Installing a midrange control on a Fender

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mcuguitar, May 15, 2008.

  1. mcuguitar

    mcuguitar Gold Supporting Member

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    I have a Fender Custom '64 Vibroverb with the 15" speaker and the stock Diaz mod. I want a midrange control on this amp, as I can't get enough midrange w/the stock bass and treble controls. Please don't tell me to buy an eq pedal. I want a midrange control installed. Who is qualified on the northwest coast to do this with a proven track record, and under a month turn around times? I've got gigs all through the spring and summer, and I don't want to be without this amp for 2 to 3 months. If anyone has this info, and can recommend someone really good, I'd be very appreciative. Thank you, Jon in Idaho.
     
  2. VikingAmps

    VikingAmps Member

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    It's a very simple thing to do. All you have to do is replace a resistor with a pot. You do need to find a hole for it tho.
     
  3. mcuguitar

    mcuguitar Gold Supporting Member

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    If you have a schematic for a 2006 Vibroverb Custom with the 15" speaker, with the resistor circled that I need to replace, that would really help. Jon.
     
  4. Blue Strat

    Blue Strat Member

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    It's the resistor between the right lug of the bass pot to ground.
     
  5. soldersucker

    soldersucker Member

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    Change it to 10k and you have a traditional Blackface(SR etc..)mids on 10" sound takes about 25 min and costs 20 cents.Know in some parts as the "nashville mod.Looke here for how to make it a true 25 k adjustable mod- http://www.mercurymagnetics.com/pages/articles/ToneQuest/TQRJul03MM.pdf
     
  6. mcuguitar

    mcuguitar Gold Supporting Member

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    Thanks guys! It sounds pretty straightforward. I'd like to use the 2nd input jack hole on the Vibrato channel as the place to mount the midrange knob. Best place to order quality diodes, capacitors and 20K pots?
     
  7. Blue Strat

    Blue Strat Member

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    Not sure why you need diodes for a bass control mod, but www.mojotone.com should have everything you need.
     
  8. SatelliteAmps

    SatelliteAmps Member

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  9. mcuguitar

    mcuguitar Gold Supporting Member

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    Thanks Mike and Adam. I ordered a 25K CTS pot from Mojotone as well as a few capacitors for another project. I might have a few questions...one of the main ones is where to mount the pot...I could use input #2 on the Vibrato channel, but I'd need a smaller knob so that the there isn't any interference with input #1 or the bright switch to the right. Any body have anything lying around that would look sweet and fit there, or suggestions? Jon.
     
  10. soldersucker

    soldersucker Member

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    Use the extension speaker jack it's a set and go deal for most of us and retains the stock appearance.
     
  11. mcuguitar

    mcuguitar Gold Supporting Member

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    Thanks solder...I will!
     
  12. willhutch

    willhutch Supporting Member

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    I just completed mod 15 minutes ago on the SFVR. Comments:

    1) I used a 50K pot, as that is all that Radioshack had in stock. I mounted the pot through the extension jack hole.

    2) The job was quick and easy.

    3) Duh - don't forget to reconnect the output jack. The most time consuming part of the job for me was figuring this out and executing. (I'm an amateur).

    4) Results:
    a) It adds a new dimension to the tone. You can dial in some beefiness that the amp doesn't normally give. It is quite useful at low volume, where BF an SF Fenders can sound glassy. I fills out quite a bit.
    b) The effect of the bass control is greatly reduced with the mid control added. It seems as though the higher you crank the mid control, the less effective the bass knob becomes. With the mid pot full open, the bass control doesn't do much, if anything. The bass frequencies are there, but not controlled by bass control.
    c) I only implemented this mod on Channel 1. It doesn't affect the reverb channel.
    d) I modified the amp years ago to get reverb on both channels. One beneficial side-effect is that the channels are now in phase and can be jumpered. I prefer the sound of the amp run this way. With jumpered channels, I've found that the effect of the mid control is dependent on the relative volume settings of the two channels. Since chanel one has the mid pot, the pot is more effective when channel one is louder than channel two. Shouldn't be surprising, but I wasn't considering this aspect of performance when I installed the mid pot.

