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Thread: strange brown concert vibrato circuit help please

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    Supporting Member Randall's Avatar
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    strange brown concert vibrato circuit help please

    I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

    Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

    http://ampwares.com/schematics/vibrasonic_5g13.pdf

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    It's weird, because it WAS working fine.....

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

    Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

    http://ampwares.com/schematics/vibrasonic_5g13.pdf
    This may help you identify it better and possible use the 6g12 schematic.

    https://www.premierguitar.com/articl...cert_4x10_Amps
    nosaj

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    Supporting Member Randall's Avatar
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    "This may help you identify it better and possible use the 6g12 schematic."

    Yes, I've already been down that road. That's where I learned that the two schematics in 1960 were very close.

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    It's weird, because it WAS working fine.....

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    Old Timer olddawg's Avatar
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    What do you mean by “recap”? ..... Power supply and bias circuits only? Imho 90% of Trem circuit problems are do to old caps.

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    Last edited by olddawg; 05-18-2019 at 07:17 AM.

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    Wow, that's complex. Is that a harmonic tremolo?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

    Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

    http://ampwares.com/schematics/vibrasonic_5g13.pdf
    How about posting some tube voltages to see if they are conducting?

    nosaj

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    Supporting Member Randall's Avatar
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    "What do you mean by “recap”? ..... Power supply and bias circuits only? Imho 90% of Trem circuit problems are do to old electrolytic caps."

    All electrolytics, PS, bias and cathodes. AFAIK there are none in the trem circuit. There are only five on the main board, all cathodes.

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    It's weird, because it WAS working fine.....

  8. #8
    Supporting Member Randall's Avatar
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    OK jason, I'll play. With speed and Intensity at zero on the trem tube:

    1: 132v, fluxuating slightly
    2: 0.03v fluxuating slightly
    3: 1.7v

    6: 180v
    7: 0.1v
    8: 1.7v

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    It's weird, because it WAS working fine.....

  9. #9
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    Are there 5 preamp tubes? Or only 4? You should have 2 trem tubes. And unfortunately, ANY cap in the trem circuit could be bad... Which IIRC, there's a crap-ton of them...

    Justin

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    OK jason, I'll play. With speed and Intensity at zero on the trem tube:

    1: 132v, fluxuating slightly
    2: 0.03v fluxuating slightly
    3: 1.7v

    6: 180v
    7: 0.1v
    8: 1.7v
    Is this V3 or V4? Can you post for V4 also?

    Thanks,
    nosaj

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  11. #11
    Supporting Member Randall's Avatar
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    Yes, there are a LOT of Astron caps in this one. Five preamp tubes. The schematic doesn't distinguish which tube is V3 or V4, but if we go with typical right to left, what I posted would be V3 and here is V4:

    1. 334v
    2. 0.09v
    3. 3.8v

    4. 330v
    5. 0.3v
    6. 3.8v

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    Last edited by Randall; 05-18-2019 at 08:24 PM.
    It's weird, because it WAS working fine.....

  12. #12
    Don't forget the joker g1's Avatar
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    Suggest changing all non-electrolytic caps in trem circuit. Like olddawg said above, most trem issues are the caps.

    Also, even though it is not an issue in this circuit, something to remember is that warming the bias on a bias type trem can reduce the effect.

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    "Suggest changing all non-electrolytic caps in trem circuit. Like olddawg said above, most trem issues are the caps"

    Well, I have measured them, and most of the Astrons are drifted out of spec, some by 50% or more. I wonder how fussy I need to be about the actual values used here, like 0.25uF and .03uF and .05uF?

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    It's weird, because it WAS working fine.....

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    Don't forget the joker g1's Avatar
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    .22, .033, & .047 would be the standard value replacements.

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    Well, I have measured them, and most of the Astrons are drifted out of spec, some by 50% or more. I wonder how fussy I need to be about the actual values used here, like 0.25uF and .03uF and .05uF?
    If foil caps show increased capacitance, this typically indicates that humidity has entered the interior via micro-cracks. Consequently it's likely to find significant leakage.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Helmholtz View Post
    If foil caps show increased capacitance, this typically indicates that humidity has entered the interior via micro-cracks. Consequently it's likely to find significant leakage.
    Which in the state of Fl would be a very likely possibility.

    nosaj

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Let me have a stab at decoding how this works. The key principle here is that the gm of a triode and thus the voltage gain, varies with cathode current. The low frequency oscillator feeds the upper half of the 7025 on the right and also feeds an inverter, bottom left. The output from the inverter feeds the other half of the 7025. The output is taken as the sum of the outputs of these two triodes. The voltage swing on these two triodes will be in opposite phases so the sum will have no net DC change. The instrument signal is fed to just the lower one so that its output is amplitude modulated by the oscillator signal. The top triode gets just the very low frequencies of the input signal by virtue of the low pass filter formed by the 47k and the 5nF. This will reduce the amplitude modulation at low frequencies, presumably because it sound better.

    Click image for larger version. 

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    Supporting Member Jazz P Bass's Avatar
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    A good read here on the circuit topology & function: https://music-electronics-forum.com/...ad.php?t=10295

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