Problem with tone controls. Please advice.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JimLamme, Jan 19, 2005.

  1. JimLamme

    JimLamme Member

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    Hi there,

    I just bought a DrZ Maz18 NR and with the limited time I had to try it, everything seems fine.

    The one problem I have now is the bass tone control doesn't have any increased response after 9 o'clock. From 7 to 9 its noticeable change in bass but not after 9. The movement of the pot seems ok.

    I looked at the bass pot and see the letters B250K and 4F1 on the back of the pot. It has the same feel as the rest of the other pots in that its smooth when turned.

    Anyone can tell me if its normal for the range to be so limited? Can anyone advice so I can decide if I should send it back? Its a minor issue but I would like something to at least work as it should be when new.

    Thanks.
     
  2. fullerplast

    fullerplast Senior Member

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    FWIW, that was my experience also with my Maz Jr and Maz 38 (I no longer have either amp). Most of the useful range of the bass control was in the first 30% of the pot's range. I found the mid and cut control to be the most useful with those amps. Bass and treble had limited useful range IMHO. That's not too uncommon with other amps either.
     
  3. John Phillips

    John Phillips Member

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    It should be possible to get a more useful sweep by replacing the Linear (that's what the 'B' means) pot with a Log(arithmic) or 'audio taper' (A) pot. It won't affect the total amount of bass available, or anything else about the amp's sound - if you stick with a 250K - just the sweep of the bass control.

    With a Linear pot, about 1/3 of the total track resistance is between 0 and 3 (9 o'clock, roughly). With a Log pot, 1/3 of the resistance is between 0 and about 6 or 7.

    (With a Linear pot, half the resistance is at half-turn; with a Log, 1/10th is at half-turn.)

    NB - this is assuming conventional wiring of the control, not a 'backwards' configuration, where a Log pot will actually make things worse.

    Hope that makes sense!
     
  4. JimLamme

    JimLamme Member

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    Hi fellas,

    Thanks for the info. Although I still think its peculiar that a linear pot is used for the control, I at least understand that its normal when using a such a pot and that its common in amps. That made a lot of sense and is what I suspected. Hmmm... I wonder if I should change to a log pot?

    fullerplast, now that you've mentioned it, the mid and cut controls do seem to be the most useful with these amps. Did you sell the 2 Maz because of this reason?

    Thanks & Regards.
     
  5. fullerplast

    fullerplast Senior Member

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    Jim- Before you change the pot, why don't you run the question by Mike (Dr Z) via email? Offhand, I don't know why he would choose a linear pot over an audio taper pot for this application. He may have a good reason or he may offer to change it for you.

    As for selling the amps, I just found other EL84 amps I liked much better. It's just personal preference. Mike builds some very fine and good sounding amps.
     
  6. Wakarusa

    Wakarusa Member

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    The behavior you're seeing may also be implicit in the tone stack's design. Don't know what values/setup Dr Z has used, but a good example is the "classic" Fender tweed tone stack (56K slope, 22nF mid & bass caps, etc).

    If you sweep that setup you'll notice that bass boost beyond about '5' on the bass control occurs primarily below 100Hz. So for guitar you don't really hear that much difference.
     
  7. JimLamme

    JimLamme Member

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    I tried asking asking Mike about it and he gave me a couple suggestions about changing tubes and trying out the bass tone control while varying the other setting to see if that helps. None of them really helps.

    Its hard to tell from an email but I think he wasnt too happy when I ask him about it. Maybe because I bought it used. Told me to "take it back to the unauthorised distributor I bought it from". He did give me a few suggestions but I didnt want to ask anymore so I just thanked him for his time.

    Anyway, I guess I'll just learn to live with it or like it. Maybe its a combination of tone stack and the pot.

    Thanks for the response guys. You people have been great.
     
  8. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    I'd say you're reading the response correctly.

    Personally, I believe a smart/caring builder understands the importance to all concerned of maintaining the performance and value of his products throughout their lifecycle.
     
  9. ekp

    ekp Member

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    When one analyzes the tone stack you can readily see the problem, The Fender style uses a 100k resistor from the input of the tone controls down to the bass and mid controls. The bass control is often 1 meg. This means that once the bass control reaches 100k the rest of the travel is less than 6 dB. Depending upon the pot, that can be in the center or less, particularly if the mid control is up too.

    One way of dealing with that problem is to replace the bass control with a smaller value and potentially replace the bass cap with a larger one... You could probably use the extra treble anyway. If not, put a compensating resistor inseries with the treble pot...

    Have a great day, Eric
     

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