6100 no current on V9 and V10 any ideas?

Discussion in 'The Workbench' started by ntrueman, Jul 13, 2020.

  1. ntrueman

    ntrueman New Member

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    Hi there, hoping you can point me in the right direction, here's the problem and what I have done so far:
    Problem: V9 and V10 show no current passing when I connect bias probes to all four tubes, V8 shows 44mA and V11 shows 52mA.
    All four tubes light up and get hot. Original tubes were SOVTEK 5881/6L6WGC but 1 broke in transit so tube depot provided new matched set of JJ 6L6GCs as replacement tubes.
    The amp passes signal OK and output is not distorted as seen on a scope.
    1. Swapped bias probe and tube over and the problem stays with the sockets. V9 & V10 always have no current.
    2. Checked the pentode/triode switch and its set to pentode. Tested switch with amp off and switch appears to be operating normally.
    3. Checked the fuse for V9/V10 and this is OK.
    4. Voltages to earth on all four sockets seem almost the same 3 is 478v, 4 is 475.5v, 5 -47.6v 8 is 0.011. Heaters to earth 2&7 are approx. 3.5v ac
    5. All screen resistors are 470ohm 5w and good.
    6. Checked the low power switch: set to high and tests OK.
    7. R272 looks a bit cooked but measures correct at 100K
    8. AMP had an extra 5K6 resistor on pin 5 of V11 (removed since schematic error)

    Did I miss something obvious?
    Don't want to remove circuit board unless I have to but running out of obvious things to try.
     

    Attached Files:

  2. anitoli

    anitoli Well-Known Member

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    Are you 100% sure there is nothing wrong with the probes?

    Pull the outer pair, does the amp work?
     
    thetragichero likes this.
  3. ntrueman

    ntrueman New Member

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    I had moved the probes about and it stayed with the socket so not the probes. But I just looked closely at the sockets and it looks like they are a bit loose. Outside two are fine. It's like the inside pair have been used a lot. The fuse holder for that pair looks like its seen a lot of use too. Obviously some history there. I'll try re-tensioning them first and if that doesn't work I'll change them. You led me too this because after I checked the probes I wondered what else could cause a connection issue, so thanks buddy appreciate the instant help. btw. when I looked at the probes pins they seemed a fraction thinner than a normal tube pins and obviously that made connections even worse. This is the first 6100 I've worked on and it's certainly got a lot of options to throw you off the scent. I nearly broke my motto of "try the easy things first" :) Thanks again!
     
  4. anitoli

    anitoli Well-Known Member

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    No problem, hope you get it sorted out, keep us posted.

    Check the solder joints on the tubes they can fracture over time and have poor connection.
     
  5. South Park

    South Park Well-Known Member

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    You need to put that 5.6 k resister back in . Do you have voltage to plates and bias on all four tubes . Taking out that resister might have cooked something on that board .
     
  6. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    If you say no current being drawn at all then what is the voltage on the cathodes, high, low or nothing at all
     
  7. anitoli

    anitoli Well-Known Member

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    There is a typo on the schematics where V11 has TWO 5.6k gridstoppers in series. It's not necessary as there already is a grid stopper on the board for each tube. This typo occured after the switch to 6l6's it wasn't on the eL34 schems. His problem is on V9-10 the inside pair.

    He says this about voltages:

    4. Voltages to earth on all four sockets seem almost the same 3 is 478v, 4 is 475.5v, 5 -47.6v 8 is 0.011. Heaters to earth 2&7 are approx. 3.5v ac

    So all the sockets are getting juice but dead on the inside pair? Put the outside pair on V9-10, does the amp work now?

    [​IMG]
     
    Last edited: Jul 14, 2020
  8. Marcell

    Marcell Active Member

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    Do you measure anything on the fuse?
     
  9. anitoli

    anitoli Well-Known Member

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    +1.
    R272 is burnt according to OP. Check the output fail fuse for V9-10. If it's blown it means either of those two tubes are no good.
     

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