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Thread: Marshall Artist, extremely low volume output.

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    Marshall Artist, extremely low volume output.

    Hi, I have an old 80s Marshall Artist, 30 watts, 2 EL34s. The amp had worked fine for a number of months but now it has a super low output. I have determined the preamp section works fine because I ran the effects send to another amp. Both channels operated as they should. I substituted the phase inverter and the power tubes from another working amp and the problem persists. Here are the voltages from the power tubes:
    V2 V3
    P3 488V 489V
    P4 486V 487V
    P5 -49V -49V
    P6 488V 488V

    Any assistance would be appreciated.

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    Supporting Member Randall's Avatar
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    Have you tried a cable between FX send and return?

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    It's weird, because it WAS working fine.....

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    Yes. Doesn't change anything.

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    Anyone else think that bias voltage on pin 5 is a bit too negative for EL34's?

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    Quote Originally Posted by mfreqmaster View Post
    Hi, I have an old 80s Marshall Artist, 30 watts, 2 EL34s. The amp had worked fine for a number of months but now it has a super low output. I have determined the preamp section works fine because I ran the effects send to another amp. Both channels operated as they should. I substituted the phase inverter and the power tubes from another working amp and the problem persists. Here are the voltages from the power tubes:
    V2 V3
    P3 488V 489V
    P4 486V 487V
    P5 -49V -49V
    P6 488V 488V

    Any assistance would be appreciated.
    please report also the PI tube voltages
    you also have a couple of opamps in line from the return jack to the tube,it could be some failure or simply a fractured soldering.

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    Quote Originally Posted by m1989jmp View Post
    Anyone else think that bias voltage on pin 5 is a bit too negative for EL34's?
    i'd say to turn off an el34 you must go up to -80V

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    PI voltages: p1 434v, p2 7v, p3 31.5v, p6 221v, p7 18.5v, p8 31.5v

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    Quote Originally Posted by mfreqmaster View Post
    PI voltages: p1 434v, p2 7v, p3 31.5v, p6 221v, p7 18.5v, p8 31.5v
    The resistor connected to pin 1 is most probably shorted,once replaced the voltage will be similar to pin 6

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    I suspected that too, but measured and it is 87k ohms. I will check the solder joint to that pin next.

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    Quote Originally Posted by mfreqmaster View Post
    I suspected that too, but measured and it is 87k ohms. I will check the solder joint to that pin next.
    reflow also pin3

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    Are both sides heater lit at PI tube position?

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    Lifetime Member Enzo's Avatar
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    I'm with g1, it is most likely the pin 1 side of the tube is not conducting, usually because the heater is not lit.

    I have never in my life seen a shorted resistor.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Enzo View Post
    ......I have never in my life seen a shorted resistor.
    Ditto!

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    Me three. Although I did see one once that had decreased substantially in resistance. So I never say never about that anymore.
    Although I still never expect to see a complete short.

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    Supporting Member The Dude's Avatar
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    The only time I've come close was when I measured a 100K resistor and it measured 10K. It was while teching at a manufacturing facility. After encountering the same part problem several times, we checked the batch. It turns out the manufacturer marked a batch of resistors wrong. Still,...... they didn't short- I don't recall ever seeing a shorted resistor.

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    Lifetime Member Enzo's Avatar
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    Those didn't short, they didn't even reduce resistance, they were just marked wrong.

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    I measured the heaters. Pin 9 was 5.6vac. Pins 4-5 were 1.8vac. So g1 called it there. Traced the voltages back to the power transformer. One of the heater terminals is showing 6.5 vac while the other is showing 2.5 vac. Would this mean my power transformer is defective?

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    How did you measure?

    While I would call it unusual to get numbers like yours, all the tube cares about is the voltage across the heater terminals. So to get the BEST measurement, you'd put one probe terminal on pin 9 & the other on pin 4/5. That should be 6.3V +/-10% or so.

    If you're keeping one probe on ground and checking the two sides of the heater individually, that may explain your numbers. Also, the tubes don't really care whether that 6.3V is AC or DC.

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
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    PS: got a schematic?

    Justin

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Yes, you were right. I still had one probe connected to ground. I have sine placed one probe on p9 and one on p4-5 and have a reading of 6.3 vac. This is the schematic i am using: https://drtube.com/schematics/marshall/3203pwr.gif

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    The problem still persists and i continue to have that high voltage reading on p1

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mfreqmaster View Post
    PI voltages: p1 434v, p2 7v, p3 31.5v, p6 221v, p7 18.5v, p8 31.5v

    I'd expect pin 2 to read about the same as pin 7. This makes me suspect that C12 is leaky. Try disconnecting the inner conductor of the input cable that goes to it and remeasure pin 2 and pin 7. BTW the grid voltages are obtained measuring between cathode and grid to avoiding loading the grids with your meter and getting silly results.

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    Measuring the 6.3V at the socket terminal is just the first part. It doesn't necessarily mean the voltage is getting to the tube pins. That is why I asked if you can see both sides lit. You might have to turn off the lights.

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    I turned off the lights and looked at the 12ax7. It appears to be glowing on both sides of the tube.

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    Ok. I'd suspect the coupling cap like nick suggested, but the plate voltage is high, rather than low which is what I would expect from a leaky cap.
    So still suspicious of that side of the PI tube not conducting. Have you checked the resistance of the pin1 plate resistor?

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    Quote Originally Posted by g1 View Post
    Ok. I'd suspect the coupling cap like nick suggested, but the plate voltage is high, rather than low which is what I would expect from a leaky cap.
    So still suspicious of that side of the PI tube not conducting. Have you checked the resistance of the pin1 plate resistor?
    Quite right. I see I missed the high voltage on pin 1 - thx. The cathode is 31.5V and the total tail resistance is about 15K so the tail current is 2mA. 2ma across the 100K on pin 6 drops 200V so there really is no current at all in that pin 1/2/3 section. Could the socket be bad so that pin 3 is open? Pin 2 is still odd.

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