question about the series capacitor

Discussion in 'Shock Brother's DIY Amps' started by stinga11, Dec 9, 2019.

  1. stinga11

    stinga11 TDPRI Member

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    Greetings, First of all forgive me for making a new thread for this nonsense but I killed myself searching online and I get nothing.

    I want to know the following; If I have two capacitors in series but in the middle I have a resistor to ground is it still in series? will it divide the total capacitors value?

    0505.png

    EDIT
    I am looking at the schematic of a ab763 and I realized that before the phase inverter there are two capacitors in series but that does not make much sense because then you would change the value of both. But it has a 47k resitor to ground, I thought that had to do with it.
    barreto schematic reverb2019.png
     
    Last edited: Dec 10, 2019
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I'm not sure what your question exactly is? The capacitors still connect at that center point electrically. They don't *know* there is a resistor there.
     
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  3. stinga11

    stinga11 TDPRI Member

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    I am looking at the schematic of a hoffman ab763 and I realized that before the phase inverter there are two capacitors in series but that does not make much sense because then you would change the value of both. But it has a 47k resitor to ground, I thought that had to do with it.
    https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf
     
  4. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Caveat: I withdrew from Physics III...

    The capacitors are not in series in the sense of calculating their capacitance the ordinary way 1/(1/C1+1/C2).

    I think that what you've shown is part of a second order high pass filter, which is two cascaded first order high pass filters. It is missing the second resistance to ground, which is probably they very next thing in the circuit from which you chopped out this piece.

    I understand (dimly) that second-order high pass filters work like the familiar first-order high pass filter, except the slope at which the filter attenuates is steeper.

    The 47k to ground is a large load for the preceding gain stage, and is a representation of the original AB763's tremolo control. On the original AB763 there is also a mixing resistor between the two capacitors you are focusing on, but the Hoffman redesign omits it.

    You could always ask the people at EL34 world for their analysis and real-world experience w/ this amp.

    My best guess based on woefully patchy knowledge is that the large cap and the 47k resistor to ground are pretty much like a real Fender AB763 (a high pass filter that lets through all guitar frequencies), and then the small capacitor feeding the phase inverter (which has an input impedance of roughly 2M ohms and forms a high pass filter with that little cap (it cuts bass intentionally), will cut bass more aggressively than in the Fender AB763. But either the Hoffman people like this side-effect or aren't even aware of it.

    :confused::confused::confused:
     
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  5. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Mods please delete
     
    Last edited: Dec 10, 2019
  6. wabashslim

    wabashslim Tele-Holic

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    You're talking about a 47K resistor but showing a 500K. Then you're referring to the schematic that doesn't match up with your little sketch at all.

    I agree with tinfoilhat, you should get in touch with Hoffman and see if he can understand your issue.
     
  7. stinga11

    stinga11 TDPRI Member

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    My question is about the capacitors, if they are in series or not? Whatever the thread is updated to avoid confusion.
     
    Last edited: Dec 10, 2019
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  8. stinga11

    stinga11 TDPRI Member

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    .
     
    Last edited: Dec 10, 2019
  9. stinga11

    stinga11 TDPRI Member

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    .
     
  10. stinga11

    stinga11 TDPRI Member

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    Thanks for answering, So the individual value of each of the capacitors will not be affected?
     
  11. shortcircuit

    shortcircuit Tele-Meister

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    I wouldn't trust that schematic. It shows the tail of the PI connected to the input signal. The 47 ohm resistor is in parallel with the 47K load resistor.
     
  12. Snfoilhat

    Snfoilhat Tele-Afflicted

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    They are affected by their connection to the other with respect to how the signal is filtered, just not in the way you asked if they might be in the first post.

    Screenshot from 2019-12-10 07-40-42.png
    To the best of my understanding, this is what's happening in the circuit between the final gain stage and the phase inverter input,

    1÷(6.3×√(47000×0.0000001×2000000×0.000000001)) = 52 Hz

    If you only had one high pass filter, the 0.1 μF cap and the 47k resistor to ground, then the cut-off frequency would be 34 Hz and the slope would be 6 dB/octave.

