Mystery hum in '67 Deluxe Reverb re-build

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by booj, Feb 11, 2008.

  1. booj

    booj Member

    Messages:
    173
    Joined:
    Jan 18, 2008
    I decided to completely re-build my '67 Deluxe. Stock it out as it were. Upon completion, I started by turning it on with no tubes. OK. Put the Rectifier in. Good DC. Nothing bad. Power tubes in. Running nicely. Good voltages. Driver tube in. Then HUMMMM. A slightly dirty 60 HZ. That is about as loud as touching the tip of a guitar cord with a working amp on "3". Voltages on the Driver are about 20 volts above the voltages listed on the schematic, which I attribute to the rest of the tubes not being plugged in. I have since,
    1. Replaced the Driver tube. zip
    2. Disconnected the 220k resistors from their preamp feed lines. Nada
    3. Removed the two .1ufs to the power tube inputs. That made it quiet
    4. Replaced the .1uf coming from the plate of the driver side that is connected to the feedback circuit. Hum is back but half as loud.
    5. Replaced the bias cap. No
    6. Swapped out the O.T. with a known good one. No change.
    The fiber board has been lifted twice. Once after de-mod and again just recently. It has been carefully cleaned and the eyelets have been reflowed. Tis old man is stumped.:horse
     
  2. PRNDL

    PRNDL Member

    Messages:
    502
    Joined:
    Aug 25, 2007
    Location:
    SW Florida
    I'd begin with checking for proper grounding, especially in the power supply.

    Is there a center tap on the heaters?

    Use a screwdriver to check all the solder blobs, and measure the resistance to ground at each, as well as on the boards.

    Did you take out the "death cap"?
     
  3. soldersucker

    soldersucker Member

    Messages:
    2,400
    Joined:
    May 20, 2006
    Rebuild? stock it out? it would help if we knew what you did.
     
  4. booj

    booj Member

    Messages:
    173
    Joined:
    Jan 18, 2008
    Allow me to clarify. Stock out means (to me) put back to stock condition. I want the "vibe" with all its flaws that the amp produced when I bought it used in 1971. Everything being the same value and the schematic followed part for part. That includes the death cap, the two wire ac cord, and the non-polarized ac plug.
    I'll moderize it, but I first want it back the way it was when I bought it. (for $75!!)
    I'll check the grounds (again), and looking at the filament wiring, I see there are no resistors from the lamp connectors to ground. Could be an issue. The original schematic shows a center tap from the filament winding. This P.T. came from C.E. Distribution, and although it has a center tap for the H.V. lines there ain't no C.T. for the filaments. Dig. I'll fix that. If it don't work I'll add it to my "tried this" list.
    Thanks for taking the time to try to help. Much appreciated!
     
  5. Bonenut

    Bonenut Member

    Messages:
    249
    Joined:
    Mar 30, 2005
    Location:
    Northern Cal
    That's your problem right there! You need either a heater center tap from the tranny or artificial center tap using two 100ohm resistors.
     
  6. BLC

    BLC Member

    Messages:
    87
    Joined:
    Jan 13, 2007
    Location:
    Two hours North Of Atlanta. Where they made that m
    Put in 2 , 100 ohm resistors on the heater wires to ground. This was originally done on the back side of the lamp.
     
  7. booj

    booj Member

    Messages:
    173
    Joined:
    Jan 18, 2008
    Did it. Done. Extremely low residual noise (hum) with the channels cranked. Donation made to the Gear Page. Old man very happy.
     
  8. booj

    booj Member

    Messages:
    173
    Joined:
    Jan 18, 2008
    Please read the text in full, John, and include (I'll moderize it, but I first want it back the way it was when I bought it.)
    I believe there's only one way to do that, even though it will be changed later.
     
  9. booj

    booj Member

    Messages:
    173
    Joined:
    Jan 18, 2008
    Thanks for the input. I had it wired with a grounded line cord when I was doing all the 120 or so mods that went along with experimenting for the perfect amp. Then I got smart and simply built an amp with its own chassis and all.
    Basically I want to see exactly what the amp does as a 1967 amplifier. (Exactly). It won't be leaving my home untill it gets grounded line cord.
    Please don't send the polaritiy ploice after me!!:MMM
     

Share This Page