Vibro Champ Gain Mods?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Rick1114, Jul 11, 2008.

  1. Rick1114

    Rick1114 Member

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    I'd like to get just a little more gain out my BF Vibro Champ & wanted too ask if anyone could comment on these Champ tweaks I found on Premier Guitar's site written by Trace Davis of Voodoo Amps:

    "Let’s start with a word of caution with regard to adding more gain: When you increase the gain on these amps, you inevitably sacrifice the overall clean headroom the amp would otherwise have. So, please keep that in mind when making changes to the circuit. Gain mods: The most common mod would be to increase the plate resistors on the 12AX7’s (pins 1 and 6). I’d recommend starting with the first plate resistor, as that might be all you need. The stock plate resistors are 100K’s, and you can use any value from 110K up to 220K. Other gain mods would be to lower the cathode resistors in the preamp. The stock cathode resistor values (pins 3 and 8) are typically 1.5K’s, and you can go as low as 820 ohms. "

    The full article is here:
    http://www.premierguitar.com/education/200607_educationcenter_tubetalk.asp


    I might just try the first bit with plate reisistor because I'm just looking for a bit more "oomph" from the amp for blues. It just had a cap job and a new Weber speaker, so it's getting there - what do y'all think?

    Rick
     
  2. soulohio

    soulohio Member

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    Rick, don't do it. i have a beeyootiful old silverface Vibro Champ and the clean headroom is at a premium as is. Don't do these mods unless you can easily reverse them. If you want more gain use a Bad Monkey.
     
  3. Lyle Caldwell

    Lyle Caldwell Member

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    I see a lot of people recommending lowering cathode resistors, without mentioning that the cathode resistor works in conjunction with the cathode bypass cap value.

    For example, if you have the common Fender pairing of a 25uf cap and a 1.5K resistor, you have a certain amount of gain set up, but also with a specific high pass filter at 4.24Hz.

    If you change the 1.5K to an 820, then the gain increases and the filter cutoff is now at 7.76Hz. That's not really a big difference, in the Fender.

    But if it's a Marshall, let's look at it again.

    Say it's a Plexi with a .68f cap and a 2.7K resistor. That gives a filter cutoff at 86.65Hz.

    Change that 2.7K resistor to a 1.5K and now the filter cutoff is at 155.97Hz. That's a big difference, as it reduces the volume of everything below the low A string.

    So if you wanted to increase the gain on that Plexi cathode stage while not affecting the lows, you'd want to change the cap as well as the resistor.

    Using commonly available caps, the closest match would be from a 1uf cap run in parallel with a .022uf cap, resulting in a value of 1.22uf, which gives a cutoff of 86.93Hz. As caps have a pretty loose tolerance, that's realisticly going to sound the same as the stock 86.65Hz cutoff, only you will have more gain from that stage.

    Back to Fenders, they all have cutoffs at 4-8Hz. Your speaker cannot reproduce these frequencies, and we cannot hear them. But the fingers on the strings do produce them, and the amp gamely wastes gain on these frequencies, which the plate or cathode follower coupling cap then removes anyway. So you can get a good bit more clarity and waste less power from a tube stage if you increase the cap value to get a higher cutoff frequency.

    For example, the classic Fender 25uf and 1.5K combo has the cutoff at 4.24Hz. If you change that 25uf to a 4.7uf, the cutoff is raised to 22.57, which is still an octave below a bass guitar's low E string. Change it to a 2uf and the cutoff is at 53.03Hz, which is almost an octave below a guitar's low E string. These are both very usable values that don't change the perceived low end from the amp, but you aren't wasting gain on subsonic crud that will never go through your speakers anyway.
     
  4. fullerplast

    fullerplast Senior Member

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    Probably one of the easiest and more popular gain mods for the VC is simply the tone stack lift. Easy to switch in and out also. You may still want to reduce the size of the cathode cap with this mod also to reduce flubbiness, but try one thing at a time.

    It's been speculated that this was actually Clapton's Layla setup.
     
  5. plexi67

    plexi67 Member

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    what fullerplast posted.. another would be to lift the feedback,both of those suggestions will be the easiest,and least intrusive. meaning its easy to put back to stock if you wanted to.
     
  6. Axekisser

    Axekisser Member

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    I added variable negative feedback and variable tone stack (mid lift/soul control) on my Champ. Just added a pot inline to the standard feedback resistor and a pot in place of the grounding resistor on the tone stack. The amp really pops now and sounds much bigger than 5 watts and an 8" speaker. I did have to drill a couple of holes but it's a '79 Champ. I would think before doing it to a BF model.
     
  7. Pencho

    Pencho Member

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    Is there a way to wire in the tone stack lift mod to a push/pull volume control for easy enable/disable? I'd like to have some extra gain on tap for my 66 VC but only if I can switch back and forth easily. Thanks.
     
  8. JJman

    JJman Member

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    Yes. Use the switch contacts on the pot to break the connection of the "mid resistor" to the bass pot. I leave the mid resistor's ground connection intact and disconnect it's other end for inserting the switch on my Fenders. The mid setting on the VC is already on "15" but changing that path to infinity does add mids and gain.
    .
     
  9. Pencho

    Pencho Member

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    excellent, thanks JJman!
     

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