Step 6: Can It Have an AUX Input?

  The amplifier section of the radio amplifies whatever signal its given, so since i now had a free band, the FM will now be the AUX input. I wanted the AM to still work and be able to switch between the two, also i wanted to use the volume on the radio to control the audio level. 
  some people used AM transmitters or FM transmitters and even bypassing everything directly to the speaker to do this, but thats just not the same as the sound actually being produced by the radio. plus it wouldnt have that old radio sound. i spent 3 days reading up on this and which tube i hopefully can hook it up to. i came across this website: http://tubedata.tubes.se/sheets/093/6/6T8.pdf which is for the 6T8 tube, which is the preamp tube. this tube has 3 inputs two are for the AM/FM detector and the third is beginning of the audio part. 
  i used the schematic to find the FM input post on the bottom of the tube and put a wire on that and another wire on the audio input post. then to be sure i turned it on and put it on a multi meter to make sure i wasnt hooked into power, and it showed 0 volts. next step, i ran the wires to my cd player and plugged it in, turned it on, and like it was meant to be, perfect clear sound coming from the radio speaker. yay! i turned it to AM with the cd player still going, and it was just AM so my new input wasnt on the AM circuit. turned it back to FM and it was there and the radio controlled the volume like i wanted so everything works like i wanted it.
<p>I don't know if anyone else mentioned this, but &quot;weak heaters&quot; don't exist. The heater can be open or shorted to another tube part, but if it isn't glowing brightly, all that means is the heater isn't as exposed. All tubes are made different, and some are more visible than others. </p><p>http://www.thetubestore.com/Resources/Matching-and-other-tube-info/Blue-Glow</p>
<p>ok, I have the same preamp tube to hook up to and I can get a signal from my CD player no problem on FM. However, once the tubes warm up, the FM signal comes in and over powers my CD sound. Thoughts? How would one still have FM but be able to put another input into the radio? I don't use the AM, is there an avenue to using that part for amplification?</p>
<p>I think your problem is that the incoming FM signal isn't being stopped like it needs to. I'm not sure of what radio you have but the main reason it worked easy on mine is the FM doesn't work at all for some reason so I used that to my advantage and put my own signal into it. If your AM isnt used much I would find that circuit and either take out the tube that makes the AM work so you can put your Cd player on that or cut the wire from the AM circuit to the preamp. It should be a fairly easy procedure. The AM side wont sound any different than the FM side with the CD player because it's the AM signal from the radio station that makes it sound like it does and not the circuit. So you can either do that which I would personally do (who needs AM right) or you can keep it on the FM side like it is but you'll need to separate the FM circuit from the preamp with a switch or something. Hope that helps!</p>
Hi there, thanks a lot for the instructions. I have also an old radio and I was wondering if it is possible to connect my phone to it so I can play my mp3's with that warm sound of the tubes. Now I know the answer. <br> <br>But I still have some questions: <br>Do we need to convert the digital sound from the mp3 player (iPod or not) into an analog signal? <br> <br>The output signal from a mp3 device (iPod or not) is not too strong for the old radio?
The headphone jack on any device is an analog signal, You can even use the line out jack if your device has one. As far as the signal being too strong for the radio, i havent seen any reason to be alarmed by it. cranking up the sound on the ipod just makes the radio go louder like it would with a newer one. I usually have the ipod volume all the way up :).
Their is another trick to the DIODE tubes. Most of the BAD or dead diod sections can be tricked into working. I used to use 1n4004 diodes to replace the detector that was weak or bad. I just soldered them into the circuit. I also used to take the 6H6 metal tubes apart and unsolder the pin ends to remove the small glass tube inside the metal shield. I then would replace the diode sections of the tube with 1n4004. Then used a 5 watt resistor to replace the filiment ( can't remember the value ). This also worked for alot of other tubes. In some cases you can use a 1n4146 for some detectors. These fixes worked better than a new tube.
This is awesome!!!!! I have been looking for a tutorial like this for a long time - always wondered if it was possible. I am definitely going to have to try this - thanks so much for sharing it!
You should be careful, some of the old tube radio chassis are connected to the Hot AC line, and would fry your iPod or mp3 player used for input
Yes, it's often a good idea to re-wire it with a 3-prong plug. <br>
http://www.angelfire.com/electronic/funwithtubes/AA5-6.html<br>Her actually is a good article.<br>
An isolation transformer, mounted externally would probably be the best
i do like the idea of an isolation transformer for safety and piece of mind. wouldnt the chassis in this case be similiar to a neutral connection? ive connected one wire of car speakers to the vehicle chassis this way when i didnt have enough wire and it worked in a similar way. luckily for me everything seems to be working fine with this circuit. &quot;knock on plastic&quot;
Worse yet, with the frame of the jack connected to the chassis in this type of radio there is a SERIOUS SHOCK HAZARD. And a 3-wire plug won't help. What is needed to make the project safe for radios with one side of the line connected to the chassis is to run the power line through an isolation transformer like this one:<br>http://www.newark.com/triad-magnetics/n68x/isolation-transformer/dp/18M9252?in_merch=Popular Isolation Transformers&amp;MER=PPSO_N_P_Isolation_None<br><br>That way neither power line wire connects directly to the radio, but goes through the transforme first.
I like how you have kept so much of the original components, and only changed a few capacitors. I would have been tempted to rip out all the old components and replace them with new ones.
There's no point in replacing the other parts. Carbon resistors, mica, and ceramic capacitors don't degrade with use or age. With a unit this old you may as well replace all the electrolytic capacitors because of there age On new equipment, it may be worth testing the capacitors first, but most people reading this don't have an lcr meter to do that (hehe got a really nice one given to me).
One of the interesting things about the older tube circuits is the richness in tone quality that the capacitance of the tube impart. I have rebuilt older T.V.s inparticular an Advent Video Beam 1000. These were the projection sets sometimes used by GoundRound in their restaurants in the 1970 into the '80s. They have an giant 8' screen which is great to watch.

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Bio: I love building things, and I'm a twin, we both are electrical engineers and inventors. Feel free to ask anything else
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