1974 mod #1987 50 watt lead

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by dad, Jun 6, 2005.


  1. dad

    dad Guest

    Recent amp works great!
    awaiting new speakers for cab ,pulled the chassis and wanted to check bias settings.. found all connections in the entire amp to appear original with the red dye on them.No resistors have ever been added to #1 &#8
    lugs on the power tubes.
    Plate readings at present are 341 on both ,is there any way i can get the readings without adding the resistors ???

    Is this what the bias probe type adapters do?

    thanxx
     
  2. Swarty

    Swarty Member

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    Yes, and 341V is suspiciously low.
     
  3. dad

    dad Guest

    ------------------------------------------------------

    Swarty when i checked the plate voltages i was grounding from the chassis ! is this a proper ground?

    I checked both of them 2wice also as I had thought they would be higher ..

    thanxx
     
  4. Swarty

    Swarty Member

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    It is proper. FWIW, I have a '71, and after putting new filter caps in it went from 370V to 410V.
     
  5. dad

    dad Guest

    I was a little curious after seeing the voltages most list ..

    Not going to change any thing in this just yet as the boy is still learning and the amp works fine for now. I am going to get a weber bias tool so i can monitor things more easily and later we will replace the original power cord and do a few small changes,If it requires new caps to bring the voltage back up then we will do that also.
    This amp has been played like this for a long time and has no apparent problems ,So it seems it must be some what normal for now .
    thanxx again .
     
  6. RussB

    RussB low rent hobbyist Silver Supporting Member

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    If that amp amp still has the original filter caps you really should change them (or have them changed by a pro) ASAP!

    That amp has been around for a long time, and that's as bad as it gets for an electrolytic capacitor. That low voltage may be an indicator of very leaky filter caps...for God's sake dad, service that amp! It's too valuable NOT to, don't wait 'till a cap fails, it could take a nice stock transformer out with it!
     
  7. dad

    dad Guest

    I wasnt aware of that RussB !

    Well before we put the new speakers in his 4x12 i will see about getting all the things done we need ,just didnt want to change anything if i really didnt need to as none of the circuits appear to have been changed /resoldered or anything.

    SO we should:

    Replace all the caps ,while we are putting in the new power cord and have the resistors added @# 1 & #8 on the power tube sockets for obtaining bias readings, have the amp biased.
    Any other reccomends for service while we have the amp down ?


    thanks guys ,
    yes the old girl has been around since 74 and i would like to see the boy get as much service from this amp as possible.
     
  8. pfrischmann

    pfrischmann Member

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    There's more.
    Are there 1K 5 watt resistors soldered between two of the pins on the output tubes? (I think it's 3 and 5 but don't quote me on that)
     
  9. dad

    dad Guest

    -----------------------------------------------------------------
    There are 2 resistors in the pic below on the power tube sockets.
    I believe they are on 4 & 6.

    They are fat green ones marked (74 W 08 ) With an Ohms insignia encasing the w ,same on each one.

    Duh ! i guess i had better add the link! : http://img.villagephotos.com/p/2005-5/1019293/powertubebottom.jpg
     
  10. pfrischmann

    pfrischmann Member

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