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Thread: Valveking mh 20 negative feedback question

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    Valveking mh 20 negative feedback question

    Need some help here.

    If i lower or raise the R38 resistor in the schematic bellow, wich i think is the presence resistor, what results will i have?


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    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by Rod View Post
    Need some help here.

    If i lower or raise the R38 resistor in the schematic bellow, wich i think is the presence resistor, what results will i have?

    http://music-electronics-forum.com/a...60-vk-20mh.png

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    It would help if you posted link to full schematic. In any case the negative feedback circuit shown in your drawing is fairly complicated with the signal going through R61 and R23 (bypassed w/ C59) before reaching C23 & R38 going to ground.

    What exactly are you trying to accomplish?

    Steve A.
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    Attached is the full schematic, i think that the amp has a little to much presence, i want to improve this, and im triyng to see if changing the R38 resistor value in the negative feedback loop will increase or lower the presence.
    Attached Files Attached Files

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    Raise the value to lower the presence. Try a 2k or 5k pot. Or you can raise the C23 cap to make it bypass more mids.
    R78, R38 and C23 form a cathode bypass for highs, whereas R77+C5 form a full range bypass.

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    Quote Originally Posted by kmensik View Post
    Raise the value to lower the presence. Try a 2k or 5k pot. Or you can raise the C23 cap to make it bypass more mids.
    R78, R38 and C23 form a cathode bypass for highs, whereas R77+C5 form a full range bypass.
    HI!

    I raised the vale of the R38 to 1.5K in the negative feedback loop, and it raised the presence, it did the oposite of what i wanted.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Rod View Post
    HI!

    I raised the vale of the R38 to 1.5K in the negative feedback loop, and it raised the presence, it did the oposite of what i wanted.
    Looking at the circuit I think that your result is impossible. There must be an error in perception or the circuit alteration.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    change c23 to .1 or 100n will lower the frequency?

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    OK I'm going to check it, I think that the presence could be also of the El84 tubes, if I change something in the power section, could I get less highs?

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    "I think that the presence could be also of the El84 tubes"

    Not sure what you mean.

    when you say too much presence, you are talking about high frequency response right?
    may not be what you think, depends on the tube health mabe they are worn and loosing lows? just asking because its not clear what your trying to accomplish.

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    I was thinking for example if I change the bias of the El84 tubes, maybe I could get less highs.

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    I would not mess with the bias, if it were me, I would try some values around the c23 .1 should lower the freq a bit.
    if r38 at 1.5k "went wrong way" try 1k ohms and see? ( that should brighten things even more ) as its a 180 now.
    according to the schematic i am looking at.

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    What kind of potenciometers should i put in the place of the R23 and R38 negative feedback resistors, Logaritmic or linear?

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    Supporting Member Jazz P Bass's Avatar
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    I would use a linear taper.

    Easier to control.

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    If change the location of the negative feedback loop resistor R78 just to after R23//C59, will this change the presence of the poweramp?, in what way?


    ValveKing2_20MH.pdf

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    If I change the location of the negative feedback loop resistor R78 just to after R23//C59, will this change the presence of the poweramp?, in what way?

    There must be someone that can help me with this question.


    ValveKing2_20MH.pdf[/QUOTE]

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    Bent Member Chuck H's Avatar
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    The circuit is complicated because there is a cathode bypass cap dependent on input from and voltage division by the NFB circuit as well as the NFB circuit depending on that cathode circuit for it's voltage division. Further this with the 220n cap being interactive between the circuits as well. I think moving R78 to the location you indicated would increase HF bypass on the cathode by reducing resistance between the cathode circuit and the 220n to ground. Increasing treble, though not technically "presence". Adding 820R to the exiting 220k (moving R78) prior to the 220n circuit does very little to change the voltage division there so I wouldn't expect any notable change to the actual presence affect on HF.

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    Just one more question,

    if i have more negative feedback entering in the preamp(for exemple if i keep the R78 value) i will have less destortion, more bass, more headroom, but what also what will happen when i raise the volume is that it seems that the sound becomes more thinner. Could this thinner sound when raising volume due to more negative feedback enter in the preamp ?

    What else could be the problem of the amp becames more thinner when i raise the volume?


    I dont know why, but it seems to me that little amps with just only 2 powertubes are very trebly, dont have enough punch and bass.

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    Bent Member Chuck H's Avatar
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    Your description of what happens to the sound with more NFB and higher output is likely perception due to the Fletcher Munson curve (what happens to your ears with changing volume). More NFB does not mean more bass. It means cleaner bass. Another affect of reduced NFB at higher outputs would be more distortions. This (perceptively) can "fill out" the sound and make it seem less thin perhaps.?. But there are other circuit parameters in play (like bias, load impedance and phase errors) that also affect the effect of NFB, especially when an amp is pushed close to or into clipping, and how it is likely to be perceived. Regardless of what is actually happening. With too many circuit specifics interacting it's impossible to apply blanket analogies about what more NFB and less NFB sound like. That is circuit and amplifier specific to some degree. But since most guitar amps follow a similar topology the NFB level can often be recognized in it's contribution to tonal character. Your circuit, however, is not as typical. And I find trying to predict affects of it's adjustment tricky because the NFB and the earlier stage that begins the loops cathode bypass circuit are sort of shared. In fact, R78 is NOT the feedback resistor. R23 is. The shared circuits between the NFB loop, presence circuit and cathode bypass circuit for V3 is comprised of C5, C23 and even C59 and R38, R77 and R78. All six of those components can be said to be in the cathode bypass circuit for V3 and also in the NFB/presence circuit. Since the two circuits affect each other relative to phase and voltage potential, but the cathode bypass circuit for V3 also operates independently for that stages frequency response that "sees" the loop and feeds the loop, well, you can see it's complex. Too complex for me to just look at it and make predictions on circuit changes and too complex to be used as a model for what changes to a NFB loop sounds like.

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    Last edited by Chuck H; 06-14-2018 at 02:40 PM.
    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "If you build it, it will hum..." Justin Thomas

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    Old Timer J M Fahey's Avatar
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    A couple details:
    1) that is a "trick" NFB circuit.
    Not only smooths output sound, lowers impedance (increases damping) and lowers dstortion, which is its main purpose, it also equalizes the power amp.
    You have a Bass boost and a Hi Mids/Treble one.

    2) C59 in series with R61 boosts Bass from 450/500Hz down, stopping at about 80Hz. (R23 in parallel with C59).

    3) C23 in series with R38 boosts mid/highs above 800Hz or so.
    Increasing R38 will decrease boost as mentioned by Chuck H so "it will have less presence".
    Just to have a clear idea of what this does, lift the grounded leg of R38 and add a switch there, so you instantly switch from full to zero Presence boost, leaving all else the same.

    4) in general, most complaints about "too bright" , "too much highs/presence/buzzy/thin" sound , also called "non pedal friendly" come from amps being played at low volume, alone, at a home situation, etc. ; once you play with others (specially after getting a few cymbal crashes a few inches away from your ears) nothing is"too bright" any more.
    That "excess Presence" is probably what helps that amp cut through the mud soundwall.

    Try the "kill presence" switch I suggested above .

    Or play alongside a loud drummer

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    Juan Manuel Fahey

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    Supporting Member Steve A.'s Avatar
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    Another point... this is an amp head so the speaker used is another factor. I am a big fan of Vox-style Cut controls to match the output to the speaker used. It is usually fairly easy to add to practically any push pull amp because you can always trace the circuitry to the output tube grids. I like to use a pot with a switch so that I can remove it from the circuit if desired.

    Steve A.

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