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Thread: Tone stack effect on volume

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    Member E biddy's Avatar
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    Tone stack effect on volume

    I just finished my second 5f10 Harvard build, but I made some changes. I replaced the tone stack from the 5f10 with the Blackface tone stack. The new amp is considerably quieter than the first one that I built. The first one I built starts to breakup at about 8, the new one is clean all the way to 10. Could the difference be in the tone stack? Tonight I was planning on wiring in the original tone stack and seeing what it does before I look for a problem somewhere else, but I was curious to see what people think.

    The way I have it is a lot like a blackface Princeton, minus one amplification stage. Was that put in there because of the gain reduction from the blackface style tone stack?

    https://guitarscanada.com/index.php?...hem-gif.21061/

    https://tubeamplifierparts.com/schem..._schematic.gif

    Thanks,

    Eric

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    Last edited by E biddy; 02-11-2019 at 10:47 PM. Reason: Added links to schematics and changed one to a 5f10

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    Supporting Member eschertron's Avatar
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    Does the 5f10 have a tone stack similar to the 5e3 single tone knob? That design is considerably less lossy that a FMV 2 or 3 knob design.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    Member E biddy's Avatar
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    Quote Originally Posted by eschertron View Post
    Does the 5f10 have a tone stack similar to the 5e3 single tone knob? That design is considerably less lossy that a FMV 2 or 3 knob design.
    Yes, I think it is the same one.

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    Supporting Member eschertron's Avatar
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    Are you familiar with Duncan's TSC (tone stack calculator)? I use it often to compare the loss between different stack designs. A BF stack may lose 20-30dB, while a single-knob maybe only 6dB. Quite a difference to make up!

    If you turn the mid knob to max, how close to breaking up can you get?

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    Last edited by eschertron; 02-11-2019 at 10:02 PM. Reason: typo
    If it still won't get loud enough, it's probably broken. - Steve Conner
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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
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    Old Timer J M Fahey's Avatar
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    If set close to "flat" (which requires a wonky knob setup, it is not simply 5 5 5) loss is almost 20 dB.
    When set by ear to more usual values, some moderate boost , itīs still some 10 dB down.

    So you lost that much gain and the reason youīd actually need an extra triode.

    Many old designs used little or no NFB; so if you use it now you need even higher raw gain factored in, to make up for losses.

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    Member E biddy's Avatar
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    Quote Originally Posted by eschertron View Post
    Are you familiar with Duncan's TSC (tone stack calculator)? I use it often to compare the loss between different stack designs. A BF stack may lose 20-30dB, while a single-know maybe only 6dB. Quite a difference to make up!

    If you turn the mid knob to max, how close to breaking up can you get?
    The AB763 doesn't have a mid pot, it is a fixed 6.8K resistor.

    Unfortunately Duncan's TSC doesn't have a model for the single knob tone control to compare.

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    Quote Originally Posted by J M Fahey View Post
    If set close to "flat" (which requires a wonky knob setup, it is not simply 5 5 5) loss is almost 20 dB.
    When set by ear to more usual values, some moderate boost , itīs still some 10 dB down.

    So you lost that much gain and the reason youīd actually need an extra triode.

    Many old designs used little or no NFB; so if you use it now you need even higher raw gain factored in, to make up for losses.
    I built this with switchable NFB and switchable cathode/fixed bias. When I take off the NFB it does get a bit louder and I get some breakup at 10.

    The other thing I was wondering about is the second triode stage isn't bypassed. How much gain could I pick up by bypassing it like the 3rd stage of the AB763 Princeton?

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    Quote Originally Posted by E biddy View Post
    The other thing I was wondering about is the second triode stage isn't bypassed. How much gain could I pick up by bypassing it like the 3rd stage of the AB763 Princeton?
    OK, I just figured it out and it should be about 6dB, so that should get me about part way there.

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    Old Timer J M Fahey's Avatar
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    If you want to play "clean" you are already there, since you get light crunch on 10.

    If you want to play somewhat dirty or adding sustain (daīBloozī setting) you can add clean gain between input jack and first triode, adding a single FET stage or even an .... Op Amp !!!!

    No, you will NOT turn it into a "soulless clinical buzzy abomination" by any means, that first stage will add little gain (say 3X to 10X), will not distort or colour sound at all , just make the first triode think you have real hot pickups in your guitar.

    Far simpler easier cheaper than adding an extra triode which by the way can be too much.
    Even $50k-a-head Dumble does that!!!!

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    Quote Originally Posted by J M Fahey View Post
    Far simpler easier cheaper than adding an extra triode which by the way can be too much.
    Even $50k-a-head Dumble does that!!!!

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    Quote Originally Posted by J M Fahey View Post
    If you want to play "clean" you are already there, since you get light crunch on 10.

    If you want to play somewhat dirty or adding sustain (daīBloozī setting) you can add clean gain between input jack and first triode, adding a single FET stage or even an .... Op Amp !!!!

    No, you will NOT turn it into a "soulless clinical buzzy abomination" by any means, that first stage will add little gain (say 3X to 10X), will not distort or colour sound at all , just make the first triode think you have real hot pickups in your guitar.

    Far simpler easier cheaper than adding an extra triode which by the way can be too much.
    Even $50k-a-head Dumble does that!!!!
    Is it as simple as something like this? Op amp gain stage.pdf

    Edit: I just saw you said in front of the first stage.

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    Not quite. Tube amps are typically high impedance, meaning that each stage should have high input impedance (often around 1M) to not load down the preceding one (or when used as first stage the guitar PUs). Your example shows an inverting circuit having an input impedance of only 1k. You should use a non-inverting op amp circuit or a Fet.

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    Old Timer J M Fahey's Avatar
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    Agree.
    You found a very low impedance Op Amp gain stage.
    Too busy now but later Iīll draw a single Op Amp gain stage , non inverting, High Z input (1M) and fed from existing tube +V (200 something volts) so you donīt need an extra supply.

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    Last edited by J M Fahey; 02-12-2019 at 03:33 AM.
    Juan Manuel Fahey

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    I found a wiring mistake in the tone stack, and it added some gain to the amp. It is still quieter than the 5f10, but sounds good. The negative feedback and bias switch work great.

    Quote Originally Posted by J M Fahey View Post
    Agree.
    You found a very low impedance Op Amp gain stage.
    Too busy now but later Iīll draw a single Op Amp gain stage , non inverting, High Z input (1M) and fed from existing tube +V (200 something volts) so you donīt need an extra supply.
    I'm pretty happy with the sound now, but I would still like to see this. Also, this amp does have a 50V tap. Would that make something like this easier?

    Click image for larger version. 

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    Old Timer J M Fahey's Avatar
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    Yes, but checking the amp schematic I found input stage is built out of a single triode.

    So **in this particular case** you might replace that single triode with a 12AX7 and have a "real" extra triode gain stage with little investment.

    I bet transformer can feed extra 150mA filament, and you might enlarge the current 7 pin socket hole to accomodate a 9 pin one.

    In fact, they might *already* have a noval compatible hole there (since chassis are often built for various models) ; the FET suggestion was to avoid punching a new hole and taxing the transformer, but it looks like "here" itīs not the case.

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