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Help Request: Vibro Champ Voltage, Bias and Tremolo Fixes

Discussion in 'Shock Brother's DIY Amps' started by brucerbc, Jan 24, 2021.

  1. brucerbc

    brucerbc TDPRI Member

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    Posting help request for Vibro Champ build thread posted here.

    Pictures and layout posted in the build thread. Voltages and bias calculations posted below.

    Issue is that I have sound, volume control and tone control, but no tremolo, some voltages more that 20% off from Fender layout, and bias well below recommended 95% plate dissipation.

    I’m not seeing the connection in these issue, so would appreciate any advice on fixes (e.g., is cold bias alone enough to cause voltage and tremolo issues or are there several issues here)?

    All voltages relative to ground.

    2021 01 24 - Voltages.JPG

    6V6GT Bias (per Tube Bias Calculator at https://robrobinette.com/Tube_Bias_Calculator.htm)

    Design Dissipation

    Tube Type: 6V6GT 14 watts

    Plate-to-Cathode Voltage: 358.6 DC volts (pin 3 to pin 8)

    Calculated Tube Max Design Dissipation = 14 watts

    Calculated Cathode Bias:

    • 85% Cool = 33.2 DC mA
    • 95% Average = 37.1 DC mA
    • 100 % Max Safe Dissipation = 39 DC mA

    Tube Dissipation Using Cathode Resistor Voltage Drop

    Number of Tubes that share cathode resistor = 1

    Voltage Across Cathode Resistor = 30.9 DC volts

    Cathode Resistor Value = 1000 ohms

    Total Cathode Current = 30.9 DC mA

    Calculated Total Plate Current = 29.2 DC mA

    Calculated Plate Current per Tube = 29.2 DC mA

    Calculated Plate Dissipation per Tube = 10.5 watts

    Plate Dissipation per Tube % = 75%


    Changes from Fender AA764 Layout

    220K Filter Cap Drain Resistor

    Cathode resistor raised from 470 ohms to 1K

    Bypass cap raised from 25uF/25V to 50u/50V

    One of the .01uF tremolo caps to .02uF

    Put negative feedback on a switch

    Put 4R and 8R OT taps on a switch

    OT: Hammond 1760C

    PT: Hammond 290AX


    Changes considered, not yet done

    Fit a 470R (2 or 3W, wirewound) screen-grid resistor to the 6V6 (move the wires from pin 4 to pin 6 and fit the resistor between 4 and 6)

    Fit a 330pF cap from 6V6GT pin 5 to pin 8

    Fit a 1K5 to 470 K resistor from 6V6GT pin 1 to pin 5
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Check all the caps and resistors in the trem circuit. The ceramic discs can go bad. One resistor on mine came apart. You have to lift one side to measure. You didn't spray deoxit in the trem jack? The guy who bought my 68 did that, turned it into a 14mohm resistor. Took an EE to sort that.

    The trem on these is quite shimmery and not strong. Usually needs to be on 6 to be really noticeable.

    I used a 110 volt primary step down on my 68 and 78 VCs and the plate voltage and B+ at the first cap can junction were 20% high.

    If you drop bias a lot of the tone falls off IMO.

    Your bypass caps are good?
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    ^^^ this for sure. Hang in, you'll get this.

    I know you have pics in your build thread, lots if I recall. But it helps the sharp-eyed pros here to put a few inline in this thread too. Just a few specific shots framed to show max detail. I'm thinking clear shots of the trem jack, the trem array on the board, and the wiring to the pots. Since the pot leads are hard to see, I'd do as much DMM continuity/resistance/ground testing as possible. Bad solder joint? Bad pot? Bad component? Bad jack? Etc...

    Specifics I wonder about:

    Note what @Dacious says about cool bias. Are you saying your bias is 75%? Most folks want at or even above 100% in these cathode-biased amps. I'm not sure it'll affect your trem, but it's important. And I don't always see great results when folks increase their cathode resistance a great deal. Did you do this to get bias down??
     