    Questions:
    1) Why is bass control's effect reduced as the the mid pot is turned up?

    2) Can I do the same mod to the other channel by repeating the process? Can I remov the 6.8K resistor from the other bass pot and connect it to the midrange pot?

    3) How does the resistance of the pot affect the tone of the amp? I imaging a smaller pot, 25K, would make the effect more subtle. As i is, the effect is not overpowering. It yields usable tone throughout the sweep of the pot, including wide-open.

    4) The amp seems a little buzzier than it was before. That is, the buzz you get when you take your hands off the guitar seems louder than it used to be (but I could be hallucinating). Any tips for routing the wire(s) that runs across the chassis? Is it subject to noise introduced by proximity to heater wires?
     
  13. alltone

    alltone Member

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    A 10k or 25k pot will bring the tone closer to the design parameters and will return your bass pot to a useful state.The mid pot you are adding replaces the 6k8 resistor. A higher resistance will lessen the effect of your tone stack but will increase the gain in that channel. A length of shielded cable with the shield grounded at one end only for your mid pot may reduce your buzzing problem. Cheers, Doug
     
  14. drgonzoguitar

    drgonzoguitar Member

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    Hey Will!

    To answer your questions....

    1) Why is bass control's effect reduced as the the mid pot is turned up?

    - Download the Duncan Tone Stack Calculator:
    http://www.duncanamps.com/tsc/

    - Plug in the values and move the virtual BMT controls...You will see how an increase in the mid range could push lows out

    2) Can I do the same mod to the other channel by repeating the process? Can I remov the 6.8K resistor from the other bass pot and connect it to the midrange pot?

    You would need another mid pot.

    3) How does the resistance of the pot affect the tone of the amp? I imaging a smaller pot, 25K, would make the effect more subtle. As i is, the effect is not overpowering. It yields usable tone throughout the sweep of the pot, including wide-open.


    See answer 1...again play with the tonestack to get a visual representation. Mess with values! Its is the safest way to experiment with tone stacks.

    4) The amp seems a little buzzier than it was before. That is, the buzz you get when you take your hands off the guitar seems louder than it used to be (but I could be hallucinating). Any tips for routing the wire(s) that runs across the chassis? Is it subject to noise introduced by proximity to heater wires?


    If you place the mid pot in the back of the chassis, you are going to have more noise. There really is no way around it. The closer it is to the rest of the tonestack, the less noise you induce.

    On another note....if you actually drill a hole into a '64 chassis---I am coming over to take your tools away!
    :eek:
     
  15. willhutch

    willhutch Supporting Member

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    The tonestack calculator is really interesting. I can see how the resistance of the midrange pot affects the interaction of the tone controls. Basically, the larger the value of the midrange pot, the less influence the bass and treble controls have.

    I will experiment with a different value pot for educational purposes. However, having jumpered channels, I can use the tone controls on channel two to adjust highs and lows. The big mid pot on channel one adds gain to that channel, which is cool.

    I also like the idea of a shielded wire grounded at on end. Can I get a length of this at Radioshack?
     
  16. VikingAmps

    VikingAmps Member

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    If you like the range of the 50K pot but don't like the way it makes the bass unadjustable with the mid up you can put the original resistor back in and run the pot in line with the cap that normally goes to the junction of the bass pot and resistor. Just disconnect the cap from the bass pot there add the mid pot in series. 100K works fine in this application too and you may want to lower the value of the resistor to 2.7K or so for more range.
     
  17. drgonzoguitar

    drgonzoguitar Member

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    If you have a guitar cable you are willing to sacrifice, you will get your cable with ground!
     
  18. willhutch

    willhutch Supporting Member

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    Yeah. I realized this as soon as I postedmy last comment about buying shielded cable. The deed is done. It helped some.
     

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