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Thread: Epiphone Galaxie EA-33RVT adding grounded plug

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    Epiphone Galaxie EA-33RVT adding grounded plug

    This looks pretty straight forward , does anyone see a problem with adding a grounded plug to this amp , by simply using the chassis for ground , or is C17 the dreaded deathcap. I apologize if my schematic didn't post
    Click image for larger version. 

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    Last edited by shortcircuit; 03-04-2018 at 04:12 PM. Reason: info

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    Supporting Member Jazz P Bass's Avatar
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    On the schematic it does appear that all 'grounds' are common to the chassis.
    So attaching the mains safety wire to the chassis will work.

    And yes, remove C17.

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    Quote Originally Posted by Jazz P Bass View Post
    On the schematic it does appear that all 'grounds' are common to the chassis.
    So attaching the mains safety wire to the chassis will work.

    And yes, remove C17.
    Thank You !!

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    and be sure to run the hot to the fuse, then switch and tie the neutral directly to the other power transformer primary. Then install the ground to the chassis with the ground wire long enough that if the cord is pulled out it is the last wire to disconnect from the circuit.

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    Quote Originally Posted by davohilts View Post
    and be sure to run the hot to the fuse, then switch and tie the neutral directly to the other power transformer primary. Then install the ground to the chassis with the ground wire long enough that if the cord is pulled out it is the last wire to disconnect from the circuit.
    the neutral connects to the other primary through S1 correct?? " then switch and tie neutral directly to the other power trans primary " threw me off , I just want to be clear. Thanks

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    Lifetime Member Enzo's Avatar
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    The schematic shows one side of the mains going to the switch and the left side of the primary, and the remaining side of the mains going through the fuse to the right end of the primary.

    What he is telling you to do is run the left side of the primary directly to the neutral mains wire, and then move the switch to the other side of the primary between fuse holder and transformer.

    So mains current comes in through hot wire, through the fuse, then through the switch, then through the transformer, and back out to the mains neutral.

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    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    The schematic shows one side of the mains going to the switch and the left side of the primary, and the remaining side of the mains going through the fuse to the right end of the primary.

    What he is telling you to do is run the left side of the primary directly to the neutral mains wire, and then move the switch to the other side of the primary between fuse holder and transformer.

    So mains current comes in through hot wire, through the fuse, then through the switch, then through the transformer, and back out to the mains neutral.
    Thanks for making my note clear. It is always good practice to connect the hot side of primary to the fuse and then to the switch and connect the neutral directly to the other side of the PT primary. Then if the fuse blows you don't have line voltage past the fuse.

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    Quote Originally Posted by davohilts View Post
    Thanks for making my note clear. It is always good practice to connect the hot side of primary to the fuse and then to the switch and connect the neutral directly to the other side of the PT primary. Then if the fuse blows you don't have line voltage past the fuse.
    Success !! Thanks to all for your support . Next thread will be biasing this amp. Thanks !!!!

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    Supporting Member Jazz P Bass's Avatar
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    The output circuit is 'cathode biased'.

    Measure across R26 (270 ohm cathode resistor).
    Stated on the schematic: 17 Vdc across that resistor is the expected idle voltage.

    You may want to lift the cathode bypass capacitor (C18) to see if that is affecting the bias voltage at all (it should not).
    Or simply replace it.

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    Quote Originally Posted by Jazz P Bass View Post
    The output circuit is 'cathode biased'.

    Measure across R26 (270 ohm cathode resistor).
    Stated on the schematic: 17 Vdc across that resistor is the expected idle voltage.

    You may want to lift the cathode bypass capacitor (C18) to see if that is affecting the bias voltage at all (it should not).
    Or simply replace it.
    Thank you Jazz P Bass , amp is on hold waiting for tubes .

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    Quote Originally Posted by shortcircuit View Post
    Thank you Jazz P Bass , amp is on hold waiting for tubes .
    Measuring today with old tubes one 6aq5 is 80%, one at 100% on tube tester ,Replaced C18 with a 25v 25uf cap, Resistor R26 in circuit 303ohms , voltage across R26 at idle 18.80 ohm , Voltage pin 5 on 6aq5 320VDC , pin 2 on 6aq5 18.55VDC so everything is running hot?

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    Last edited by shortcircuit; 03-13-2018 at 08:51 PM. Reason: +

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    Quote Originally Posted by Jazz P Bass View Post
    The output circuit is 'cathode biased'.

    Measure across R26 (270 ohm cathode resistor).
    Stated on the schematic: 17 Vdc across that resistor is the expected idle voltage.

    You may want to lift the cathode bypass capacitor (C18) to see if that is affecting the bias voltage at all (it should not).
    Or simply replace it.
    Measuring today with old tubes one 6aq5 is 80%, one at 100% on tube tester ,Replaced C18 with a 25v 25uf cap, Resistor R26 in circuit 303ohms , voltage across R26 at idle 18.80 ohm , Voltage pin 5 on 6aq5 320VDC , pin 2 on 6aq5 18.55VDC so everything is running hot?Click image for larger version. 

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    Last edited by shortcircuit; 03-13-2018 at 09:49 PM. Reason: +

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    if you feel comfortable you might try the following to check the bias of the amp.

    1. Measure the voltage drop on the cathode resistor to ground. Write the value down.

    2. Divide this voltage by the actual measured value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

    3. Measure the voltage on the plates of the power tubes to ground. Write this down.

    4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

    5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6AQ5s in Class AB, if it is over 9 watts, then you need to install a higher value cathode resistor. If it's 7.5 watts or less, you need to install a lower value cathode resistor.

    6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.

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    Quote Originally Posted by davohilts View Post
    if you feel comfortable you might try the following to check the bias of the amp.

    1. Measure the voltage drop on the cathode resistor to ground. Write the value down.

    2. Divide this voltage by the actual measured value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

    3. Measure the voltage on the plates of the power tubes to ground. Write this down.

    4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

    5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6AQ5s in Class AB, if it is over 9 watts, then you need to install a higher value cathode resistor. If it's 7.5 watts or less, you need to install a lower value cathode resistor.

    6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.
    Thank you! came up with 9.33

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    Looks pretty close, it would be interesting to check with a set of matched tubes. Good job.

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    Quote Originally Posted by davohilts View Post
    Looks pretty close, it would be interesting to check with a set of matched tubes. Good job.
    Have a new set of tubes, will post results later on , waiting on some caps I ordered , as limited as they were I miss our RadioShack

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    Quote Originally Posted by davohilts View Post
    Looks pretty close, it would be interesting to check with a set of matched tubes. Good job.
    After installing new tubes and a few caps my readings increased from 9.33 to 9.48 , raised R26 from 270 ohm to 384 ohm ( 330 + 50 ohm ) and am now at a happy 8.85 Thank You !!

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    Great job, I love gibsons even though they are such a pain to work on! Enjoy!!

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