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Peavey Windsor Doug Hammond Mod Phase Inverter

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  • bobloblaws
    replied
    Originally posted by Enzo View Post
    So now your voltages are restored to more reasonable levels.
    Yes, please see attached.

    I'm measuring 91 watts at the output, not bad.

    But I'm seeing some crossover distortion so I will attend to the power tube bias.

    Thanks again!

    Click image for larger version

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  • Enzo
    replied
    So now your voltages are restored to more reasonable levels.

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  • bobloblaws
    replied
    Originally posted by Enzo View Post


    I don't recall, did we verify pin 2 is NOT shorted to ground?
    You called it. I'm an idiot. You see in the lower right of the diagram where it says "5 point terminal strip - mount to existing PCB standoff? Well I did exactly that and the middle lug is connected to where it mounts and the standoff I used is connected to the chassis. Thanks for patiently working though this with me, much appreciated!

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  • Enzo
    replied
    OK, the following is VERY unusual...but it happens. The pins in a socket have two ends. Up top is the part that grips the pin on a tube base. And underneath is the part that solders to the circuit board. The pins can break in the middle. This results in a gripper part that grips real well, and a bottom part that is soldered in real well, but the two are not connected. I am assuming when you take voltage readings that you are probing the "bottom" end of the pins. Pull the tube and probe the top gripper part of pins 1 and 6. With no tube, you should read pretty much full B+, like you already do on the bottom end of pin 1. If my very rare condition exists, you would read full B+ on the underside, and zero or very low voltage at the gripper.

    Same can happen at pin 3, only no voltage with tube gone. Pins 3 and 8 might be connected below, but measure resistance to ground from each pin, 2 and 8. Both should read the same.


    I don't recall, did we verify pin 2 is NOT shorted to ground?

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  • bobloblaws
    replied
    Originally posted by Enzo View Post
    Are BOTH heaters glowing on that PI tube?
    Yes, definitely. Both tubes I tried test fine on my tube tester as well. I verified the heater circuit and that both elements were glowing. I have 6.3 VAC across pins 4/5 and 9. I checked that the tube pins are all connected and I re-tensioned the clip for pin 1. Connection between pin 3 and 8 is sound. Got me stumped.

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  • Enzo
    replied
    Are BOTH heaters glowing on that PI tube?

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  • bobloblaws
    replied
    Originally posted by Enzo View Post
    If the PV B+ is 120v lower than the MArshall, that right there might exoplain why the voltages are lower than expected.



    Your B++ is only 270v to start with and you measure 268 at pin 1. Pin 6 is not "so much lower", pin 1 is so much higher. There is no voltage drop across R45, which means that triode is not conducting. No conducting, means high plate voltage and low cathode voltage and no amplification.
    That is weird, I even tried swapping in another tube last night. I'll try that again and compare the voltage on pin 1.

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  • Enzo
    replied
    If the PV B+ is 120v lower than the MArshall, that right there might exoplain why the voltages are lower than expected.

    I don't know why the voltage on pin 6 is so much lower than pin 1.
    Your B++ is only 270v to start with and you measure 268 at pin 1. Pin 6 is not "so much lower", pin 1 is so much higher. There is no voltage drop across R45, which means that triode is not conducting. No conducting, means high plate voltage and low cathode voltage and no amplification.

    Leave a comment:


  • bobloblaws
    replied
    Originally posted by Enzo View Post
    Can you inject a signal at the point the loop used to be? I am looking to bypass everything C13 and before to just test the power amp.

    One way to characterize level is to listen at one point and listen at the next without touching the controls of the tracer. If it gets louder, we have gain in the stage. If for example we have a good strong signal at the master or at C13, but at the power tube grids it is much quieter, that tells us the signal is not being amplified.

    Just thinking aloud: the cathode voltage of the PI comes from the current through the tail, If it is half what you expect, that could mean one side is not conducting. ANy chance pins 3 and 8 are not well connected on that PI?


    Higher than normal plate voltages and low cathode voltages mean the tube is not conducting as much.


    What are your B+ voltages here? And what is B+ in your Marshall model?

    Got a scope? Good. Can you apply a test signal to the regular input and see a good signal at the master or at C13. Don't turn up the scope vertical from then on. The signal will not shrink in the PI or power stage, so if you were having to turn it up to see, then we have missing signal level.
    As far the scope goes I had better luck with my old analog scope than I did with my digital one, but the bottom line is, there is in fact a net signal loss from V3 pin 2 to the power tube grids that I can detect with the audio probe, instead of the expected amplification. I believe the connection between the cathodes is solid but I had not re-measured V3 pins 1 and 6 once the tube started conducting where apparently it hadn't been previously, so the updated diagram I uploaded actually need more updating. Providing that now. I don't know why the voltage on pin 6 is so much lower than pin 1. R66 measures 100K in circuit.

