Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

New Hoffman AB763 is quiet. TOO quiet

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 8, 2017.

  1. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    So I've got a one channel Hoffman AB763 on the bench. Finished up all the wiring, so I fired it up for the first time last night.

    Pilot light came on fine, filaments lit up, but no sound whatsoever through the speaker. No hum, no hiss, no nothing. It was late and I was tired, so I didn't check voltages or go through the schematic verifying connections or any of that stuff.

    But does that fact (the total lack of hum/hiss through the speaker) lead you to think of one thing over others? Grounding issue?
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Output jack wiring, or the shorting contact is not opening when a speaker cable is inserted. Check power tube plate voltage and see if you hear anything through the speaker. A few pics will help, too.
     

  3. nathanh

    nathanh Tele-Meister

    Age:
    32
    199
    Oct 13, 2016
    DFW
    I had this happen to me when I forgot to connect the NFB resistor to the output jack.
     

  4. Platefire

    Platefire Friend of Leo's

    Age:
    70
    Nov 3, 2003
    North Louisiana around Many
    Does it have a standby switch, was it on??? that will silence everything.
     

  5. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    Well, I didn't get any time to look at it last night.

    I'm thinking speaker jack too. I screw that up every time.
     

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    If you plug the speaker into the Aux speaker jack by mistake you won't get much signal to the speaker.
     

  7. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    OK, I think I may have figured it out. I currently have two jacks, the main and an extension (both switchcraft shorting jacks). When I wired it up, I wired the NFB resistor (820K) to the extension jack, not the main. Electrically I believe this is the same as not connecting the NFB at all.

    I’m not in front of the amp to check/correct it, but do you agree that would cause the issue I’ve described?
     

  8. nathanh

    nathanh Tele-Meister

    Age:
    32
    199
    Oct 13, 2016
    DFW
    If it's off coming from the tip it shouldn't matter which jack it comes off of as they're wired together. If you wired it from the Ground then it definitely shouldn't work. Got any pictures?

    EDIT: I was looking at the wrong layout. Yes, connect that puppy to the correct jack!
     

  9. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    only one shunt to ground should be used here on the primary jack. leave the aux shorting tab open
     
    nathanh likes this.

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    ^Yep, remove the jumper between the ground terminal and switch terminal on your aux jack.
     

  11. nathanh

    nathanh Tele-Meister

    Age:
    32
    199
    Oct 13, 2016
    DFW
    How did this turn out?
     

  12. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    It hasn't, as of yet. I redid the speaker jack wiring- took out the extension jack entirely, I've never used one in my life. Still nada.

    Haven't really had time to troubleshoot any more. Kids, job, etc.

    I'll have to do the old highlighter on schematic trick. I'll try to post some voltages too in a day or 2.
     

  13. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    OK, I'm back. I'm sure you were all in suspense.

    Amp still totally silent. I come bearing Voltages-weird ones. This is basically a one channel Super reverb. I haven't connected the NFB yet:

    Wall Voltage- 125V
    V1: Pin 1- 463V, Pin 3-390V, Pin 6-463v, Pin 8-393V
    V2: Pin 6-474v, Pin 8-393V
    V3: Pin 1- 465V, Pin 3-393V, Pin 6-465v, Pin 8-393V -Got some crackles through the speaker when touching the probe to pin 6
    V4: Pin 1- 232V, Pin 3-2.33V, Pin 6-475v, Pin 8-241V
    V5: Pin 1- 265V, Pin 6-262V, Pin 3/8--107v- Got some crackles through the speaker when touching the probe to pin 3/8
    V6: Pin 3-475 V
    V6: Pin 3-475 V

    Just using the super reverb schematic as a reference, These all look very high except for the phase inverter and the power tubes.

    Any thoughts based on that, especially the fact that the voltages on the 6L6s look reasonable? Does the fact that I'm getting some crackles mean the speaker jack isn't grounded?
     

  14. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    Something's very wrong as I've never seen 300V+ on a cathode of a preamp tube. Not even entirely sure how that's possible, if the cathodes are grounded to the chassis. Check that your chassis isn't 'hot' as in HV flowing through it. Check all your PT connections.

    I feel as if your chassis is getting ~475V from a tap on the tranny, and that's why the preamp cathodes are all seeing close to 400V. The first stages should be roughly around Negative 1.5V DC if I'm not mistaken
     

  15. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Need pics. There's obviously something really amiss here, like a major wiring issue or a missing ground or three.
     

  16. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    It's almost like you don't have tubes in there. No current being drawn, everything is settled at B+. Does seem like a ground issue.
     

  17. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    So im sitting here at my daughtets lacrosse tournament thinking about my PT. I'm using a Mojo 762 (see attached)

    For the secondaries ive got:
    Green: to heaters
    Yellow: recto pins 2 and 8
    Red: recto pins 4 and 6
    Red/yellow: ground (maybe this is my issue- should this be no connection?)
    Red/blue-to bias point
    Orange (faraday shield)- not connected.

    Should i connect orange to ground? Remove the red/yellow?
     

    Attached Files:


  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Your PT wiring looks correct.

    Yes, connect Orange to the chassis.

    The Red/Yellow high voltage center tap must be grounded to the chassis.

    Are your power tube cathodes connected to ground? With the amp off measure the resistance between the power tube pin 8 to the chassis and to the Red/Yellow wire.
     

  19. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    First off, thanks so much for the help. All of you guys on this board are the best.

    What do you mean by the red/yellow center tap must be grounded to the chassis? Is there a center tap "between" the Red/Yellow and the red?

    So, if you look at the Hoffman AB763 one channel schematic (attached), the PT cathodes are wired to a 1 ohm resistor and then to ground.

    I checked resistance as you suggested:
    V7- pin 8 to chassis- 1.0 ohm; Pin 8 to red/yellow wire- 0.8 ohm
    V8- pin 8 to chassis- 1.0 ohm; Pin 8 to red/yellow wire- 0.9 ohm

    These were taken with the red/yellow wire still connected to ground. Is that correct?
     

    Attached Files:

    Last edited: May 21, 2017

  20. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Your resistance readings look good so you have continuity between the power tube cathode pins and the PT center tap (the red/yellow wire is the center tap).
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.