My recent build is a little stiff...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guiltless, Jan 29, 2012.

  1. guiltless

    guiltless Member

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    I have not had a chance to do anything yet unfortunately. I will open it again tonight and take more measurements as well as trying to switch the OT leads.
     
  2. LarryN

    LarryN Member

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    I would try switching the speaker leads. I think you'd get that big noise if you switch the OT leads, but maybe that's what you meant.
     
  3. guiltless

    guiltless Member

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    Ok, so I took the following measurements today with 113v at the wall in my garage. I also measured the Jr High where the church meets and the wall voltage is about 113v-115v. Please note that my DMM stopped working on me so I had to revert to an old, but accurate, analog meter. So all of the voltages are approximate. The higher the #, the more approximate given the different analog scales.

    Also, I forgot to mention that I have really only played the amp live on the Normal side.

    The first number is in SS rec mode and the 2nd is with the 2x5V4G's.

    B+ - 460, 440

    V1: 5F6-A
    1 - 214, 202
    3 - 1.5, 1.4
    6 - 215, 205 (Unused with the Grid to ground)
    8 - 1.5, 1.4 (Shared Cathode)
    V2: 5F6-A (2nd triode is a cathode follower)
    1 - 184, 174
    3 - .95, .95
    6 - 312.5, 295
    7 - 184, 174
    8 - 185, 176
    V3: (BF AB763 Normal Channel)
    1 - 235, 222
    2 - 1.75, 1.7
    3 - 236, 225 (I get a huge pop through the speakers when measuring this pin)
    8 - 1.7, 1.6 (The inboard leg of the resistor is scratchy when I move the probe on it)
    V4: PI
    1 - 210, 201
    2 - 46, 45
    3 - 82, 79
    6 - 189.5, 182
    7 - 42.5, 42.5
    8 - 82, 79
    V5 - V6
    3 - 460, 440
    4 - 460, 440
    8 - 0 I have no clue why I cannot get a cathode reading. No redplating and the amp sounds fine. This is really weird.
    Also, the amp seemed to sustain perfectly fine at home, in the garage. The one thing I couldn't do today, that I did on sunday, is crank it with my FD2 in Comp Cut mode. This makes me wonder if the issue is with the preamp of the Normal channel rather than the output section...

    I also realized something that I did (or rather didn't do) is use a coupling cap after the 2nd triode of V3 into the 220k mixing resistors and into the PI. I didn't think it was necessary as I have a .1 coming off the 5f6-a side and there is no reverb to potentially get voltage. Could the lack of this cap be the reason for the really lout pop when measuring the plate of V1?

    I am concerned about my lack of ability to read a cathode voltage. I am using a pair of 1 ohm 1% resistors in that spot, and the amp wouldn't work if there wasn't a ground connection.
    It might be a little difficult to see here but I have a lead running from Pin 3 and Pin 8 of the output tube sockets to a spot on the right of the pic where I have test points. I wouldn't think there would be any interaction there, but I could be totally wrong.
    [​IMG]

    I'm sorry LarryN, I did not switch the speaker leads as I am using a friends Z-Best cab with about 20 screws holding the back on. We are supposed to be swapping back this week so I can have my open back 1x12 so I will definitely switch the leads then.
     
  4. guiltless

    guiltless Member

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    I should clarify that the wires running from pin 3 and 8 do not end at the same spot but rather at their own test points. Each tube has a test point for Plate Voltage and Cathode Voltage. I did this so I could know where my voltages were on each output tube without opening it up.
     
  5. LarryN

    LarryN Member

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    I was wondering about pin 3! Recheck your bias resistor wiring. ground>1 ohm resistor>pin 8>test point, correct?
     
  6. guiltless

    guiltless Member

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    Correct! I actually checked directly at the pin as well, just to make sure.
     
  7. LarryN

    LarryN Member

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    mv setting and one test point to ground?
     
  8. LarryN

    LarryN Member

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    Getting bias voltage?
     
  9. guiltless

    guiltless Member

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    yep! Bias voltage is present and the amp sounds good.
     
  10. guiltless

    guiltless Member

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    I am actually a little more interested in the loud pop when measuring pin 6 of V3 and the scratching when measuring pin 8. That definitely does not seem normal.
     
  11. LarryN

    LarryN Member

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    Strange!?! There has to be an explanation. I'd check the test points, meter and resistors separately. That cathode voltage has to be somewhere!
     
  12. guiltless

    guiltless Member

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    I probably won't be able to get to it again until Wed night :-/
     
  13. LarryN

    LarryN Member

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    Pin 6 is high enough voltage to pop when measuring. I'm not sure about the static on 8.
     
  14. guiltless

    guiltless Member

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    It's definitely more than a pop. It scared the crap out of me the first time it happened. It is very, very loud.
     
  15. LarryN

    LarryN Member

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    I'm lost on that one. Sorry.
     
  16. guiltless

    guiltless Member

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    Thanks for the help! And to answer TweeDLX's question, I am running standard AB763 PI values. 22k tail and a 470r cathode resistor.
     
  17. guiltless

    guiltless Member

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    So I found the reason that I could not measure the cathode voltage... My meter was not so obviously dying. It now will not read any voltage. I'm not too bummed as it was cheap and I won it in a contest. It worked well for a while. I am borrowing my father's Blue Point DMM for the time being and am able to measure everything perfectly.

    The real unfortunate side effect of this was that I went to play the amp at church and did not realize that I was biased WAY to hot and I blew one of the GE 6l6's that came with the amp. They were old anyway so I am taking to opportunity to give KT66's a try. The PT is designed for 4 6l6's so it should be fine with 2 kT66's.
     

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