Add Phono Input To Tube Amp
After fixing up the cabinet to this radio and enjoying staring at it for awhile, it is finally time to take a whack at the chassis. It worked fairly good as it was but it had a hum that was always in the background which gets pretty annoying after awhile. Its also not good for the tubes and what not when it does that anyway. The FM band on this radio doesnt work either. These issues will all be fixed in this instructable plus a couple of other things to make it more 'usable' for newer technology with an IPOD/ aux input.you need some tools for this plus some safety common sense 1: multi meter 2: soldering iron 3: solder 4: scrap wire 5: wire snips 6: new capacitors!
7: earphone jack 8: a drill and a 1/4 inch bit. In order for you to know what to replace, an inventory must be made of what needs to be replaced. Since all radios are different you need to figure out exactly what your radio has inside. You can do this by either looking directly at the capacitors and hopefully the numbers will be still visible.
You can also look at the service manual for a schematic for your radio. You can get one here: If they have a schematic for your radio, it will be like mine and list specifically every capacitor and resistor value and any other bit of info about your radio. Mine even tells me how to calibrate the AM and FM which would seem impossible without the manual. The manual also show component locations to help you locate the parts.
Next, get a piece of paper to write down the values and voltages you need. I also wrote down conversions from microfarads to picofarads to help when i go to select the new ones. From what ive done research on and what the great people on this website have told me, the paper and wax capacitors are the ones that need to be changed, which is underneath the chassis.
As far as the humming noise goes. That is the electrolytic capacitors in the power supply circuit, mine was located in a metal can on the top of the chassis which contains 3 capacitors inside it actually. Make sure the radio is unplugged before you go poking at things! On most chassis, the metal case is the negative part. But while you're in there, look around and see if there is any burnt out parts or wires that need attention.
I originally thought this would be a fun part of the restoration project, but I was wrong. I say that because there is alot of things you have to be careful of as you chose the new caps. Mainly the voltage ratings, polarity, and values. I was sweating up a storm and doing lots of conversions to get what i needed. The selection wasnt good as i hoped but its the best around besides the internet. I bought my supplies at FRY's electronics in Las Vegas.
If you replace a cap that has a lower voltage rating than what is called for, you will come to know its wrong as the radio powers up and you hear the caps explode! So be careful and write down what the voltage ratings are.
You can always go above the rated voltage but NEVER lower. A capacitor will only take in the voltage its given. Example: if the rating for a cap you need is 100v, you can get a 150v, 250, etc. And it will work the exact same as a 100v.
Polarity is also something to look out for, especially in polarized electrolytic caps. These are the ones cylindrical in shape and have a black stripe on one side.
If these are installed backwards, it will also explode! The power supply caps i used to replace the can caps are the only polarized caps i used in this whole restore. Keep in mind though that there are non polarized electrolytic caps out there too. They look really similar in appearance but dont have a strip on them and the box they come in says non polarized.
You can install these in any position. Polarity doesnt matter and they wont explode if the voltage value is where it needs to be. Values are important in the fact that the radio wont work correctly.
These values are listed in microfards on the old paper and wax caps but are abbreviated 'MFD'. When buying new caps, you can buy ones in picofarads too if you do the conversions for them. It is the same capacitance if you did it right. Thats why i wrote it in different values for the same cap on the paper. You can look up a conversion table on google. On my radio parts list in the manual online, it says i can go +/- 20% which means i can go 20% below or 20% above the rated value and it will still work the same. Older radios arent so specific i guess.
When youre at the store or online buying these things and youre like me and cant find half of the value and voltages you need, dont freak out. With capacitors you can parallel them and put them in a series to get the value you need. Say if you need a 20mfd cap 250v, you can get two 10mfd 250v caps to get the 20mfd value you need.
Add Phono Input To Tube Amp Converter
Keep the voltages the same if possible or higher. Just because they are in parallel the voltages is only as good as the lowest one.