    Last edited: Jan 24, 2021
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Oh, and regarding your target voltages, it's important to look at the voltages on both the original VibroChamp schematic:

    vibro_champ_aa764_schematic.png

    and the later Bronco AB764, which in fact was identical to many (? most) VibroChamps.

    bronco_ab764_schem copy.png

    Which Fender are ya gonna believe: B+ 355, or B+ 410? Yes, the transformer changed (sorry, I used to know when) but the 410 B+ was notoriously accurate for many years. There's a lot I don't know, but I'm not sure the 355 was ever accurate, even with the 125P1B PTs.

    So. Note *both amps have a 470 bias resistor.* So your 405 B+ isn't quite a bit too high -- it may be almost perfectly vintage. Given the PT you're using, it's what you'd expect. So one more argument to run a 470 bias resistor.

    Now, full disclosure -- my 1967 VC ran the notorious ≥ 400 B+ on modern wall voltage, and sounded a bit too bright, clean, hard-edged, so I put a dropping / sag resistor between the rectifier and the reservoir. But I'm still running a 470 bias resistor and dissipating above 100%. And yes, again, this may well have nothing to do with your trem problem, and I may have missed something obvious that means I'm all wet. But FWIW...
     
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  5. dan40

    dan40 Friend of Leo's

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  6. Linkjr

    Linkjr Tele-Meister

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    Have you tried a continuity check using layout and all components involved in the trem circuit? Even with hot bias I could notice trem on or off in my vc

    Sorry to throw this in but it can happen- have you tried foot switch turning trem on /off?
     
  7. D'tar

    D'tar Friend of Leo's

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    Any progress here?

    I would recommend at least halving your 6v6 1K bias resistor. I would use the stock value. YMMV

    Your voltage chart indicates some oscillation happening in the trem circuit. On your board, I cant' tell where the yellow wire from the intensity pot terminates. it should go to v1b cathode. Ohm from the trem pot yellow wire lug to v1p8.
     
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  8. brucerbc

    brucerbc TDPRI Member

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    Thanks all for replies and suggestions! I've been under a pile of work, but hoping to get back to the amp tonight and will post suggested pictures and investigation results.
     
  9. NTC

    NTC Tele-Meister

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    Try the stock cap values in the trem first, then raise them one at a time. It may be going REALLY slow and you don't notice it. Also, is the oscillator oscillating?
     
  10. brucerbc

    brucerbc TDPRI Member

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    Update: cathode resistor and cap returned to stock 470 ohms and 5uF/25V.

    Voltages way better ... mostly:

    2021 01 28 - Voltages.JPG

    Bias now running a too hot 120%, so will need to lay in a range to resistors to get closer to 95%.

    Unfortunately, still no tremolo regardless of the speed and intensity pot positions or whether tremolo jack is shorted.

    So on to the helpful troubleshooting tips:

    I've checked all the caps and resistors several times (again) as well as tested values and circuit continuity. All good, but no tremolo.

    Thanks for pictures suggestion:


    20210129_004453[1].jpg


    20210129_004515[1].jpg


    20210129_004538[1].jpg


    I've done as much DMM continuity/resistance/ground testing as I can think of and still no joy.

    Bias now 120%, but still no tremolo.

    Yes, thanks though! Done all that. Still no tremolo. Urgh.

    Oscillation still there in voltages after bias change, but still no tremolo

    Yellow wire from the intensity pot terminates at V1, pin 8, as you noted.

    Any other pointers or suggestions would be really appreciated. I'd like to get this one in use!

    Thanks.
     
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  11. D'tar

    D'tar Friend of Leo's

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    Does the speed pot have effect on the oscillation? If you plug in the footswitch and cycle the switch, does the oscillation stop/start as expected. If yes we may assume V2a is functioning as designed so we can focus on V2b and the injection point.
     
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  12. jmp81sc

    jmp81sc Tele-Meister

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    It looks like your using an isolated jack for your trem switch. I think that needs to be grounded since it is isolated from the chassis. I think the way it works is that the switch interrupts the ground connection to turn off the trem.

    Nice looking build otherwise.
    John
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I think the trem jack is the one on the left here.

    18C7622E-FD84-42A4-ACD3-B7A91D35CC2C.jpeg
     
  14. brucerbc

    brucerbc TDPRI Member

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    ^^This is correct. I've been using a 1/4" plug and test leads to (try to) turn the tremolo on/off, but hearing no discernable effect whether tip is grounded or not.
     
