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Old "Kay" guitar amp question

Discussion in 'Audio' started by Bigdrums, May 5, 2018.

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  1. Bigdrums

    Bigdrums

    1
    0
    May 5, 2018
    This is a very old guitar amp probably from the late 1940's. I believe it is single ended in that it has an 80 rectifier tube, a 6C5, a 6N7 and a 6L6. The socket for the 6L6 says 6N6. The amp works but I can see on my variac amp meter the current draw goes down instead of up when I strum the guitar. This seems odd to me as all other amps I've tested this way increase the current draw. It also seems to have a large current draw at idle, (no input signal) at .9 amps. The amp has had some work done, perhaps a lot as much of the circuit has been rewired and new capacitors. What if anything is out of the norm here?
     
  2. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    Does the amp go?
    If the amp goes and the guitar sounds good through it, your measurements are very educational.
    :)
     
  3. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The current consumption seems to be excessive even on your low voltage:).(100W?). What is getting very hot?

    I have no data on the 6N6 so it could be a rare valve.

    The 6L6 is running in class A and its current should not change with signal level unless you are running into distortion. The bias required is 13V or more and the valve current should be about 55mA. To attain this bias, I would expect a 220Ω resistor in the cathode (pin 8). A cathode voltage over 13V will need investigating.
    Or does it have fixed bias with the cathode earthed?

    The grid coupling capacitor could be leaky, If it has not been replaced, it is well past its sell by date.
    If there is a capacitor across the cathode resistor, check this for leakage.
    If the valve is working at its maximum with no signal, then it cannot take more current and the average will drop.when a signal is applied.

    Any measurements with power on are hazardous but since you have a Variac I assume you know what you are doing.
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The 6N6 is a double valve with a zero bias output stage. No way can this be replaced by a 6L6 without circuit modification.
    Running a 6L6 without bias can ruin the valve, output transformer, mains transformer and rectifier.

    There is a description of the 6N6 on radiomuseum.org
     
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