I didnt need to put any in a series to get a value i needed but if you need to the option is there. One last thing, a good thing about capacitors, it more or less doesnt matter what type of cap you use as long as its value and voltage is what you need. I used polarized electrolytic in the power supply because thats what it originally had, and i mostly used ceramic and mylar caps for the rest of it simply because the ceramic caps handles alot of voltage when others dont.
I used non polarized electrolytic ones where i could too. I also purchased a decent soldering iron and solder while i was there. Those battery powered cold heat ones dont seem to work for this project.
Now that you purchased all the parts you need to replace we can begin the cutting out of the old capacitors. Just grab and pair of wire snips and dig in. You should do one at a time. Just because you have the schematic doesnt mean if you cut them all out at once you will stand a chance of knowing where even one goes much less the value thats supposed to be there. Start simple and easy. Not me though, i started with the hardest ones i could, those being the power supply circuit caps in the metal can.
Three of them in that thing. Not the same value either. On the side of the can it has symbols and values next to the symbols.
Underneath the can next to the wire post should be a symbol matching one on the side so you know what cap goes to which wire. Not in my case. There is symbols on the can as you can see in the picture but no symbols underneath. In my can there is 20, 20, 40 mfd 250v caps inside. Funny and true story, i spent an hour at the store looking for just these values that they didnt even have. They didnt even have the values i could parallel or series them to work.
I did however find 47mfd 250v caps there. So now i had one cap that would work within 20% i could use. But inpatient me, i got to the point where i just said screw it and i got three 47mfd caps for the power supply.
I read that if the capacitance is too low the radio will hum which is why it was doing that in the first place because the old ones dried out. And if the capacitance is too high, more power will flow through the circuit which is also not good. I desoldered one wire of a post and soldered it onto the positive leg of the new cap (keeping in mind the polarity) and did that for all three now that they are the same value. Then i soldered the negative legs of all three together and soldered those to a wire which was then soldered to the chassis (chassis being the negative). The metal can is a negative common for the old caps inside you should always power up your radio after each cap you do to make sure its working properly and not going to explode etc.
When i did this i didnt hear any noise or hum come from the speaker like i usually do. So i turned the volume up and all i heard was crystal clear audio!
No hum whatsoever! My gamble with those caps paid off. No issues at all. Once these are soldered, make sure they are secure and wont contact the chassis or other components. Once again, be careful when testing it after each new cap, there is DC voltage in here when powered up.
I measured it at 142vdc in some spots and anywhere above and below that in certain places. DC with a little current can kill! For me, after doing that, the rest was so easy! I only have to deal with one at a time now and not three.
I started in the amplifier section of the radio using the non polarized caps where i could. The old wax caps arent polarized as far as i can tell and it would be a huge headache to figure out how to put in polarized ones correctly. I used the mylar (red ones) here too, and a few where power is sent through to circuits from the switches and transformers. I used ceramic as coupling caps between tube posts. To start doing this, start with one cap and snip off the leads right at the cap to leave some lead to solder on your new one. (makes it easier to relocate if you forget and and also easier to solder). I double checked the value on the old wax cap to match up my new one.
I then twisted each new cap leg around the old lead and soldered it on. These are non polarized so it dont matter which way its put in. As you can see in the picture i paralleled a couple of ceramic caps to get the value i needed.
There is also some original ceramic capacitors in here, and im told they dont need changing unless they are broken or burnt. Why did they not put ceramics everywhere instead of those wax and paper ones? Feel free to comment on this i would like to know! I just kept moving one capacitor at a time working my way through each section and then finished on the other side of the radio on the am/fm circuit caps. There is also some RF chokes and transformers in here, it would be a good idea to ohm these out with a multi meter to make sure they dont have an open connection somewhere.
Check wires too, as you can see in the picture they are the fabric covered kind that could short against the chassis if a hole is rubbed in it. Mine checked out great and the material is still really strong. I did some research on my tubes in the radio. I noticed a couple of tubes had a very weak heater in them.
One tube has 3 heaters in it actually, and another one has 2, while the rest of them have just 1. The tube with the 3 heaters in it is the 6T8A tube. This one from what i read is the AM/FM detector and amplifier tube.
