NAD 1966 Blackface Band-Master

Discussion in 'Amp Central Station' started by HD28, Sep 15, 2021.

  1. HD28

    HD28 Tele-Meister

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    Thank you!
     
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  2. HD28

    HD28 Tele-Meister

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    6l6 resistors
    V6 grid stopper carbon comp 1550 ohms
    V6 screen grid ceramic brick made in Mexico 440 ohms
    V5 grid stopper 1694 ohms
    V5 screen grid 449 ohms
    Should these be changed out?
    Thanks again Wally!
     
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  3. Jared Purdy

    Jared Purdy Friend of Leo's

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    I love reading your replies. Encyclopedic. Always a teachable moment.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    Thanks, Jared….just sharing some tidbits I have picked up reading about and looking at amps.
     
  5. Wally

    Wally Telefied Ad Free Member

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    HD28, like most things people debate the question of matching stoppers. I prefer them to be matched. Those two screen grid stoppers are not a bad match, but they are low numbers for 470ohm resistors. I order these in batches and go through the batch matching pairs…writing the value on the side of the ‘brick’. I don’t think Inhave ever seen anything below 460 ohms or above 480 ohms when looking at nominal 470s. .
     
    Last edited: Sep 27, 2021
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  6. Old Plank

    Old Plank Friend of Leo's Gold Supporter

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    Cool, nice amp, enjoy it!
     
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  7. Sparky472

    Sparky472 Tele-Holic

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    Congrats! I have a lot of love for the Bandmaster.
     
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  8. HD28

    HD28 Tele-Meister

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    I finally had some free time to install the parts on the band-master.
    3 prong plug
    Desoldered the death cap
    Changed the grid stopper and screen grid resistors to matched pairs.
    Changed all e-caps
    Found a broken solder joint at the 100 ohm nfb resistor
    Changed the 820 ohm nfb resistor because I thought it was toast. Because it read 98 ohms. The new one reads 94 ohms once installed. Lesson learned.
    Wrapped the tremolo roach in some high tech lineman tape I had
    Changed all 3 tremolo caps to .022 orange drops
    Brought up to line power with variac
    Adjusted the fixed bias to 69% on both 6l6’s. 450-455vdc 46-49mA
    Heater 6.9vac
    All caps read less than 0.002vdc on the ground side except V1 P6 0.13vdc
    Voltage readings on all tubes are acceptable.

    Except V3.
    P6 412vdc, P8 0.114vdc, P1 460vdc, P3 0.002vdc
    I swapped out the old Sylvania 12ax7 for a GT I had.
    P6 396vdc, P8 0.04vdc, P1 445vdc, P3 0.0vdc

    the trem is slow and strong
    Very nice breakup at volume of 5

    Where should I start troubleshooting V3?
     
  9. Wally

    Wally Telefied Ad Free Member

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    I take it that you are speaking of the 1500ohm and 470ohm resistors on the power tubes?? If so, both of these applications are grid stoppers. One is the control grid stopper….the 1500ohm connected to pin 5 control grid. The other is the screen grid stopper connected to pin 4 screen grid. When speaking of the grids and applied ‘stoppers’ in beam tetrodes or pentodes, we need to maintain proper terminology, imho.

    As for the tremolo circuit tube….V3. You say the circuit is functioning. Your cathode voltage readings show no voltage there. I don’t understand how the circuit functions without having proper bias voltage applied to that cathode?? Are you sure about those voltage reading?
     
  10. HD28

    HD28 Tele-Meister

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    Yup
     
  11. Wally

    Wally Telefied Ad Free Member

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    One would wonder how it is working without the control grid being negative in relation to the cathode.
     
  12. HD28

    HD28 Tele-Meister

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    I don't either Wally. I will measure these again and report back...
     
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  13. HD28

    HD28 Tele-Meister

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    Short the trem Jack and voila. Voltage galore!
    Thanks Wally.
     
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