BFSR Noise Issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TimmyP, May 5, 2016.

  1. TimmyP

    TimmyP Member

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    It is (except for a few coupling cap values) an AB763. It has a constant low-passed white noise. I have it down to V4B (removing V1, V2, V3 have no effect. I swapped in a known good tube, replaced the plate and cathode resistors and the cathode bypass cap (which was only a year old), replaced the 3M3 and 10pf on the grid, and replaced the output coupling cap. I lifted the board to check for foreign matter beneath it, and I heated it up to see what happened (nothing). Grounding V4B's grid to ground stops the noise.

    Help please.
     
  2. swiveltung

    swiveltung Member

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    Goes away with the reverb turned all the way down right?
     
  3. Blue Strat

    Blue Strat Member

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    First things first. I suspect that this isn't a Fender amp but a home built amp. Am I correct? If so, you should mention this in your first post as the troubleshooting method is different...especially if the amp has never worked correctly (without the noise you're reporting).

    So, before we go on, please enlighten us.
     
    zenas likes this.
  4. UsableThought

    UsableThought Supporting Member

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    Second the vote for what this really is if it isn't quite an AB763.

    And also, for those of us (me) who are interested in noise problems & would like to follow/lurk, what do you consider V4? Looking at the AB763 schematic, I see two 7025s for input tubes; a 12AX7 for the effect; and a 12AT7 for phase inverter into a pair of 6L6GC's. OK, but then you say -

    - but crawling through the schematic & also a layout, I don't see these values for resistor or cap on any tube's grid or anywhere at all for that matter. Maybe they're there, and I'm just not seeing them; if so, apologies. Either way it would be helpful if you can identify the tube you suspect by function as well as abbreviation. Plus if the circuit has in fact been modified to have non-stock values in many places rather than just a coupling cap or two, maybe you can post a schematic showing mods?
     
    Last edited: May 6, 2016
  5. Blue Strat

    Blue Strat Member

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    Half of V4 is the reverb recovery tube, bottom of schematic after reverb tank, and the other half is the wet/dry mixer...top, 3rd triode from the left. The 3.3M/10pf combo feeds the grid of the top triode on the schematic.

    I once had a similar noise problem and found the problem to be the 3.3M resistor...first time ever. What are the odds that the replacement 3.3M was also noisy? A real hair puller for sure.
     
    UsableThought likes this.
  6. pdf64

    pdf64 Member

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    There are also 220k and 470k resistors in the grid circuit; maybe one of those has gone noisy?
    Do the Vdc on that triode's terminals check out ok?
     
  7. TimmyP

    TimmyP Member

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    This is a real BFSR, not a clone.

    Problem is after the reverb control (which is down, and does not add appreciable noise when up).

    Plate voltage is kinda high on V4 (I forget what it was), but the other plate voltages are normal for on old amp on modern (120++) mains.
     
  8. Blue Strat

    Blue Strat Member

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    Remove the 3.3Meg and 10pf cap. That'll tell you whether the noise is coming from that leg (the dry signal from V2) or the leg coming from the reverb output.
     
    pdf64 likes this.

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