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Bass Issue?

Discussion in 'Solid State' started by tube-a-lou, May 16, 2018.

  1. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Hi all,

    I'm restoring a 60's Thomas Vox Pacemaker guitar amp, and for me it has a touch
    to much bass, so I have no schematic. In between the treble and bass (pots) controls there
    is a 1uf cap, it was a electrolytic which I changed to a poly and it has bass now (more) plus on the
    bass pot on one outer and middle tab is a 100ohm resistor. If I change the cap would it change the
    deepness of the bass and which value should I attempt?
     

     

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  2. elnaldo

    elnaldo Addicted Member

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    hello. we need a schematic to advice on that subject. Increasing a capacitor can increase the bass or tame it, depending of the application.

    you have usually coupling capacitors between the amplifier stages that act as hi pass filters, decreasing the value will cut the bass. Using some on-line calculator like this one you can check the cut freq. http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php NO AFFILIATION

    Is it a solid state amp or a tube amp?

    I'd stay with the original design and turn down the bass pot.
     
  3. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    It's a solid state, I found this it's from one of there other models but they seem
    to use the same circuit in all the models just with the reverb, which mine does not
    have. This has the preamp section with the two tone controls.


    fullsizeoutput_6177.jpeg
     
    Last edited: May 17, 2018
  4. elnaldo

    elnaldo Addicted Member

    Messages:
    7,391
    Location:
    Buenos Aires GMT-3
    I see. C1 is a hi-pass filter, reducing C1 will drop the bass getting into the amp.

    Then C11 cuts the bass out of the volume control to the power amp.

    C5 cuts the bass going to the tone control

    If you just want to cut the bass without any other mod, replacing any of those caps is the easiest way. I think , to really hear a difference, you could start with 1/10 of the value and listen, to adjust later to your taste.

    If you want full bass, but changing the way the bass control works, you need to modify the tone control circuit. Remember, capacitors let high freqs to pass, and block or offer high resistance to lower freqs. So any time you increase a capacitor value, you let more low freqs to go that way. And, reducing a capacitor, less low freqs can go that way.
     
  5. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I started at C1 first dropping it from .1uf to .047 it's a coupling cap so that's easy. C11 on mine is
    a .2uf don't know if I could find a lower cap than that but will try, C5 is a electrolytic and I think
    that was replaced to a 2.2uf by someone else. My main problem is the the bass control does not
    have much control over the bass I could go 0-3 and it's fine after that 4-10 just boomy.

    Thanks for the help it really great!
     
    Last edited: May 17, 2018
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,126
    Location:
    uk.. the middle bit
    dont know much about this circuit but would c9 r13 play a part here ?
     

     

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  7. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    That's right on the bass pot lug's too. it's the 1uf and 100 ohm resistor, would that effect the tracking of the bass control. Here's a picture of the inside of the amp.


    P1070043.JPG
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,126
    Location:
    uk.. the middle bit
    watch c9 as non polar ..not they are dangerous or anything .. :D
     
  9. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I took it out last night and put a 1uf poly cap in and woof!. What I really want is a
    smoother taper when I increase the bass control.
     
    Last edited: May 17, 2018
  10. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Okay tonight I removed the 1uf cap, I put in a .0047 and that made a big difference the
    bass is more adjustable now. I'm still going to try a .01,.022 and .047 the .047 is about
    halfway so it should make some difference. If anyone has any ideas feel free to add
    something.
     
    bigbobster likes this.
  11. elnaldo

    elnaldo Addicted Member

    Messages:
    7,391
    Location:
    Buenos Aires GMT-3
    Check out this site. Vox and Fender tone controls explained, I think all your answers are there. Read the basics and scroll down to the "Tone stack bass". https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm

    Seems that the bass is cut by C5 and R10-11-13 making a RC high pass filter, being R11 variable, the cutoff freq changes. The resistance will change from 1.5Kohm to 11.6Kohm http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

    There are ways to parallel a resistor to a pot to make it smoother.

    I see your bass pot is 10K , but the "output" is in parallel with 100 ohm, so one half is always less than 100ohm. You are by-passing C9 with the bass pot.

    Imagine you have the pot at 1/2, you have 5K at one side, and less than 100 ohm on the other side (the other 5K in parallel with 100ohm). That's 50:1. With the pot at 10%, you have 1K at one side , and <100 ohm at the other side. that's 10:1, and you said at that level things are OK.

    Seeing this, I think a resistor between the wiper and the end connected to C9 will make it smoother. Imagine you have a 5K resistor in parallel with that half of the pot, when the pot is at 50% (5K), you have 2.5K at that half (5K pot, parallel with 5K resistor), and <100 ohm at the other half, that's 25:1, a bit better than the 50:1 ratio without the resistor. A smaller resistor will decrease that ratio. For example, a 2.2K, when parallel to the pot at 50% (5K) will give you 1.52K, that's 15:1 ratio against the 100 ohm resistor. The problem is that the max resistance will be always lower than the resistor value, so it will never reach 10K, so the bass will never be at max.

    Doing this will have unpredicted results in the tone control slope. But I'd try a couple of resistors to see if you find a good balance. I think 5K is a good starting point.

    Also, reducing C5 should change the range of the control. I'd try less than 1uF (0.68 perhaps). This shouldn't affect the treble, since high freqs should pass that capacitor.
     
    tube-a-lou likes this.

     

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  12. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I did change C9 to a .0047uf and heard a big difference, if this route doesn't work I'll start trying
    the resistor change. I was surprised tonight when I changed it the sound opened more and
    the bass control doesn't get deeper till it starts to hit 6 on the knob. Before the switch I could
    not go past 4 on the dial it would get to boomy. I'll read the info thank you
     
  13. elnaldo

    elnaldo Addicted Member

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    Location:
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    Check what's happening at midrange freqs when you change C9. From what I see, reducing C9 offers more resistance to low freqs, so they go through the potentiometer, which is in parallel. At a point, the pot completely bypass the capacitor, so perhaps this is the easier solution without cutting the bass response.

    Before your change, more low freqs could bypass the pot, perhaps that gave less control.

    Taking a second look at it, I see the effect is similar to what I suggested, a resistor in parallel with C9, so signals find less resistance at the pot.
     
    Last edited: May 18, 2018
  14. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Okay, It's really hard to change anything on the board, there's to many wires to remove and
    it's a big job, so it's better to change things outside. I alway's wait till I have to do a major job
    on it like replace the rest of the caps and such these amps aren't known for easy access. So
    I'm hoping this plan of action works.
     
  15. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Well I took out the 2uf cap and C5 at put in a 68uf but that had no effect still bassy I even tried
    raising the resistor on the bass pot to 5K nope nothing works there.
     
  16. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Further update, I replaced C11 from a .22uf to .068uf the bass got weaker but the noise
    level increased.
     

     

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  17. elnaldo

    elnaldo Addicted Member

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    Hello. I think you were on the right track decreasing C9. Or, what I suggested was to connect a 5K resistor in parallel with C9. Replacing the 100 ohm resistor with 5k should increase the bass.
     
  18. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I goofed! the reason why C11 was noisy was because I put in a .0068uf not a .068uf,
    this is what caused the noise I guess it was to weak. The C11 is for the driver section
    and when I changed it the bass did get weaker so I might try a .1uf 100v.
     
    Last edited: May 20, 2018
  19. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Making headway this morning, changed the .22uf C11 cap to a .1uf and the bass is a bit less will have
    to try a .047uf down from the .1uf and see what it does.
     
  20. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Yep, C11 is the cap to change the bass, I just changed the speaker to a ceramic type and
    no bass!
     

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