Haze 40 Bias Voltage Issue

Discussion in 'The Workbench' started by PapaJ, Jan 11, 2018.

  1. PapaJ

    PapaJ New Member

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    Hi everyone, hoping to get some help here. Recently acquired this Haze 40, had a bad power tube so I ordered a matched pair of EH EL34s to pop in. Go to set the bias and I get no voltage between pins 1 & 2 of con3, and about 40mv between 2 & 3. No amount of fiddling with the bias pots changes the reading between 1 & 2. Any ideas where to start? Nothing obviously burnt on the board, plate voltage measures about 460V. If I measure resistance between the pins I get 1.3 ohms between pins 1 & 2 and 2 & 3, and double that between pins 1 & 3. Thanks in advance for any help.
     
  2. Micky

    Micky Well-Known Member VIP Member

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    Swap the power tubes and measure again.
    If it follows the tube it may be bad.
     
  3. PapaJ

    PapaJ New Member

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    I will try that and report back. Would be unfortunate as they are new tubes.
     
  4. PapaJ

    PapaJ New Member

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    Ok, switched tubes around. No change in readings.
     
  5. thanasisg6

    thanasisg6 New Member

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  6. thanasisg6

    thanasisg6 New Member

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    if is this the correct schematic you can not mesure on pin 1&3 negative dc voltage ,you should mesure only dc ma between 1&2 and 3&2 with the negative probe on pin 2.In this case you should read 39ma wich means 0.039V (V=R*I)on R26,R42 and your EL34s will have the correct bias voltage.Be careful the 39ma is the appropriate reading for the AB CLASS FIXED BIAS and for 70% max safe dissipation ,for 460V anode and 25w EL34 anode dissipation.You can mesure the negative bias voltage if can varying ,in the front end of R39,R50.If there is not varying negative voltage ,you have problem on bias power supply.BE CAREFUL THE HIGH VOLTAGES THEY ARE LETHAL!!!!! good luck
    https://robrobinette.com/Tube_Bias_Calculator.htm
     
  7. thanasisg6

    thanasisg6 New Member

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    check also the D9,D13 .If you can unsolder each one end ,and check for sort circuit
     
  8. PapaJ

    PapaJ New Member

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    Thanks for the reply. Everything I've read about biasing these amps says to read the mv across the pins, which will equal the mA due to the 1 ohm resistor, and Ohm's law of course. I will check those diodes if possible.
     
  9. Micky

    Micky Well-Known Member VIP Member

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    Correct you should read no voltage between pins 1&3.
    You should read mv (millivolts DC, not ma...) between 1&2 and 2&3.
     
  10. PapaJ

    PapaJ New Member

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    I do get voltage between pins 1&3, about 35mV with the negative lead on 1 and positive on 3.
     
  11. Micky

    Micky Well-Known Member VIP Member

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    Check the diodes previously mentioned, as well as the big 5W resistors R56 & R58.
     
  12. PapaJ

    PapaJ New Member

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    R58 is open, guess that's a good place to start. Unfortunately don't have a stash of 5W resistors sitting around. Will have to wait until I get a replacement. Thanks for the guidance.
     
  13. PapaJ

    PapaJ New Member

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    I lied, just found a couple of 5W 500ohm resistors in my parts drawer. Can put them in series to replace the 1K in R58, if they will fit.
     
  14. thanasisg6

    thanasisg6 New Member

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    of course!!!!!!!!!!!!
     
  15. PapaJ

    PapaJ New Member

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    Just wanted to follow up on this thread. Replaced R58 and everything is good to go. Got the bias set with no issues. Thanks everyone for the help!
     
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