    B+ for the PI node (they call it B++) is 270. I'm also uploading one of the Marshall diagrams I used for reference showing 390V on that node.

    Click image for larger version

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    JCM800_2204.pdf

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  • Enzo
    replied
    Can you inject a signal at the point the loop used to be? I am looking to bypass everything C13 and before to just test the power amp.

    One way to characterize level is to listen at one point and listen at the next without touching the controls of the tracer. If it gets louder, we have gain in the stage. If for example we have a good strong signal at the master or at C13, but at the power tube grids it is much quieter, that tells us the signal is not being amplified.

    Just thinking aloud: the cathode voltage of the PI comes from the current through the tail, If it is half what you expect, that could mean one side is not conducting. ANy chance pins 3 and 8 are not well connected on that PI?


    Higher than normal plate voltages and low cathode voltages mean the tube is not conducting as much.


    What are your B+ voltages here? And what is B+ in your Marshall model?

    Got a scope? Good. Can you apply a test signal to the regular input and see a good signal at the master or at C13. Don't turn up the scope vertical from then on. The signal will not shrink in the PI or power stage, so if you were having to turn it up to see, then we have missing signal level.

    Leave a comment:


  • bobloblaws
    replied
    Originally posted by Enzo View Post
    The master has no effect on what you see/hear at the power tube grids? That sounds like crosstalk. WHat kind of level is the signal at those grids?
    I'm not sure how to quantify the level. I'm using a simple audio trace and my scope can't seem to detect a signal, at least not in the way that I'm accustomed to. Maybe I need to flip the probe to X 10.
    Originally posted by Enzo View Post
    Plug a signal into the FX return, what comes out?
    As I said, the FX loop has been taken out of the equation.

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  • bobloblaws
    replied
    Originally posted by Enzo View Post
    So right at C13, signal on one end and not the other? Check that the no signal end is not shorted to ground. And if not, then I sure would try a new cap in that spot.

    That FX loop circuit is weird. If we connect something to the FX loop, with the loop turned on, it works. To turn off the loop, they added relay SK102 to short across it. Unfortunately, that sets up a feedback loop sending the return signal right back through the send. So, many complaints, and I don't doubt the factory modified a bunch of them.


    Remember, you don't HAVE a MArshall. I don't see any reason the voltages in this Peavey would match a JCM800 model. I would look for reasonable voltages, not exact duplicates. Right off the bat I'd be surprised of the B+ voltage was the same in both amps.

    If both halves of the tube are heating, and you still have zero volts on the grid, that lends credence to my shorted to ground notion.

    There IS NO NORMAL grid DC voltage when measured to ground. When someone says 28v, that means in HIS amp with HIS meter. Look at your MArshall drawing. It shows 45v on the cathodes - I'll believe that. It shows 28v on the grid. That grid is 17v more negative than the cathode, Imagine how poorly that stage would perform if it was really biased that far down. The problem is your meter. It has an internal impedance. The grid voltage comes from the bottom end of R16. I'd expect maybe a volt drops across that resistor. So 44v for discussion. Theree is no grid current, so that 1meg resistor does not drop any voltage while in use. The real grid voltage is then 44v. But when you go to measure, your meter impedance and R17 form a voltage divider. like this: 44v, through R17, and through your meter. SO it reads artificially low, REAL low. But meters are not all the same, so there is no way I can predict that your readings will match his.

    You want to knw the real grid voltage? Measure DC voltage across cathode to grid directly. SUbtract that from the cathode to ground voltage and you will have the grid to ground real voltage.

    HAving explaained all that, if I am, servicing one, I will check to ground, and if I see something in the range of half the cathode voltage, I'll consider it good, I don't need to calculate the "real" voltage.

    And when I blame your meter, there is nothing wrong with it, my nice FLuke acts the same way. Rather it is a limitation of your gear and procedures you must be aware of.
    Yes, I wasn't expecting exact duplicates, but I would have expected in the ball park. For example, my cathode voltage is 19V as opposed to 45, yet my plate voltages are quite a bit higher than the nominal JCM800 counterparts. So am I wrong to think that my recorded voltages indicate a problem? They seem lower than reasonable to me.

    Thanks for the tip about the meter impedance setting up a voltage divider. That didn't occur to me. As it happens, I went ahead and measured direct from grid to cathode and the grid is 18V more negative than the cathode.

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  • Enzo
    replied
    The master has no effect on what you see/hear at the power tube grids? That sounds like crosstalk. WHat kind of level is the signal at those grids? Plug a signal into the FX return, what comes out?

    Leave a comment:


  • bobloblaws
    replied
    "I can audio trace the signal to the power tube grids but I'm getting nothing at the speaker output and nothing past C13 responds to the master volume control."

    Sorry for the confusion.

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  • Enzo
    replied
    OK, then what IS current status? I can only respond to what you present.

    Leave a comment:

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