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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

5F2A Weirdness...Troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by keithb7, Nov 24, 2017.

  1. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Team, your comments are appreciated. I am reinvigorated and pumped to get back on this.

    The only downer is my demanding full time day job is holding me back. Being a part time amp hack I forget more than I’ve learned it seems.

    I do have an O-scope. I have played with it a bit. It’s still fairly foreign to me. I also have a signal generator I can pump into the preamp. What I still need it a bigazz 100W wire wound resistor dummy load. Its painful hearing 1,000 hz Wave through the amp. I’d like to dig the scope back out and learn more with it. If ya’ll will guide me through it.

    I did use my DMM last night to pick up the milli-volt AC signal from strumming the guitar, at V1 input. It’s there.

    More to come.
     
    Last edited: Dec 1, 2017
    t guitar floyd likes this.

  2. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    I should point out that in my amp fuel'd rage this week, I:

    1. Burnt my face with my soldering iron. LOL. It can happen!
    2. Took 480 volts DC through my right index finger. Off preamp stage. Amp was unplugged. I had drained caps about 40 times before this incident. It only takes once.
    3. Walked away, left with a reminder when I look in the mirror...Stupid hurts.
     
    dr_tom, jsnwhite619 and Axis29 like this.

  3. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Ow again. You’re a good guy, Keith! Be careful up there.
     

  4. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Alright. Wounds licked. Repaired and slammed another amp out this week. No more screwing around. Back to this 5F2A. I took some voltage measurements tonight. See attached schematic. I see something that tweaks me, but I am not sure why I am seeing this symptom.
    See pic below:

    Far left 8uF cap in power supply. 293V measured. V1 stage 1, through 100K resistor, voltage drops to 182V. Yet second stage of V1, through neighbour 100K resistor voltage only drops to 275V. Both resistors measure in at 99.8K ohms. Why am I getting different voltage drops here? Something wired wrong maybe?

    [​IMG]
     
    King Fan likes this.

  5. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    How much voltage drops across plate load resistors is directly proportional to the current through them.

    V1a

    293V - 182V = 111V
    111V = I * 100K
    I = 1.1 mA

    V1b

    293V - 275V = 18V
    18V = I * 100K
    I = 0.18 mA **** really low!

    Is your 1500R bias resistor on V1b really 1500R? Is its ground connection good?
     
    Last edited: Dec 6, 2017 at 2:39 AM
    King Fan likes this.

  6. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    That's normal; the tubes are conducting different currents through the plate resistors, due to differences in their characteristics.
    Edit: while differences are normal, that's a really large difference, as snfoilhat pointed out.
     
    Last edited: Dec 6, 2017 at 2:14 AM
    King Fan likes this.

  7. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Maybe it's a 15k instead of 1500? Or maybe that triode is wonky.
     

  8. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    what are the cathode voltages?
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I looked back through the thread just now. You said you replaced the NFB resistor, that's the 22k from the speaker jack right?

    Check the junction of the 22k, 1.5k, and lead from V1 pin 8 very closely. If that joint is compromised, or the 1.5k is toast, that would cause your issues.

    I'd lift the NFB lead at the speaker jack and verify that circuit. That lead should measure 23.5k to ground, and V1 pin 8 should be 1.5k to ground.

    Since the power tube arced, the cathode capacitor on it may also have been damaged by overvoltage. Clipping a new one in around it, or temporarily replacing it, would also be something I'd try if getting V1b under control doesn't solve this.
     
    King Fan likes this.

  10. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Folks I appreciated the positive feedback. Amp pun intended.
    I will look into each and every comment as I am still trying to find the stubborn gremlin in this:

    It measures 1.484 K ohm.... Wait a minute! Hold my calls. I see something...

    [​IMG]


    Hooked up wrong. Doh. I checked everything so many times. My board is not laid out like the Weber layout in post #10. The 22K NFB resistor into the 1500K resistor, is to the right of the 6V6 cathode cap and resistor. Threw me off, and I was too stubborn to see it. I had the 1500R connected to the turret at the coupling cap to the right of it.

    Thanks folks for putting up with a couple of cranky sessions lately. Amp is now better than it was. I rewired it with cloth push-back wire. Reduced voltage now by way of zener diodes.
    New resistors. 2 new coupling caps. A 470R 5W grid resistor added. It sounds great.

    Humbly knocked back a few pegs, gained more experience and tried a few new things. All in all a good experience.
     
    Last edited: Dec 6, 2017 at 7:58 PM

  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Congratulations, Keith!
     

  12. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Congrats on finding the problem, relief at last! That pic of the shocked infant cracked me up, perfect choice.
     
    D'tar likes this.

  13. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Way to hang in there, Keith. Your little snafu pales in comparison to some of my ‘creative’ wiring.

    Gonna show us the new work and cloth-covered wire?
     

  14. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    360
    Feb 2, 2015
    Brick, N.J. USA
    Congrats.
    Lesson for all of us on not seeing what is plain in front of us even though we checked it over and over again.
    I do it at work all the time on AutoCad drawings.
     

  15. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

    Jan 2, 2007
    Virginia, USA
    I knew you'd get it sooner, rather than later! You da man!
     
    keithb7 likes this.

  16. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Congratulations on finding the problem. Like someone told me a while back, the problem is somewhere in that bread pan. Ya just gotta find it!

    I'll bet that amp is getting a workout again. :)
     

  17. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Red, represents the rage that I was in. All red. Diode pack is near PT as seen.

    How did this silly thread hit 2000+ views? Lots of y'all support. Thanks.

    [​IMG]
     
    Last edited: Dec 7, 2017 at 10:11 PM
    dan40, King Fan and RLee77 like this.

  18. jmp81sc

    jmp81sc Tele-Meister

    161
    Feb 6, 2015
    Orange County
    Hey Keith congrats on your build and trouble shooting success.

    I am going through a head scratcher trouble shoot right now and can't wait to have my DOH moment.
     

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