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Twin Reverb - Amp circa 2005 Trem issues

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  • sparkies
    replied
    Originally posted by Helmholtz View Post
    The bias feed resistors don't drop voltage, so have no influence on bias and balance.
    I know they don't drop voltage. They screw with the signal's zero crossing and cmrr when they are different resistance. Plus only one voltage moves the branch of two grid circuits at the same time. The example one is a standard way of doing the same thing without a tapped bias pot.

    Besides that, matched tubes for amps wear evenly when they are matched tubes.

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  • Helmholtz
    replied
    Turning the balance pot lowers the grid bias for one tube while increasing the grid bias for the other one.
    The bias feed resistors don't drop voltage, so have no influence on bias and balance.

    Leave a comment:


  • sparkies
    replied
    Click image for larger version

Name:	bias branch.jpg
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    This is better bias circuit

    Last edited by sparkies; 02-11-2024, 07:58 PM.

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  • sparkies
    replied
    Originally posted by Helmholtz View Post
    The balance pot is very useful to minimize (B+ and bias) ripple hum.
    Buying "matched" tubes doesn't always ensure balanced operation in an amp.
    Also power tubes may age at a different rate.
    There is a separate one for hum, In the original control's design, the control is 10K resistance and center tapped at 5K of resistance which is fed by the Bias supply's voltage divider's top resistor, that the wiper completes the voltage divider so when the control is in default centered position, the wiper is at zero trim.going one way or another sets a different voltage drop between them. If I want to do the same thing instead what is done in the AA763 I would just the potentiometer connected as the voltage drop in a AA763, wiper on one grid +matching feed resistor, then either next to one side of the potentiometer put the other grid circuit

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  • g1
    replied
    There was no such thing as matched tubes (at least at common retail level) when the 'output tubes matching' pot was in use, that's why they included it.
    As far as the objection to bias current running through the wiper in this design, that is the same as pretty much every other Fender bias pot ever used. It is not a typical problem spot.

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  • Helmholtz
    replied
    The balance pot is very useful to minimize (B+ and bias) ripple hum.
    Buying "matched" tubes doesn't always ensure balanced operation in an amp.
    Also power tubes may age at a different rate.

    Leave a comment:


  • Chuck H
    replied
    Originally posted by sparkies View Post
    But the real question is why did they build it into these amps anyways? Because you buy matched tubes to go into the circuit.
    At the time of the balance circuit they may have been attempting to cut costs (or solve a problem they were seeing?). Maybe some bean counter for Fender determined that it could be cheaper to just include the balance circuit so they could plug in any two tubes and balance them rather than pay for matched tubes for each amp. Or maybe Fender was already in the practice of just plugging in any two tubes and started seeing unacceptable imbalances. For a long time in those days any two tubes from the same "batch" were probably close enough to matched. The "matched tubes" thing for guitar amps is much more important now when tubes are all over the place for individual spec. So now "matched" is almost all you ever see. There have been a couple of really high performance amps that required bias balancing to work properly (some Ampegs I think).

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  • Chuck H
    replied
    Originally posted by Tom Phillips View Post
    Sparkies already posted one example of Fender using the 56k cathode resistor in the AA763 tremolo circuit. Note also that, in the next generation AB763 circuit, the signal to the grid of the 12AX7 triode that drives the neon bulb is taken from a different point in the LFO feedback chain. Maybe this is why Fender felt that they needed to change the gain of the bulb driver.

    [ATTACH]n993857[/ATTACH]
    Oh snap! I missed that. At a glance I caught the 100k value adjacent to it. Sorry sparkies. My bad. Thanks Tom.

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  • Helmholtz
    replied
    Seems we lost the OP.

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  • Tom Phillips
    replied
    Originally posted by Chuck H View Post
    I haven't seen this. Perusing all the Fender LDR tremolo circuits new and old I can't find any that are using a 56k cathode resistor for the actuator triode. Can you post a link?
    Sparkies already posted one example of Fender using the 56k cathode resistor in the AA763 tremolo circuit. Note also that, in the next generation AB763 circuit, the signal to the grid of the 12AX7 triode that drives the neon bulb is taken from a different point in the LFO feedback chain. Maybe this is why Fender felt that they needed to change the gain of the bulb driver.

    deluxe_reverb_AB763_schem.pdf

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  • Helmholtz
    replied
    Originally posted by sparkies View Post
    I said you can put the balance control there instead.
    No, won't work.
    Balancing the power tubes requires varying individual bias.

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  • sparkies
    replied
    Originally posted by Helmholtz View Post

    This:


    That is if you want the control.Stop with your horse shit with me.

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  • sparkies
    replied
    Originally posted by Helmholtz View Post
    Different value bias feed/grid leak resistors will not change power tube bias, as these resistor don't carry a DC current, so don't drop voltage.
    Again bad advice.

    You're Jumping into conclusions
    I never said it changed that. I said you can put the balance control there instead. Not that it needs it anyways.

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  • Helmholtz
    replied
    Originally posted by sparkies View Post
    What are you really referring to?
    This:

    But if you still want balance, I would substitute the 220K feed resistors with a 500K potentiometer.

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  • sparkies
    replied
    Originally posted by Chuck H View Post
    You beat me to it Helmholtz All that can do alter the grid load for either side of what should be a balanced circuit.

    And I'm not sure what's meant by "one of the hand drawn ones" WRT the schematic. Aren't they all hand drawn? Is this just more nebulous wording to divert the issue?
    They were hand drafted up to the 80s.

    Why you are ripping on the balanced control substitution?
    Granted, its not like the one I built into tube compressors.
    But the real question is why did they build it into these amps anyways? Because you buy matched tubes to go into the circuit.
    Last edited by sparkies; 02-11-2024, 02:56 PM.

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