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Looking for less low-cut in Randall RVC (like RD5C)

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  • Looking for less low-cut in Randall RVC (like RD5C)

    I couldn't resist buying a Randall RVC tube combo amp on clearance at GC for $199. The only thing I don't like about it is that there is too much low-cut happening. From what I've read, this is to keep things from getting too muddy at high-gain, but I don't ever turn the gain past 11-o'clock, if even that. If I plug my guitar directly into the effects return, there's plenty of low-end, so I just want to try to adjust the amount of low frequencies that must be filtered out before that. The amp has a single tone knob, and a three-way switch to select a mid-boost, flat, or scooped-mid preset. All of the presets exhibit the same lack of lows, so I figure there may be a single cap that I can swap for a different value to see how it sounds with more lows. I haven't been able to find schematics for this relatively newer amp online. How do I go about finding which capacitor I need to change?

  • #2
    Okay, I started working on tracing the PCB to create a schematic. I've only got as far as the first stage of the first pre-amp tube, but figured I'd post it in case it helps. This is the first schematic I've tried to write up, so hope it's alright so far.Click image for larger version

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    • #3
      The first stage bypass cap (.68) is a good place to start.

      You can go as high as 25uf.

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      • #4
        After almost a year, I got around to trying the change recommended above as a start. I swapped C20 and C23, the cathode bypass capacitors for V1a and V1b. They were both .68uf 100V polarized electrolytics. I replaced them with 10uf 25V non-polarized electrolytics and was really surprised by the dramatic difference. The bass response improved significantly, at the expense of some gain. For my use, this is actually a plus as well, since the amp had more gain on tap than I needed before, and now it actually has a usable clean range (at least at decent home volumes). I figured I should at least post a reply to share what I found. If anyone has one of these amps, I highly recommend making this modification.

        Now that it's actually producing some lows, I may want to swap out the tiny output transformer, though, we'll see.

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        • #5
          0.68uf as a bypass capcaitor effectively shunts all bass frequencies to ground.

          It's good if you have a high gain situation, as the bass notes sound bad when you are looking for uber distortion.

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          • #6
            I like to think of it as the gain of the 12AX7 being reduced at bass frequencies because a 0.68u cap doesn't fully bypass the cathode resistor at low frequency. And why use an electrolytic for 0.68u?
            Last edited by Dave H; 05-02-2015, 11:13 PM.

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