No Grid POP! (Trouble shooting a P2P 5F2A)

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Oct 16, 2021.

  1. James Knox

    James Knox Tele-Afflicted

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    So, I’ve been having some fun learning to build point-to-point, using a schematic rather than a RobRob Layout.

    On this latest 5F2A, I fired it up and it sounded “not right”. Guitar audio is “slightly weak” compared to a couple others. I also noticed that a single note “fades off” rather than sustaining. It almost sounds the note hits a limiter and falls off.

    I chopsticked around checking all solder joints, took readings of all resistors, swapped power and preamp tubes, and checked voltages. Plate to Cathode is 329V, plate current 36.4V, dissipation 12watts for 86%.

    Started the Trouble Shooting Procedure of “Popping the Grids” and I got nada on the 6V6. Here are my voltage readings:

    B1 356
    B2 299
    B3 244

    12AY7
    1. 120
    3. 1.86
    6. 123
    8. 1.80

    6V6
    3. 347 plate
    4. 297 screen
    8. 18.09 Cathode

    Why no pop?

    Thanks in advance for for help!

    Here is a pic:

    89904DFE-211F-4E31-9024-1DDD7A7D1956.jpeg
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I can't see well from the photo. 6V6 Pin5 looks iffy from way over here.

    Double check what is connected to each tube pin... again.

    Do you have ground connections to the chassis at the input jack, 8uF cap, and reservoir cap areas? The chassis paint looks like it has been scraped off at the 8uF terminal strip. How about the other grounded parts?
     
  3. D'tar

    D'tar Friend of Leo's

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    IME.... probing a 6v6 grid or plate usually produces a "scratch" at best. You dont have the 50x amplification you are getting like on the 12ax. If you are getting guitar signal to the speaker..... the pop test is irrelivant.
     
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  4. James Knox

    James Knox Tele-Afflicted

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    I have the Grid (pin 5) jumpered to pin 1 with the 1.5k grid resistor, then to the .022 coupling cap and 220K resistor to ground.

    The Two Point grounding scheme is Power grounds to the terminal strip near the Filter Caps and Preamp grounds to the terminal strip near the preamp Tube socket. Both terminal strips are bolted to the Scraped Steel chassis and Dexoit-ed.
     
  5. James Knox

    James Knox Tele-Afflicted

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    Ok, I get scratching at the plate, fo sho. Yes, getting signal to the speaker.
     
  6. James Knox

    James Knox Tele-Afflicted

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    Here is a drawing of a similar layout. The drawing is for a 5F1, but the build is a 5F2A with a NFB Pot and switchable V1 Caps. It might be a little clearer to see than the photo... 104EE4C9-746B-47B2-8F8A-72B627BE09E1.jpeg
     
    Last edited: Oct 17, 2021
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So the solder connection on that pin5 is beautiful?:)
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Both of pins 4&5 have a good solder connection on the 12A_7?

    Guessing... I am anticipating some wonky bad solder connection that gives almost good sound but not quite.
     
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  9. James Knox

    James Knox Tele-Afflicted

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    In the pic I see that Black color, but in real life it’s real purdy!

    Looks good and tug test is tight

    You are probably right. I will be re-flowing later this morning. Soldering on the Tag Terminal Strips is kinda new to me. Big open gap AND multiple Jumpers & connections at different sequences of the build.
     
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  10. D'tar

    D'tar Friend of Leo's

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    There appears to be more in the box than on paper....

    Adjustable NFB
    Switchable bypass caps
    Multitap ot
    ???

    Try disconnecting nfb?
     
  11. James Knox

    James Knox Tele-Afflicted

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    Oops, forgot to mention that the crude drawing if for a 5F1, but the build is for a 5F2A. Sorry for the confusion. Just wanted to show what I was thinking for the main P2P Layout.

    I’ll try disconnecting the NFB.
     
  12. andrewRneumann

    andrewRneumann Tele-Afflicted

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    Hello James, hope you are doing well!

    Is the sound you are getting clean or distorted?
     
  13. James Knox

    James Knox Tele-Afflicted

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    Hey Andy - back atchu Bro!

    It’s clean low and disto hi, sounds like a regular 5F1/5F2. *slightly lower output and single note fades faster than normal. Kinda like the vol knob is twisted down after the note holds for 2 seconds.
     
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  14. James Knox

    James Knox Tele-Afflicted

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    Ok, I just re-flowed every solder joint I could reach without dismantling the V1 Cap 3-way Switch in order to get to pins 1 and 2 on the tube socket.

    As soon as a couple more of my sleepyheads start moving around I’ll fire it up and see if we got it.
     
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  15. andrewRneumann

    andrewRneumann Tele-Afflicted

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    How about when *continuously* strumming some chords?

    Does it maintain its volume or does it lose volume and stay low until you stop playing?
     
  16. James Knox

    James Knox Tele-Afflicted

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    Stays up continuous strumming - maintains volume. The “fade” not as noticeable in a confluence of multi note chords.
     
  17. James Knox

    James Knox Tele-Afflicted

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    LLC for the win!

    Playing now and seems fine - gonna do some more burn in just to make sure!

    Thanks everybody for your help!
     
  18. Wally

    Wally Telefied Ad Free Member

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    Fwiw, the time-honored ‘pop’ test is done on the plates.

    As for LLC for the win, he is always a valuable source, ime.
     
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  19. James Knox

    James Knox Tele-Afflicted

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    Thanks Wally! No wonder I got no POP on the grid, lol. The screen makes more sense.

    I got the idea it was the grid from RobRobs site…

    “With all the volumes turned up you should always hear a pop when probing the grid of any tube in the signal chain. Tubes that run the reverb or tremolo may not pop when probed.


    If you get no grid pop then you have a problem somewhere between that tube and the last tube that popped”
     
  20. Wally

    Wally Telefied Ad Free Member

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    James…not the screen grids but the plates. This is one of if the THE oldest test for audio amplification circuits. It has been used since the beginning of audio amplification circuits….or so says DaveFunk in his book. you start at the output and move back through the signal chain to the input. The input stage should yield the loudest pop since that pop will be amplified by the entire circuit. Anytime you find a weak or non-existent pop as you move from the output section towards the input, you have found an area of concern.
     
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