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Tube AA5 radio conv to amp

Discussion in 'Amp Tech Center' started by Pierre couture, Jun 23, 2016.

  1. Pierre couture

    Pierre couture TDPRI Member

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    guys I just got a very nice Canadian GE 1948 tube radio.

    I want to convert it in a guitar amp.
    I know how to do it... But there seems to be several safety issues with these radio without isolation transformer.

    If I add an isolation transformer and plug a 3 pin plug in the wall.
    Then i would hook the ground of the guitar jack to the 3pin plug ground...

    Would i be safe now?

    Pierre
     
  2. bparnell57

    bparnell57 Poster Extraordinaire

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    In isolation transformer and grounded cord would help keep you safe but it would have to be implemented properly. I cannot say this is safe. I'd advise you to try to find an old PA amplifier to convert.
     
  3. printer2

    printer2 Poster Extraordinaire

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    Do you have the model number? Would have to look up the schematic to see what could be done. Also how about a picture of the chassis?
     
  4. printer2

    printer2 Poster Extraordinaire

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    Took a quick search, probably close enough to the following.

    [​IMG]



    3-4W out, get another preamp tube and place it in front of the 12SQ7, possibly another 12SQ7. Strip it and add a few parts, can have a little Champ.
     
  5. Pierre couture

    Pierre couture TDPRI Member

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    image.png This is the schematic
     
  6. printer2

    printer2 Poster Extraordinaire

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    Same idea. First three tubes are useless for audio so you can use the sockets. The 35L6 will give you about 2W. The 12SQ7 can be thought of as half a 12AX7. The diodes you would want to tie to the cathode. The electrolytics should be replaced. Get a 120V:120 isolation transformer to feed it all. Lots of instruction on the web how to do it.
     
  7. Pierre couture

    Pierre couture TDPRI Member

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    If i need only 2W as I want a very low power amp to play in my living room, i guess I don't need to add other tubes?
     
  8. printer2

    printer2 Poster Extraordinaire

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    If you only use the 12SQ7 in there to drive the output tube it probably will not give you much gain. If you want a cleanish amp it might be enough. Could always drive it with a pedal to give it more signal.
     
  9. Crawfish

    Crawfish Tele-Afflicted

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    Build a Champ instead. Or something like a Noisy Cricket. Don't waste your time trying to convert this radio into a guitar amp.

    I see these threads a lot here. It's just way too much work to try and convert a radio - especially a transformerless set - to be an amp. You really have to alter everything to make this work properly and sound decent. It's not as easy as it seems. You're taking something that's a lot more complicated than a guitar amp and de-engineering it to use a small part of the circuit.

    Just cause it has tubes doesn't mean it will sound good. The output transformer and speaker will likely not sound good with an electric guitar. And as others have noted, you will not have a lot of gain from this circuit without extensive modifications.

    On an AA5/AA6 (which is what this one is, since it has an RF stage), one issue is that the filaments are all in series. You can't just pull tubes from the circuit - the voltages on the filaments add up to 118 volts in this circuit. If you were to eliminate the RF and IF tubes and just run the driver, the rectifier and the output tube on 120 volts, you'll blow the filaments instantly.

    It is possible to connect an input to this set - if you know what you're doing - and play an electric guitar through it. (I started to write out how to do it but decided not to since I don't want something disastrous to happen). However, I can almost guarantee you will be very disappointed in the result.

    If you want to keep the set, do what I do and restore it for its intended use - as a radio.

    -Kevin
     
    mRtINY likes this.
  10. mRtINY

    mRtINY Friend of Leo's

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    Crawfish, you beat me to it. Not only is the isolation a safety issue, those series filaments, while a clever choice for a cheap radio, are a serious limitation to reusing it as almost anything else.

    I saw a few of those oddball tubes in a bin at the surplus store yesterday. I thought hard about a couple of metal-envelope 12SQ7 tubes. But, the odd configuration and the 12v, non-center-tapped filament just made it too difficult to integrate into a guitar amp circuit....


    -tINY
     
  11. Pierre couture

    Pierre couture TDPRI Member

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    I don't want to be anoying, but I did the convertion, on an other AA5 radio as some of the component of the GE were not working.

    The end result is exactly what I was looking for, a little low power amp to play guitar in my appartment.

    The sound is clean and the form factor is small.

    The tubes are shown on the image
     

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  12. printer2

    printer2 Poster Extraordinaire

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    Glad you are happy with what you have. A Champ can put out enough sound to get you into trouble in an apartment.
     
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