DRRI preamp mod

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by cinstan, Feb 15, 2012.

  1. cinstan

    cinstan Member

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    west of Boston
    Was struggling with the issue of weak plinky tone when OD pedals are turned off and considering a clean boost pedal to add back in the missing girth (ala David Barber's "amp cooking" strategy) {the googler knows about this if you're unfamiliar}. This calls for setting the dirt pedals to unity gain and using a clean boost pedal to overdrive the amp's input.
    I tried this and was underwhelmed with the results.
    It occurred to me in a dream after eating too much chili one night: There's the normal channel preamp with it's glowing 12AX7, tone stack, etc. just sitting there unused...hmmm.
    I rewired the amp so that the output of the normal ch. preamp tube (V1) now feeds the normaled contact of the Ch 2 input jack. This allows normal operation if you plug into ch 2. It worked fine but there was less gain available than I wanted so I disconnected the tone stack (like a PAB switch in the Dumble-esque designs) and presto! I now have a real tube-based clean boost (0-25db) built right into the amp. It sounds great and doesn't take up any real estate on my pedal board. Must resolve to eat more chili in the New Year:)
     
  2. guitarcapo

    guitarcapo Senior Member

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    Basically you took V1 and made it 2 extra gain stages to the reverb channel? I'm surprised that doing that gave you so little gain that you had to bypass the tonestack. Are you feeding the signal directly into V2 or is it going through that resistor at the input jack? I would also think this idea would add noise from V1 into your signal when it isn't being used (if it's hard wired and not switchable). Also I'm surprised that you aren't getting so much gain that it's killing headroom. But if it works for you that's great. Got clips?
     
  3. cinstan

    cinstan Member

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    Location:
    west of Boston
    The tone stack loads the signal to the tune of 15db. Yes, the signal is connected to the jack, rather than V2, this way plugging directly into the #2 jack on the vibrato channel breaks the connection and the amp works normally. It's surprisingly quiet when the channels are cascaded, no noise. Sorry, no clips but it sounds like a very good clean boost adding just enough gain so that when the guitar is turned all the way up the amp is just starting to get overdriven. It was really just a volume level management issue, when I had gain cranked in from a dirt pedal, turning the pedals off resulted in weak tone at a lower volume so I could never solo with out the pedal. This way, the pedals add dirt when switched-on but there's no level change so I can use the tone of the guitar directly into the amp...very cool.
     
  4. guitarcapo

    guitarcapo Senior Member

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    I have a tube pedal that does something similar. Badcat 2 Tone. The clean setting is just half a 12ax7 to the jack.
     
  5. cinstan

    cinstan Member

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    Joined:
    Jul 14, 2007
    Location:
    west of Boston
    Yeah, same result in the end, I was just interested in doing it without adding another pedal. I never used the normal channel anyway so it was just sitting there doing nothing...too tempting to resist.

    Thinking about offering this mod as a service, I run a small amp repair business...
     
  6. guitarcapo

    guitarcapo Senior Member

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    I had thought of doing something similar to my Super Reverb but did a Diaz mod instead. When I turn down my vibrato intensity to 0 it clicks off and adds one half of a 12ax7 to the signal. I also added reverb to the normal channel with a jumper wire. Not only does this give reverb to the normal channel, I can now jumper them.
     
  7. guitarcapo

    guitarcapo Senior Member

    Messages:
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    Joined:
    Mar 16, 2010
    I had thought of doing something similar to my Super Reverb but did a Diaz mod instead. When I turn down my vibrato intensity to 0 it clicks off and adds one half of a 12ax7 to the signal. I also added reverb to the normal channel with a jumper wire. Not only does this give reverb to the normal channel, I can now combine them without phase canceling
     

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