Fluctuations in bias current help.

Discussion in 'Amp Tech Center' started by Zachgraves2086, Dec 5, 2019.

  1. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    Long story short.. I have a 66 Super Reverb I’m having some weird problems with...when I turn the treble knob past about 3, the bias current,mA jumps from 45ma up to 70ma and the amp has a low hum to it.. I have no idea where to start, the cathode bypass caps have been changed, the filter caps have also been changed but it seems to still have the original tone, coupling and other small capacitors.. I imagine it’s a coupling cap issue but before I jump in and start changing original parts. Is there something else I can check before hand? Any help is appreciated.


    P.S.... In my experimenting tonight I noticed this only happens when I have a guitar in my hands standing next to my Fluke meter that has my Eurotubes bias probe plugged in.. when I step away from it, the bias current goes back to normal... I’m lost...
     
    Last edited: Dec 5, 2019
  2. kbold

    kbold Tele-Afflicted Silver Supporter

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    Weird indeed. Perhaps a coupling cap ( in line with the signal to block DC volts) is faulty. It would be a cap between input stages and bias adjustment.

    That's the only thing that comes to mind. So, I think your imagination is correct.
     
  3. LightningPhil

    LightningPhil Tele-Meister

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    Sounds like RF feedback that’s eminating from the meter and being picked up by the guitar. You may inadvertently be making an oscillator and turning down the treble may dampen it.

    If you’ve got a scope, this should be easy to spot (connect it to a lead that’s going to the fluke). But easiest to ignore and stand back a bit.

    If this is the case, and you really want to fix it, add some grid stopper resistors to the valves to cure.
     
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  4. Dacious

    Dacious Poster Extraordinaire

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    Bias meters can cause oscillation which will create runaway current draw. It's one reason why the direction used to be do bias checking with all controls zeroed.

    I worked a a big Laney that was prone to running away with my homebrew bias probe.

    I've gone to Transformer resistance method.
     
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  5. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    Thanks for all the replies everyone.. I’ve found some other issues as well.. one of the voltage drop resistors was actually a 47.2k instead of the 4.7k... giving basically 290 volts before the 100k resistors before is hits the pre amp tubes... I don’t know if that has anything to do with other issues I’ve described but I’m going to attempt to get those lined out as well.
     
  6. Wally

    Wally Telefied Ad Free Member

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    the voltage from that node is shown as 270vdc on the AB763 schematic. I would expect a higher voltage there with modern wall voltages....maybe..depending on the time of day and usage of the grid. Are you sure about that resistance there?
     
  7. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    DB860C10-6460-4E28-B476-288F98347487.jpeg

    It’s 290 coming from the B+ voltages .. suppose to be around 410 then hit the 100k resistors which drops it to @270...
     
  8. Wally

    Wally Telefied Ad Free Member

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    Okay....my bad....I went straight to the preamp plate voltages and ignored the voltage coming off of that node. For what it is worth, the B+ is the voltage coming off of the rectifier...feeding the first filter. The feed from the first filter to the OT primary center tap is node A...unnamed in the Fender schematic. Some schemes label this as B1. That 4.7kohm resistor in the Fender circuit sits between nodes C & D...named on the schematic and on Rob’s drawing.
    I agree that you have found an error....no matter what one calls those nodes along that power rail.
    https://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf
     
  9. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    I apologize if I came off a bit crass. No intent there. I’m relatively new to the amp repair world so I don’t really know what the terminology or nomenclature of a lot of this stuff. I really do appreciate all of the help. I do have another question if you can help. All of the coupling caps look to be the originals and from what I’ve been reading, everyone says to go ahead and change them out. Since the filter caps and cathode bypass caps have been changed, would it be wise to go ahead and tackle the coupling caps??
     
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  10. milocj

    milocj Friend of Leo's Ad Free Member

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    A '66 Super should still have the blue molded coupling caps. Most people claim that those rarely leak and they are pretty highly coveted due to their being used in the blackface amps. I would only change one if you find that it's leaking and causing a problem.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    ^^^^^this. When those blue caps are missing from a Fender amp from that era, one can pre-suppose that someone pulled them to sell them for much more than the replacement caps cost. It will also affect the value of such an amp, imho.

    and....I took no offense and sense no crass attitude. You simply noted my error.
     
  12. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    0DCE0956-2B64-4E80-9F3F-2A6BC5427787.jpeg Yes it still has the blue molded caps but that big red sucker is one I’m skeptical of. I unsoldered one side from the board and checked it with a capacitance meter and it’s actually dead on at .1uf.. I’m not really schooled well on capacitors of that nature so with voltage applied does it leak by, I don’t know. I’ll fix the issues I know of being issues with it and if I have any thing else I can’t youtube my way out of or ask at the forums, I’ll take it to a pro.
     
  13. Wally

    Wally Telefied Ad Free Member

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    So far and considering that the bias fluctuation may be a non-issue, it seems that that dropping resistance needs to be corrected and then a voltage chart made...and a Sonic assessment at a good bias point....if the bias is stable.
     
  14. dan40

    dan40 Tele-Afflicted

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    In your first post you mentioned that the bias current was fluctuating with your meter attached. Is the amp performing properly otherwise when the meter is not attached? Is the only issue a low level hum or is there another problem with the sound of the amp?
     
  15. Zachgraves2086

    Zachgraves2086 TDPRI Member Platinum Supporter

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    The amp works and sounds fine now after fixing a couple issues I had found before attempting the bias adjustment.. during the bias adjustment I found other voltage issues (voltage drop resistors being incorrect and a poor solder joint on a filter cap)
    It was causing some weird humming/motorboat sounds but only when I would get close to the meter with a guitar plugged in and adjusting treble...when I had the guitar away from the amp it wouldn’t do it even when turning the treble pot every which way... I then turned the amp off, unplugged my bias probe from the amp reinserted the tube back in the socket, fired it up as usual and moved that guitar all around that amp and nothing happen all while treble pot turning... repeated the process, plugged the bias probe back in and did the experiment again... fluctuations and noise came back... repeated the unplugged bias probe experiment and the noise went away... I’m not sure if it was inducing a signal into the amp or what.. but as someone above said.. could be the meter was causing some issue with the bias probe plugged in and inducing an RF/EMF into the tube...
     
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