Bassman 5F6A Build Issues

Discussion in 'Shock Brother's DIY Amps' started by Guitarnut, Sep 26, 2016.

  1. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Hello all, I'm building my 2nd 5F6A Bassman from the kit that Dave at Boothill sells. I sourced my transformers from Classic Tone. I have everything wired and was testing voltages when I ran into a few issues.

    1. The bias tap shows 50.2 volts on the diode's cathode but only .4 volts on the anode. I've tried several diodes, flipping it around...no change in any of these scenarios. The tap when removed from the circuit meters fine just like at the cathode.

    2. The heater taps off the transformer are lighting the jewel but meter at 95V!!! Also, they meter the same at the sockets. I removed the heater wires, and it lights the jewel but still shows the same voltage. Took the wires off the lamp and still way too high. I know during pre-install metering, they showed 6.3V.

    I've been over my wiring and the circuit layout at least 10 times. I'm using a dim bulb tester thru the entire build so I don't think I shorted anything out. This is making me crazy, I can't figure out the bias and why the lamp doesn't blow at 95V is odd. I thought they were 6V lamps. I don't have any pics at the moment but will post some later.

    Any advice for trouble shooting would be greatly appreciated.

    Peace,
    Mark
     
    Last edited: Sep 26, 2016
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,367
    Joined:
    Dec 29, 2012
    Location:
    United States
    The bias voltage at the output end of the diode must be a negative voltage. Check the entire bias circuit including the 220k power tube grid leaks, something might be shorting to ground and giving you a very low output voltage.

    Are you measuring your 6.3v heater wires wire-to-wire? You should be. Do you have a 6.3v center tap or artificial center tap?

    [​IMG]

    [​IMG]
     
  3. dan40

    dan40 Friend of Leo's

    Posts:
    2,200
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Are you setting your meter to DC when measuring the anode side of the diode?
     
    King Fan likes this.
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,367
    Joined:
    Dec 29, 2012
    Location:
    United States
    Great question Dan.
     
    dan40 likes this.
  5. dan40

    dan40 Friend of Leo's

    Posts:
    2,200
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Also be sure to check the batteries in the meter. It's not unusual to see weird readings with a low battery,
     
    clintj likes this.
  6. clintj

    clintj Friend of Leo's

    Posts:
    4,957
    Joined:
    Apr 4, 2015
    Location:
    Idaho
    Quick check that's saved a few people before as well: measure something known, like a battery, with your meter. I keep a 9V taped to my chassis cradle for a reference voltage.
     
  7. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Thanks. I have a 6.3v center tap and was measuring wire to chassis. I removed the heater circuit going to the sockets and checked wire to wire...6.4v. Gotta check my wiring at the sockets.
     
    Last edited: Sep 27, 2016
  8. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Bingo!!! Something so simple and I completely spaced that the diode is a rectifier. I'm getting a small voltage drop and healthy neg VDC now...guess I was all along. Thanks!
     
    Last edited: Sep 27, 2016
    dan40 likes this.
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,367
    Joined:
    Dec 29, 2012
    Location:
    United States
    Make sure your 6.3v center tap is secure. If it isn't connected to ground you can see crazy voltages like you got measuring from the pilot light terminals to ground.

    If your 6.3v transformer wires are still disconnected you can pull all the tubes and just check for continuity between the beginning of the heater wires to each socket heater pin. Also make sure you don't have continuity between the 6.3v wires and ground.
     
  10. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Thanks! I removed the crimp on connector from the CT and soldered it to the ground bus. All seems fine now. I'll also check for continuity as you suggested. I can check adjoining pins as well.

    Peace,
    Mark
     
  11. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    A side note...I received a notice of the first reply to this thread but nothing since and I have been back between postings. Is this a common glitch. I used to get flooded with replies back in the day.

    Peace,
    Mark
     
    Last edited: Sep 27, 2016
  12. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Duplicate...deleted.
     
  13. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Thanks. New batteries. I don't build that many amps so I always start with fresh batteries on each build. Cheap insurance.

    Peace,
    Mark
     
    dan40 likes this.
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,510
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Did you turn on notifications when you first posted? I sometimes forget to.
     
  15. Guitarnut

    Guitarnut Friend of Leo's

    Posts:
    4,230
    Joined:
    Jun 29, 2009
    Location:
    USA
    Yep. I received the first reply like I mentioned above. Nothing since. I didn't get notified that you posted this question.

    Peace,
    Mark
     
  16. Mr Ridesglide

    Mr Ridesglide Friend of Leo's Silver Supporter

    Posts:
    2,152
    Joined:
    Dec 11, 2009
    Location:
    Bloomington, MN
    Is the amp up and running right now? The Boothill 5F6A I put together a few weeks ago is pretty incredible. Took me a bit to get the bias right on power tubes, but it is just about a perfect amp.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.