down their house for good measure.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 23, 2014, 11:12:46 am
content: Quote from: calexanian on October 23, 2014, 04:54:24 amNah, just an awful piezo pickup turntable, nothing hi-fi.

author: calexanian
author_rank: Super Contributor
timestamp: October 23, 2014, 03:04:45 pm
content: Quote from: Richard Crowley on October 23, 2014, 05:47:53 amI agree. There will be no quality there. I would much rather see you build your own. There is a very low cost kit sold by antique electronic supply and a number of online dealers that has all of the basic parts for a 1 or 2 watt stereo amplifier on a little pcb. You could easialy hide that inside an old radio case and add a second speaker for stereo and have something much safer and practical. I believe it uses 6bm8's or something like that. I will try to dig a link up. For $100 in parts off of eBay you could easialy build something better that that old dangerous radio.