Help with bias circuit issue

Discussion in 'Amp Tech Center' started by _Steve, Jul 31, 2020 at 5:04 PM.

  1. _Steve

    _Steve TDPRI Member

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    I feel a little bad starting another thread on the same build - im still very much learning about this world!

    Im having a very strange issue with the bias circuit of the 6G3 im building from a AO-29 chassis and transformers.

    My bias circuit is only putting out around -8VDC, when it should be around -27 to -32. I have tried swapping all the components and have now even recreated the entire circuit with new components on a breadboard outside the chassis to try to eliminate possibilities but i get the same incorrect result!

    I then simulated the circuit in LTSpice using the exact values on my breadboard and I get a correct voltage of around -27VDC.

    I must be doing a really stupid noob mistake. Can anyone please spot the differences in these two circuits?

    LTSpice (same component values as breadboard - produces correct -27V):
    LTSpice_Bias.PNG

    My breadboard version (produces -8.3VDC - AC jumpered from PT, other alligator clip is grounded back to chassis):
    breadboard.jpg
     
  2. tubegeek

    tubegeek Friend of Leo's

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    A bias circuit has its electrolytic grounded at the + terminal, not the - terminal like a B+ filter cap. Check that and also test your diode.
     
  3. _Steve

    _Steve TDPRI Member

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    Haha -27.5VDC! That did it! THANKS!! Thats 4 hours i'll never get back lol

    The cap polarity had crossed my mind so I had already double checked it with a Mojotone schematic. Look at this:
    20200731_141110 (2).jpg
     
  4. tubegeek

    tubegeek Friend of Leo's

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    Bastards!

    Drop them an email. That totally sucks.
     
  5. _Steve

    _Steve TDPRI Member

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    Well I didn't buy anything from them so I can't complain lol

    Amp is up and running - thanks for your help!
     
  6. tubegeek

    tubegeek Friend of Leo's

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    How's it sound?

    Rock on!

    And - they're still bastards if they are sharing a wrong 'un.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I've emailed them a couple times and they are usually quick to respond when you point out things like that.
     
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  8. King Fan

    King Fan Poster Extraordinaire

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    Agree, Mojo are usually responsive. But if you didn't buy from them (or in fact if you did), word to the wise -- their layouts and schematics aren't perfect. Most kit diagrams and instructions have problems. They're small shops and they employ earth humans.... :)

    Unless you can find a known-good diagram from someone like Rob, and especially for things that don't change over the years, Fender is often the best source.

    upload_2020-8-1_8-27-1.png

    But even Fender schematics have occasional 'mistakes' -- usually things that changed on the shop floor after the drawing was printed...
     
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  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Had the same problem while servicing a 66 Super Reverb not long ago. Amp had been worked on by a professional hack, me being just an amateur hack I put the bias cap in the same way the pro hack did, backasswords!
    Same results bias would only go to maybe -10, so I start poking around checking voltages and finally see the problem. It wrecked the new 100uf-100v F&T cap but it didn't explode anyway.
     
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  10. King Fan

    King Fan Poster Extraordinaire

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    Yeh, for sure, it's way easy to do. Nothing more dangerous than feeling confident that the guy who drew it / did it knew what he was doing.

    Steve, I was wondering if you'd damaged the cap -- or cap can? But I'm guessing the cap can isn't in the actual bias circuit?
     
  11. _Steve

    _Steve TDPRI Member

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    Yeah the cap can was just the nearest value I had to replicate it out of the amp. The axial 25u one in the circuit seems to be OK. I just reversed it in the circuit, checked the bias voltage and did a quick fire up and sound check before I had to quit for the day. I'll double check both caps properly on Monday morning when 'family time' is over ;)
     
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