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Peavey Windsor Doug Hammond Mod Phase Inverter

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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.
    Sorry, my last post was not logically self contained. At first I responded based on where I was at in the previous post and your inquiry, then I went and did additional testing. So " signal on the master volume side of C13 but not on the other side" did not reflect current status.

    Leave a comment:


  • Enzo
    replied
    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.

    Leave a comment:


  • bobloblaws
    replied
    Originally posted by Enzo View Post

    Oh, and I just re-read. You have signal at the master voume, and not past C13. You don't actually say you have signal AT C13. From the master volume, the signal path goes through a couple of relay contacts and/or cutouts on J4 FX

    return.
    Yes, signal on the master volume side of C13 but not on the other side. And I definitely confirmed continuity from C13 to pin2 of V3. The FX loop/relay stuff muddies the waters. The guy I bought it from told me it was stock. When I turned over the preamp board I found a bunch of jumpers and cut traces which as far as I could tell changed the loop from parallel to serial. I thought he was full of crap but he swears he bought it new and knew nothing about it. Then I found a pic on a forum showing virtually the same mod and the poster believed it was done at the factory. So who knows. Anyway, I changed it further so that neither the FX loop or the boost circuit are in play at all.

    The last time I checked whether V3 heaters were working they were. But I couldn't be positive that was the case when I jotted down those voltages so just now I went to confirm. This time I removed the tubes (V3 to V7) and pulled out the power amp board and reinstalled the tubes so I could troubleshoot from both sides of the board. Anyway, now I'm actually getting positive voltage readings in the PI circuit (albeit quite a bit lower than what I've seen documented for JCM800's.) Maybe the tube was seated improperly, I don't know. I'm still seeing 0V at pin 2 and one Marshall diagram I saw suggested 28V was normal at that point. 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. Here's the new voltage readings.
    Attached Files

    Leave a comment:


  • Enzo
    replied
    OK, you HAVE a Windsor, you are trying to alter the circuit to resemble a MArshall model. So C13. C13 is on the preamp board. You have ssignal on the preamp side of C13. Where exactly does the signal disappear. I get that it is not at pin 2 of the tube. But is it even at the power amp end of C13 itself? From there it crosses the preamp board to pin 2 of the ribbon connector. got it there? Now pin 2 of the other end of the ribbon, on the power tube board. And between that ribbon and pin 2 of the tube, there is a wire jumper in the middle of the trace. Does it make it past that?

    All that to detect a break in the path. But the path could be shorted to ground too. You have moved parts around and I guess cut traces? A mistake might have happened: a blob of solder, a part removed and not replaced. Or a wrong part installed. Or two parts swapped places. WHo knows.

    Oh, and I just re-read. You have signal at the master voume, and not past C13. You don't actually say you have signal AT C13. From the master volume, the signal path goes through a couple of relay contacts and/or cutouts on J4 FX return.

    One quick test, do you still get signal coming out the FX send jack?

    And the PI itself, I don't know what your green voltages are on th Windsor drawing, but a long tail pair with zero volts on the cathodes tells me the tube is not conducting. Are the heaters even glowing?

    Leave a comment:


  • bobloblaws
    replied
    Originally posted by Enzo View Post
    My first thought: what is wrong with it? What does it not do or what unwanted thing does it do? Without a problem it is hard to advise action.
    There is no signal beyond the coupling cap that precedes the PI. On the Marshall schematic it is C9 (22nF) and on the Windsor it is C13 (100nF) (not shown on the pic I uploaded, it's connected to "sig".) The other side of the cap is connected to the master volume wiper and I have signal there but not on the PI side.

    Leave a comment:


  • Enzo
    replied
    My first thought: what is wrong with it? What does it not do or what unwanted thing does it do? Without a problem it is hard to advise action.

    Leave a comment:


  • bobloblaws
    started a topic Peavey Windsor Doug Hammond Mod Phase Inverter

    Peavey Windsor Doug Hammond Mod Phase Inverter

    I'm in the process of converting a Peavey Windsor 100W head to a quasi-JCM800 according to the "Doug Hammond" mods. https://www.aronnelson.com/gallery/m...H/amp/windsor/

    Preamp is working fine as far as I can tell but the long tail phase inverter is not. I've tried replacing V3 and I've triple checked my wiring but I am obviously missing something. Attaching a couple of pics showing expected voltages in and around the PI circuit and my actual voltages. Anyone have any thoughts on this?

    - Bob


    Click image for larger version

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