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HELP please OTL tube headphone amp (again)

Discussion in 'Audio' started by Ian Mason, Jan 10, 2018.

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  1. Ian Mason

    Ian Mason

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    Dec 24, 2017
    Thank you to everyone for all of the help, thanks to you the valves are now glowing like the sun, there were two problems, one was that the 2 resistors were not in p3 and p6, they were wrongly attached to the two 1000uf/ 300v and the second problem which was highlighted was that the 6.3v windings were not making a good contact with the valve pins.
    However a new problem has arisen, I now have a dreadful hum (very loud) coming through the headphones, please could anyone tell me if I have wired anything wrong with regards to the audio connections, as usual I have added photos. Thanks again the progress feels rewarding.
     

    Attached Files:

  2. duke37

    duke37

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    Jan 9, 2011
    Remove the 6SN7 and earth the grids of the 6080. Does it still hum?
     
  3. Ian Mason

    Ian Mason

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    Dec 24, 2017
    I have removed the 6sn7 and the hum is barely audible where as with the tube in place the hum is very loud, how do I earth the grids of the 6080
     
  4. duke37

    duke37

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    Jan 9, 2011
    Take a lead with a crock clip at each end and clip one end to the grid and the other to the chassis.
     
  5. Ian Mason

    Ian Mason

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    Dec 24, 2017
    Trevor, please forgive my ignorance but what exactly do you mean by GRID please, do you mean one of the 6080 pins or any positive connection. Sorry
     
  6. duke37

    duke37

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    Jan 9, 2011
    In #13 I gave the pin connections of the 6080 which are the same for the 6SN7.

    Starting at the reference tag, and working clockwise, the connections are
    grid
    anode
    cathode
    grid
    anode
    cathode
    heater
    heater
    The 6080 is two valves combined, one for each channel.
    You should measure the cathode voltages to check there is no damaging current. My valve book suggests that the cathode voltage should be about 30V,

    It looks as if the trouble is with the 6SN7.This could be wrong connections or poor power supply smoothing.
     
  7. Ian Mason

    Ian Mason

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    Dec 24, 2017
    I have a new multimeter arriving tomorrow with crocodile clips so I will check the voltage then, I don't think that there are any wrong connections
     
  8. duke37

    duke37

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    Jan 9, 2011
    I am worried, I woke in the middle of the night and could not get to sleep again.

    The filaments seem to be excessively bright which may indicate far too high a voltage on all components.
    Since it has run in the present condition for a while perhaps another 2 seconds may not do any more harm.

    Measure the filament voltage. The meter must be on ACV.
    Anything over 7V, switch off quick.

    You have rewired the transformer primary. Have you done this right? The mains neutral should be connected to the wire with the black tag nearest to the core. The next white wire should be connected to the nearby wire with a black tag and nothing else. The remaining white wire goes to mains live via a fuse and switch.
     
  9. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Aside . . . . .
    Its been so o o o o o o o long since I have had a 6080 in my hands . . . .circa 52 years . . . that I even forget a normal orange glow from them .
    BUT . . . that top view of the 6SN7 has its filamants appearing to be impinging on up towards the incandescance threshold.
     
  10. Ian Mason

    Ian Mason

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    Dec 24, 2017
    Measurements have been taken for pins 7 and 8 on both valves and voltages are a perfect 6.2, I am yet to take grid measurements as I feel required to drain all capacitors.
     
  11. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    I'll still bet you want to know as fast as is possible, so go over to the 47 K volume pot on a channel and solder a short jumper wire between the center tab of the control pot and its ground tam.
    BAMMMMMMM . . . . grounding accomplished on those voltage void connections and not even getting near any B+ high voltages.
    Fire up and see if all of the HUMMMMMMMMM is now gone, or, if not . . . the DC purity of the 6SN7 supply voltage will then be suspect.

    73's de Edd
     
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  12. duke37

    duke37

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    Jan 9, 2011
    Hi Edd,
    Most of the hum had gone when 6SN7 was removed. The HT supply was still connected so I think that the problem is not in the supply.
    Grounding the grids (p1 and p4) should remove any input unless it is being input via the cathode. The heaters are working OK so I do not think that a heater wire is connected to a cathode but should be checked.
     
    Ian Mason likes this.
  13. Ian Mason

    Ian Mason

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    Dec 24, 2017
    The hum has been significantly reduced by tidying up the 6.3v wires and twisting them however it appears that I have wired the potentiometer wrongly as the volume is going neither up down and the sound in general is just poor, can anyone provide a small diagram please to show which pins that the audio cables from the valve pins 1 and 4 of the 6SN7 and from the rca inputs should be going please.
     
  14. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Ian Mason . . . . .

    The top relates to your pot terminals identifications related to circuitry connections.

    The adding of the bottom pot is a procedure for establishing a phantom variable
    adjustable center tap on that 6.3 filament winding and thus balancing out the two
    potential phases of AC that could be radiating from the filament wiring to sensitive
    1st grid circuitry.
    At pretty close to the center of pot setting, the hum should null out and abate, or
    to either side, it should start to increase.



    upload_2018-1-13_11-40-41.png

    73's de Edd
     
  15. duke37

    duke37

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    Jan 9, 2011
    I do not think there is a hum bucker pot in this.
    There are two pots on the one shaft with the centre tag of each trio connected to a grid.

    When you have the connection correct, the resistance from the grid to chassis should be minimal when the control is turned to the left and up to the value of the pot when turned to the right.

    The spare contact of the trio is the input.and should go to the phono socket.
     
  16. 73's de Edd

    73's de Edd

    2,805
    1,176
    Aug 21, 2015
    The hum bucker pot is an ADD ON that can be utilized if any hum pick up from filament wiring still happens to present a problem .
    The top pot illustration is utilized for each channels volume control.
     
  17. Ian Mason

    Ian Mason

    32
    0
    Dec 24, 2017
    Thanks Edd, you really are very helpful but unfortunately I only speak thicko, could you please tell me if the cable from the rca inputs goes into the valve base pins 1 and 4 or does it go directly to the pot, if they go to the pot then are they joined at the same pot tam as the wires that do come from the valve pins, I have a pot that 6 pins. Please explain in the simplest terms for my pea like brain. Apologies for the rubbish pictures
    Thanks again Ian
     

    Attached Files:

  18. BobK

    BobK

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    Jan 5, 2010
    The middle pins go to the grids.

    Looking at the pot from the side of the shaft, with the pins down, the signal goes to the pins on the left. The other two piins go to ground.

    Bob
     
  19. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Ian Mason . . . .


    Using YOUR dual pot below

    A s per pot " tam " . . . . . . . . .tap / (terminal) had become " tam " when ripping off text at 100 WPM and not later proof reading.

    upload_2018-1-14_13-35-29.png

    73's de Edd
     
  20. Ian Mason

    Ian Mason

    32
    0
    Dec 24, 2017
    Ha ha, thanks
     
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