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5F1 AC voltage low

Discussion in 'Shock Brother's DIY Amps' started by Clintstone, Apr 16, 2018.

  1. Clintstone

    Clintstone Tele-Meister

    150
    Jan 17, 2016
    Brandon Manitoba
    So I finally got my transformer wiring straightened out and powered up my amp, no speaker or tubes installed. My wall voltage was 119 and my light bulb limiter did not glow so that should mean nothing is shorted to ground. With my volume/POWER switch turned off voltage at the fuse is only 88 volts. Following the power wire to the back of the switch I have 88 volts on the power input terminal and 22 volts on the output (SHOULD THIS NOT BE 0V) turning the power switch on I have 33V on each terminal. Any ideas?
     
  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Do you a schematic or layout? Is this an original 5F1? If it is origina or if it is wir3d as original, the AC input would best be rewired....with the white neutral going directly to a PT primary while the black hot goes through the fuse to the switch to the other primary. In this scheme, the voltage coming out of the PT secondary is your test point for the AC voltage to the rectifier.....and that voltage is determined by the PT design as stated in the spec sheet.....given whatever voltage is coming from the wall.
    If you have the AC input wired as in the original, then the Ac voltage tested from the black feed to the switch to the white contact on the fuse should equal the wall voltage.
     
  3. Clintstone

    Clintstone Tele-Meister

    150
    Jan 17, 2016
    Brandon Manitoba
    It is a kit that I built, I wired the PT the way Rob Robinette has his layer out on his website. With the white neutral going to the PT primary and the hot black wire going through the fuse to the switch then back to the other primary.
     
    Wally likes this.
  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Well, how are you measuring the AC voltage from the black to the white? Is the connection of the white to the PT primary still accessible?? That is the only way you could check the incoming AC voltage.
    Where are you placing the probes for this check?.
    Have you checked the PT red secondary for AC?
    And...yes....with the switch in the off position, there should be no voltage on the outgoing terminal. Unplug the amp and use your ohmmeter to do a continuity test across those terminals on the switch....Off should show open/infinite resistance, On should show continuity. Check for continuity from these contacts to ground, which would be a fault.
     
  5. Clintstone

    Clintstone Tele-Meister

    150
    Jan 17, 2016
    Brandon Manitoba
    I looked at your post Wally then I read it again and realized I was trying to read the voltage with my ground probe connected to chassis ground ( can you tell I used to be an automotive mechanic) I measured across the switch terminal and it is not shorted. so I installed the tubes and speaker and plugged it back in. Nothing is on fire so this is good, I turn on the power switch and I see the rectifier tube and the power tube start to glow, this makes me happy. After about 20 seconds I start to get a loud high pitched squeal from the speakers, this doesn' sound right. I shut off the power switch, the noise fades as the tube glow disipates. I turn the switch back on, tubes start to glow and noise comes back. I recorded the sound with the camera on my phone I will try to post a video of it
     
  6. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Sounds like you need to reverse the primary leads on the output transformer. Pretty common for new builds (especially Champs for some reason).
     
    JuneauMike likes this.
  7. dan40

    dan40 Tele-Holic Silver Supporter

    958
    Aug 19, 2015
    Richmond Va
    Follow clintj's advice and your amp should be good to go. Another way to test for reversed primaries would be to unsolder the nfb wire that leads from the output jack back to the board. If the amp sounds fine (it will be a little more hairy and overdrive easier) with the nfb lifted, this means your OT leads are reversed and need to be swapped like Clintj mentioned.
     
  8. Clintstone

    Clintstone Tele-Meister

    150
    Jan 17, 2016
    Brandon Manitoba
    problem solved, I reversed the leads like you said and we have sound. I can't wait to get a cabinet built for this little sucker.
     
    robrob and corliss1 like this.
  9. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    Problem not solved unless you determined what was the reason for the voltage inconsistency. You don't sy if the voltage readings were per Wally's posts or not.

    Amps will work with all kinds of weird voltages - but it doesn't mean they are working correctly. It can also mean you may suffer an early..and expensive...transformer failure.

    So if you haven't solved that issue - do it.
     
    Wally likes this.
  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    IMHO, kits should come with some certain basic info and paperwork....such things as tube pinouts, schematics as well as a layout, and blank voltage charts. But.....that might put some off when thinking about building because such an approach could be seen as a demand that the builder have some basic knowledge that imho is pertinent to coming to an understanding as to what it is that they are building. Imho any tech worth paying will have checked the voltages during any work and as a final step of the work. IMHO, repair work should be finalized by noting...in writing to the owner—-even if it is one’s own self—-with some basic voltage and biasing numbers. This is a service for the amp’s user and is a baseline for any future work on the amp.
     
    Silverface likes this.
  11. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    ^^^^This.
     
    Wally likes this.
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