This tube does three different things, and it had a very weak heater (or i should say 3 weak heaters). And the tube with 2 heaters in it is the 12AT7 tube which is an FM detector or sorts, it had only one heater lit and the other was dead. All others were glowing a bright orange. There is an antique store in vegas that i recalled going to that had a huge basket of old radio tubes in it. I went there and spent probably 2 hours going through all the tubes looking for some replacements for my radio and i actually found every kind except the 12AT7.some tubes looked brand new. I did some research before i left to see if i could substitute different ratings for the original and discovered a couple they had there would work in place of the 12AT7. So i got one 12AX7 and a 12AU7 which the AX has a slightly higher gain than the AT, and the AU has a lower gain than the AT.
So i got one higher and one lower. The gain just makes it sound a little different i read online. I took my 'new to me' tubes and went to buy them.
There was 6 tubes i wanted and i talked to the lady who works there and got the price down to $10 for the 6 tubes. Which in my research wasnt bad at all for all them compared to buying them online. I priced them on ebay later and a couple of them were going for $10 each while others were around $2 on average. Buying those 6 tubes off ebay including shipping would have cost me almost $60. So it really does save money to do your research. Plus i didnt have to wait to get them! I came home and plugged them in, and all of them work great, as far as all the heaters lighting up brightly, but the radio didnt act any different or sound any better by me doing this.
On the bright side i got some spare tubes for the radio. It still has no FM band, for the fact that i did look up other radios on the internet of the same exact brand and model and their FM worked, so i know i missed something, but its no big deal. I came up with a better solution to the problem. 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: 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. 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. Now that i know it works i can move on and make it look professional and original. I bought a chassis mount 3.5mm headphone jack.
Stereo or mono doesnt matter since the radio is mono. I looked at the back of the chassis for an existing hole and found one a little too small for it, so i got a 1/4 inch bit and made it bigger.
Its in a semi visible spot below to antenna. I did a 'dry' run without solder to confirm it will work before i solder it on. As i did this i noticed that once the jack is on the chassis the metal on the jack is the same as the ground post on the back so it worried me a little bit because i didnt know if that being grounded to the chassis would be an issue. I put one wire on the terminal and then touched the jack to the chassis while everything was on and it worked better than just with the two original wires! Awesome, so now i only need one wire to make it work!
I tried the one wire on the number 2 post that goes to the FM but it wasnt as loud as the number 1 post. I soldered the wire onto a lead coming from the ratio detector to the number 1 post on the 6T8 tube, and tested it again for sound and it worked great, only able to hear it on the FM band and not AM.
(AM still working the same.) i guess the ratio detector is also FM? Last thing i needed to do was run the wire to the jack and solder it on and tighten it up. Confident i was now done i double checked for crossed wire and leads and then put the radio back together in its cabinet with a big smile on my face. After its all put back together the only way anyone can tell anything has been done is by an almost invisible jack below the antenna.
Other than that one wire soldered onto the tube terminal this radio now had an AUX input that almost zero changes have been made to the radio. I didnt cut any wires or alter the circuit in any way whatsoever. I could have just soldered a headphone plug on it and been done but it would always be there in view. This way i can unplug the cord like its never been there and no one would really know. And if one day the FM decides to start working again on its on by some miracle it will be able to. I tried my CD player, IPOD, and cell phone on this radio with the input and it all sounds great. It has clear sound with no hum or any annoying tones at all.
It still has the distinct tube radio sound like it should, the volume can be controlled by its own knob. The AM works perfectly like it should, no static from the volume control either.
Out of curiosity i plugged a mircrophone into the input and i could hear my voice coming over the speaker too. I dont see a use for this but it can be done. This couldnt have had a better outcome than what has happend.
Everything works as it should and almost as if it was designed with this and originally had it. All tubes work and are operating at max quality with new capacitors. Ive had this radio on for almost 3 straight days before i wrote this and it has worked perfectly, still no hum, or funny stuff. Im in the clear.
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! 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.
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: Isolation Transformers&MER=PPSONPIsolationNone That way neither power line wire connects directly to the radio, but goes through the transforme first.