  15. brucerbc

    brucerbc TDPRI Member

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    Yes. The speed pot appears to have an effect on the DC oscillation at V2 pins 1 and 6: narrower DC oscillation range at Speed 1, wider range at Speed 10.

    Yes. I've been using a 1/4" plug and test leads to turn the tremolo plug on/off. DC is still present at V2 pins 1 and 6 when tremolo plug tip tied to ground, but is steady, not fluctuating. Fluctuates again if tremolo plug tip not tied to ground

    Still no audible tremolo effect in any situation so far.
     
  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I’ll let @D'tar and the other smart folks think about the electronics here. Trem circuits have several sticking points, like starting up the oscillation, governing its amplitude, keeping it stable....

    If it was mine, tho, I’d be tempted to rebuild/rewire that whole part of the board. It may be perfect — but that's anybody’s guess. The ODs block our view and have zero advantage here. It’s easier to see what’s where if it looks more like the original. Ceramic discs are fine, or use nice small axial caps.

    59C0647E-6580-47C2-99CD-878539D99C26.jpeg

    And without pulling the board (unless you need to) you could use the visibility of a rebuild to triple check the wires are routed right and don’t short to each other. This is a particularly gnarly area of the layout. :)

    75281A27-5313-4138-9CC6-052598594DF8.jpeg

    For sure it’d be tedious and might well make no difference. So maybe only if the smart, directed, electronics approach doesn’t work. :D
     
    Last edited: Jan 30, 2021
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  17. jmp81sc

    jmp81sc Tele-Meister

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    Your oscillator is working, it shows in the voltage swings in V2, however it is not effecting the the V1b cathode voltage to create the effect. Your V1 voltage should swing also when the trem is engaged. Seams like your tremolo circuit is isolated from the rest of the amp, maybe your intensity pot?

    Keep at it, my two most difficult builds were with trem circuits.

    John
     
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  18. D'tar

    D'tar Friend of Leo's

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    To confirm proper wiring and conponent values. IMO, this is how I would proceed.

    We have multiple paths to ground so I recommend disconnecting a couple things to minimize confusion. 1, Disconnect the NFB feed at a convenient location. 2, Remove the yellow wire from the intensity pot.


    Can you give us the ohm readings for

    V1p8 to the intensity pot lug3(yellow wire disconneted end)

    V1p8 to ground

    V2p8 to the intensity pot wiper(brown wire)

    Also v2p8 to ground with intensity pot max/min

    This should give an idea of whats happening.
     
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  19. brucerbc

    brucerbc TDPRI Member

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    Thanks @D'tar @jmp81sc and @King Fan for support and suggestions. Greatly appreciated.

    Yes, I'm leaning towards rebuilding the tremolo circuit with new parts, hopefully with board in place.

    Definitely regretting going with the orange drops when ceramics were out of stock.

    Will save that as a backup option if the directed troubleshooting doesn’t work though.

    In meantime, as suggested by @D'tar, I've ...
    • disconnected the NFB feed at a convenient location (I installed a back panel switch for that); and,
    • removed the yellow wire from the intensity pot.
    Ohm readings follow:
    • V1p8 to the Intensity pot lug3 (yellow wire disconnected end): no connection ("0.L") ... or sometimes alternating between ~260k/no connection/zero
    • V1p8 to ground: no connection ("0.L") ... or sometimes zero (flashing intermittently) ... or sometimes 10M ... or sometimes alternating between ~260k/no connection/zero
    • V2p8 to the Intensity pot wiper (brown wire): no connection ("0.L") ... or sometimes alternating between ~200k/no connection/zero
    • V2p8 to ground with intensity pot max: zero (flashing intermittently).. or sometimes alternating between ~2k/no connection/zero
    • V2p8 to ground with intensity pot min: zero (flashing intermittently). or sometimes alternating between ~20k/no connection/zero

    Not sure why my findings are changing at same measurement point, different times. Maybe varies with how I probe V2p8?

    Looking forward to any interpretations of what's happening?

    Thanks!
     
    Last edited: Jan 30, 2021
  20. D'tar

    D'tar Friend of Leo's

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    This is with amp off, unplugged, caps drained right?

    The v1p8 to lug 3, i should clarify. Ohm from v1p8 to the end of the yellow wire that was connected to lug 3. So, to the wire, not the pot.
     
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