new jtm45...something is wrong with my phase inverter!

Discussion in 'The Workbench' started by laycern8, Jul 27, 2019.

  1. laycern8

    laycern8 Member

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    Hello everyone! The last time I had a problem with this build I posted it here and you all were beyond helpful (thanks again). I need some advice so I can finally get this build from the "workbench" to the "building the classics" with some victory lap shots.

    I build a jtm45. on power up, the pin values for V1 and V2 look great. The pin values for V3 (PI - 12ax7) do not. I can't figure out why. All the resistors I have checked seem to be in range, even the old carbon comps. Hoping someone can tell me which components I should check on, I'm still a novice when it comes to understanding circuits.

    The values I SHOULD have on V3 are as follows, by pin number:

    V3: 1- 205 VDC
    3 - 48 VDC
    6- 195 VDC
    8 - 48 VDC

    Instead, my values are

    V3: 1 - 201 VDC
    3 - 29 VDC
    6 - 337 VDC
    8 - 29 VDC

    Pin 6, which I know is anode #1, seems concerning. It is a 72% increase from expected value. Strangely, the cathodes are also affected, but are showing lower values than expected. Please help!

    Also, would running the amp in this state damage any components?

    Thank you all in advance. It's really a privilege to be able to access a community of people with such expertise.
     
  2. Far Rider

    Far Rider Hendrixian VIP Member

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    I'm a newbie myself at amp building, and also built a JTM45. Have you tried swapping tubes to see if there is any change?
     
  3. laycern8

    laycern8 Member

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    Far Rider yes I have. I have three new 12ax7's, a tung sol, a mullard and a JJ. I've swapped them all around but the values stay the same.

    For the record, B+ is 443VDC
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Seems like the tube is bad...
     
  5. laycern8

    laycern8 Member

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    It doesn’t fit though. No matter what tube I put in there, they all behave the same way. Low values on cathodes, super high value on anode #1.

    If it were the tube, wouldn’t I expect to see haywire values when I dropped it in the other positions?
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Something is wrong w/ the circuit board or shorted across pin 6 plate resistor.
    Since it is impossible for a resistor to short, it must be the board itself.
    (resistors can only open, and never short out)

    You would think that pin 3/8 cathode resistor was open if pin 6 shows 337 VDC.
    But pin 1 shows normal voltage, so the cathode is probably good.

    So take out the tubes, use you ohm meter to test the resistors in the PI.
    There is something up w/ a bad connection (re-solder the socket, check for a broken circuit track.)
    Because pin 6 resistor should be dropping voltage and it's not dropping.

    So look at pin 8 and see if the socket is broken off the board at that connection...
     
  7. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Have you confirmed the value of the pin 6 resistor, such as is it maybe a 10k where it should be a 100k?
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    I think from the readings that the pin 6 resistor connection must be disconnected from the socket (broken circuit track, etc...)
     
  9. neikeel

    neikeel Well-Known Member

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    There are a couple of possible reasons, given that this is a clone build.
    First post some clean well lit photographs of your circuit board.
    Second measure your screen voltage too.
     
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  10. laycern8

    laycern8 Member

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    I am totally baffled. The resistor for pin 6 reads 100k just as it should. B+ before the resistor is 363, after the resistor is 330. I swapped the resistor out for a higher tolerance 100k resistor and it made no difference.

    I have no screen grid measurement because the brown/blue primary wires on the OT are disconnected. The OT is blown. I haven’t replaced it yet because I wanted to fix this anode voltage issue on the PI first.

    I can’t compress any photos enough to get them on here, will post link

    *edit* actually the pics are still over on the other thread I made for this amp here http://www.marshallforum.com/threads/ot-problem-with-new-jtm45-build.106926/#post-1816469
     
  11. laycern8

    laycern8 Member

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    So an update, I ended up lifting the pin side of the resistor off the terminal to measure the voltage, assuming something is shorting. Instead what I found is that even with one terminal in the air, the resistor is only dropping the voltage by 30 volts, from 360 to 330. Does that make any sense to anyone?

    There is an 82K resistor right next to it that’s dropping voltage a lot more. What the hell is going on? Totally lost very confused
     
  12. Goldguitarguy

    Goldguitarguy Member

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    Can you test the current draw in mA at the B+ side of the resistor? If so, then you can calculate the correct numbers. That would tell you if the resistor is working correctly. I suspect that it’s toast.

    (Va) = Voltage reading of B+ side of resistor
    (Vb) = Desired voltage
    (Vc) = Required voltage drop
    (R) = Resistance in ohms
    (I) = Current in amps

    Va-Vb=Vc

    R= Vc / I

    Don’t forget to convert current from mA to Amps....lol
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    "I found is that even with one terminal in the air, the resistor is only dropping the voltage by 30 volts"

    That is not possible.
    With one terminal in the air, there should be NO voltage drop across the resistor.
    The FULL voltage should pass right through the resistor.
    voltage in = voltage out. No drop at all.

    The 82K resistor drops voltage because there is a load on it.
    (the load is the triode tube stage)
    Without a load, there is no voltage drop.

    Voltage in > resistor in> resistor out> load = voltage drop
    The voltage drop is caused by the load. (ohms law)

    Voltage in> resistor in > resistor out> no load = no voltage drop
    Voltage in is the same as voltage out because there is no load on the resistor.
    That cannot be changed, it's Ohms law.
     
    Last edited: Jul 29, 2019
    laycern8 likes this.
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn power off, unplug mains power.
    Take all the tubes out.
    Set meter for ohms.
    Connect black probe to pin 3 of PI socket. (do this on the tube side of the socket)
    Connect red probe to pin 8 of PI socket. (do this on the tube side of the socket)
    What is the ohms reading between pin 3 and pin 8 of the PI socket?
     
  15. johan.b

    johan.b Well-Known Member

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    You need to get readings from all pins for them to mean anything.
    But if one side isn't conducting, either the voltage on that grid is off relative to the cathode, the heater on that side is not working, you have a leaky cap or that tube is faulty
    J
     
    Last edited: Jul 30, 2019
  16. laycern8

    laycern8 Member

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    I had to take a break from the whole thing for a few days. Also, in my frustration, I ended up attempting to read the amperage on the wrong setting and fried the DCA section of my freebie harbor freight multimeter. I now have a new multimeter, but the same problem.

    The component immediately before pin 6 on the phase inverter is a 100K resistor. The bands on the resistor are brown-black-yellow, and it also tests at 100k. One side is attached to the B+ voltage line. The other side is detached entirely, and sticking up in the air. I did this to eliminate the possibility that the 47pf cap connecting the two plate resistors was bleeding. With the negative probe grounded to the chassis, I am reading two different DC voltages on either side of this resistor. There is a 30 volt difference between the two sides. I know that ampmadscientist said that this is not possible, and without knowing much in the way of electrical physics I tend to agree, since something is definitely wrong.

    This is the second 100K resistor I have tried. Both resistors have tested at 100K, both resistors drop the voltage by 30 volts. If this makes no sense, I will make a video and hopefully someone can point out whatever stupid ass mistake I must be making here. The only other possibility of course being that there is some quantum break in reality occurring at the exact coordinates of my 100K PI plate resistor (just my luck).

    Goldguitarguy, I am about to test the current with this new multimeter, I will post it here.

    Here is goes...
     

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