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  • Magnavox conversion

    Hello,
    OK, I have an old record player I converted into a guitar amp. Upon reading the (geofex)
    article I now find out it is not a good idea. I would like to ask if it is possible to change the RC
    circuit and cascade one channel into the other. I do not have the schematic or model number.
    The tubes are as follows (1-6EU7) (2-6BQ5's) (1-6CA4). It has three transformers, I guess
    one might be called primary and one for each channel. I currently have it working by just
    soldering both inputs to a 1/4" input jack and using 2 speakers via connections on the chassis.
    It is doing what is described in the article as having a pumping effect. Also as a matter of the
    safety concerns I installed a grounded power cord. Any suggestions would be great

  • #2
    Does it look like this one?
    Attached Files

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    • #3
      This is probably a stereo amp with 3 transformers. My suggestion would be to make an 18 watt Marshall clone. You will have to find or buy the correct OT (for one PP rather than two class A amps) and change the pre and PI tubes to 12AX7s. Best to gut it and start over. Make a new turret/eyelet board and use new tube sockets as well as new components. It won't be much more work or very expensive and you will be much more satisfied in the long run because you will have a decent guitar amp. Especially if those old EL84s are good.

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      • #4
        Originally posted by JHow View Post
        Does it look like this one?

        Hello,
        Sorry but I do not know how to open a thumbnail

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        • #5
          Click on the thumbnail. You will need to be logged in.

          The reason your amp is motorboating is the (lack of it's) balance control. It is located on the preamp chassis, inside the shaft of the Bass control, and when the preamp is disconnected the amp will oscillate. In the schematic you'll see that the balance works off of the Negative Feedback circuits - a really bad idea made worse when it is only half connected. But if you want to use it as a stereo amp all you need to do is replace the balance pot with two fixed resistors.

          The best thing to do if you want to use it as a guitar amp is get a good push/pull output transformer and follow Olddawg's advice to gut it, making it into an 18W Lite IIb. The only thing I will add is the 6EU7 is most likely a Mullard and will make a fine input tube if you wire it correctly (different pinout than a 12AX7).

          RWood

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          • #6
            I think if you just click on it, it should bring up the schemo. Otherwise, I can send PM you the jpeg.

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            • #7
              Hello,
              OK, Thank you very much!!!! I had to pull the schematic up on a different computer. This one is a MAC that I think came over on the Mayflower! Anyway on the (geofex.com) web site it provides information about preamp resistor and capacitor values being way out of sync with what is needed for a guitar amp. One of the symptoms being what they call a pumping effect. On the schematic you provided I can now change the values to cure it,
              I just wired both inputs together and installed speakers on the two outputs. I can see now the speakers may need to be wired in series, also the schematic has 4 speakers with some caps in the circuit.
              I am in no way disregarding your suggestions about using the parts to create an 18 watt amp, at this point I'm just not there yet, however just searching for this information has been quite a learning experience as I now have a general idea about how and why tubes and amps work.
              The people on this forum are for the the most part very knowledgeable and most times very helpful It is really great to have a place of like minds to come to, most of the time when I am at a place of business trying to buy something electrical or related to music I find that most people have no idea what I am talking about
              Anyway thank you once again!!!!!!!!!!
              Last edited by SpareRibs; 12-13-2011, 07:05 PM.

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