    If you only had the one high pass filter made by the 0.001 μF PI input cap and 2M input impedance, the cut-off frequency would be 80 Hz and the slope would be 6 dB/octave.

    In this second order high pass filter that the designers (Hoffman and Rob) created by deleting some channel-mixing resistors from the original 2-channel amp circuits, the cut-off frequency is 52 Hz and the slope is 12 dB/octave.

    Low E on a standard tuning guitar is 82 Hz. If all of the above is correct (and it may not be) then I doubt anyone who has built a Hoffman single-channel AB763 or a Robrob single-channel amp has heard much difference.

    The changes in cut-off frequency and slope are happening below standard guitar frequencies.

    Why are you asking? That may help people to help you further. Good luck
     
    Last edited: Dec 10, 2019
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  13. stinga11

    stinga11 TDPRI Member

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    I am asking because I just made a deluxe reverb and it sounds great but it has a lot of gain and I am considering adding a 220K resistor to ground to take away some gain but curiously my friend wants me to build an amplifier like my Deluxe reverb. But he wants his deluxe reverb has more profit and for that reason I am considering removing the 220k resistor and this schematic adapts very well to what my friend wants. But I don't want the values of the capacitors to change, for example if the output capacitor is .1 and the input capacitor of the phase inverter is .001 I want those values to remain the same and that's why I'm worried they are in series, I don't want the value to change.
     
    Last edited: Dec 10, 2019
  14. Wally

    Wally Telefied Ad Free Member

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    Subscribed.....
     
  15. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Let's say you want to boost the gain of the last preamp stage by eliminating that 47k load, deleting it or disconnecting it from ground.

    Then the 100 nF cap and the 1 nF cap are in series with no resistor, no T-shape, no second order filter. Just ordinary caps in series Ctotal = 1/(1/C1+1/C2). Then Ctotal would be 0.99 nF. That is so close to the size of the smaller cap (1 nF) at the phase inverter that the preceding cap (the bigger coupling cap after the preamp) doesn't really matter.

    Just try it. I think that'll be more convincing than a screen full of equations. I predict it will tighten up the low end compared to a perfectly accurate Hoffman AB763, but that the additional gain will color the sound and the behavior of the amp so much that you can't really tell what about the EQ changed.
     
  16. LightningPhil

    LightningPhil Tele-Meister

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    Is it really a .001uF? The roll off would start around 1kHz. No bass at all!
     
  17. dogmeat

    dogmeat Tele-Afflicted

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    I built that amp. I added a master volume, a 3 way NFB switch, and an extra control on the reverb to get both depth and dwell. none of those mods are required, and none will change the basic amp... its a great piece the way it is. you will probably get bigger effects in trimming the sound by switching tubes and speakers. and ya, those caps are't really in series. Lightning Phil... I run the bass pot only 1/2 way up and thats all it needs. go figger
     
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  18. Mongo Park

    Mongo Park Tele-Holic

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  19. stinga11

    stinga11 TDPRI Member

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    this is my actual amp. Sound greats but has too much gain. con un cap.png

    I sent an email to Rob Robinette and he recommended that I put another resistor to lower the gain a little and I will do that. He recommend me a 330k resistor to also compensate the plate 100k resistor of the normal channel.) con dos caps.png

    That's why I say, without the resistor(T-Shape) the capacitors are in series. But with the resistor(T-Shape) are not in series. I don't plan to remove the 47K resistor because for the amplifier that I am going to build for my friend, I would eliminate the mixing resistor and leave the 47K resistor. sin caps.png
     
    Last edited: Dec 10, 2019
  20. stinga11

    stinga11 TDPRI Member

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    No, my doubt is, when the two mixing resistors are eliminated a series circuit is created and the total of the two capacitors does not have the original values. But according to what I have understood by the explanations above on this thread, when the 47k resistor to ground that replaces the load of the tremolo potentiometer is added, the circuit is no longer in series and the capacitors have the original individual values.
     
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