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

[doublesigma]

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?

[doublesigma]

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.

[Jazz P Bass]

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

You can go as high as 25uf.

[doublesigma]

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.

[Jazz P Bass]

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.

[Dave H]

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?