Background Recently I bought a Mesa Boogie V-1 Bottle Rocket to see if it would work as a clean preamp for bass guitar. I collect and use vintage vacuum tubes, mostly for audio, so the V-1 seemed promising. Here is a summary of my initial experiences with this pedal: what I learned and did, and the results I got. Note that as of this writing, I am an electronics novice (regarding physics, circuits, soldering, etc.) [DISCLAIMER: I make no guarantees about the accuracy of my information here, or of its applicability to your pedal. If you try the modifications I describe and damage your V-1 Bottle Rocket or change its sound in a way you don't like, I am in no way responsible. Even if I say things like "I suggest", this post is purely informational; I am not recommending that anyone actually apply the information.] The stock sound The previous owner of my V-1 Bottle Rocket, and others, said the pedal had huge bass output and was "dark" sounding. The bass output is what I needed, so I bought the pedal. In use with bass guitar, I found the pedal to be as described: it had massive bass output and it sucked treble. Other descriptions I read were also true. Warmth, nuanced harmonics, and classy overdrive sounds were all there. But for using it as a mostly clean tube preamp, the lack of treble output was a deal killer for me. Even trying all my tricks with 12A?7 tube types (more on that later) and with the treble knob cranked, I could not stop the V-1 Bottle Rocket from sucking treble. Modifications: tubes [Debate about the superiority of vintage tubes to current production tubes is beyond the scope of this post. So in this post I will assume that, though there is no accounting for taste and specific applications, vintage tubes are in all ways better than modern ones. As such, when I mention tubes here, I mean vintage ones.] The V-1 Bottle Rocket has two tube sockets, called V1 and V2. Facing the pedal as for normal use, V1 is on the left and V2 is on the right. As I understand it, V1 is for input gain and optional overdrive, and V2 is for output gain. MOST IMPORTANT: To use the Bottle Rocket as a clean preamp, especially for bass guitar, do not use 12AX7 or even 5751 in V1; skip 12AT7, too. Instead, use 12AU7. In V2, which tube type to use will depend on what the Bottle Rocket feeds. For maximum gain, use 12AX7. Lower gain alternatives can include 5751 and 12AT7. I recommend not using 12AU7 in V2. Though the tubes in both sockets will govern the pedal's voice, V1 is the most impactful on the "clean tube preamp" application. Each tube variety (the combination of make, model, year of production, etc.) will sound different. Try what you have, either for discovery or to meet a sonic objective. My tube collection affords me an obscene range of sonic options. Just in case the cost of tubes is not an object for you, for using the Bottle Rocket as a clean bass preamp I suggest this strategy: Use a technical, hi-fi 12AU7 in V1; a variety known for supposed neutrality, very high headroom, and extreme but composed high frequencies. La Radiotechnique (RT), Telefunken, and lab grade Philips Amperex (7730, etc.), or maybe vintage Tungsram are obvious choices, but pre-1951 Ken-Rad is a more soulful alternative, albeit with less headroom. Siemens will also do, but earlier production only. Then use V2 to take that hi-fi audio and nice it up with a tube that is high quality, but more soulful AND has excellent high frequency output. Pre-mid-1960s GE or Tung-Sol, late 1950s RCA or Sylvania, Brimar, or pre-1970s Mullard will do fine. Again, Ken-Rad could be great, but probably not if you already have Ken-Rad in V1 (that would over-emphasize the signature Ken-Rad frequencies.) Modifications: circuit board Checked by the lack of treble output, I researched V-1 Bottle Rocket schematics and searched for comments online. Finding nothing I could use, I noticed some unused pads for trim pots on my pedal's circuit board: "1M TRIM, PRES. ADJ" and "20K TRIM, MID ADJ". I wrote to Mesa Engineering, asking for advice. Could I use those adjustments or change capacitors to get more treble output? After waiting some days for a reply, I tried the trim pots approach, soldering the specified trim pots to the pads on the back of the board. The presence adjustment only let me roll off treble, not boost it. The midrange one had no audible effect for me. Then Mesa replied. The rep had spoken with their resident V-1 Bottle Rocket tech support expert. They corroborated my experience of the trim pot adjustments. And they told me how to boost treble output by adding a capacitor to the stock circuit. (Joy!) The tech said "add a 500pf in parallel to R21 (220k). If you ever sell it, please clip it off." I thanked them profusely and promised to remove the 500pf cap if I sold my V-1. IMPORTANT: To honor my promise to Mesa, I must at least ask that you consider removing the added 500pf cap if you are going to sell your V-1 modified in this way. What I, the novice, learned included reading schematics, reading 5-band resistor color codes, and the perils of disassembling and reassembling a pedal of some complexity. I also learned that I make less good decisions at 5:00 am, when on a mission and sleep deprived. 500pf caps acquired (I chose silver mica), I disassembled the V-1 and located all its 220k resistors. Most of their labels on the board were visible, but a few were not. I chose one and used leads with clips to test the 500pf cap with it. I played bass through the V-1 in this state. Liking what I heard, and believing I heard the V-1's treble output boosted, I soldered the new cap to the V-1 and reassembled the pedal. May I say that I did a pretty good job, too; neat, clean and complete. (Refitting the LED was the hardest part for me. I was unable to wedge it in tightly, as I found it, but got it to be held in place by other parts.) A few hours later, however, I doubted myself and called BS. To prove it, I opened the V-1 and inserted another 500pf cap (it had long, radial leads) under the leads of a different 220k resistor on the V-1 board. The cap stayed put there, so I was able to play through the V-1 in this state. Success. At last, the veil had been lifted. Having found the correct resistor this time, I could hear all the small movements of my fingers on the strings, all the snap, snarl, chiming and twang that was missing in the V-1's stock circuit. All the articulation from the sound of pedals feeding the V-1 was restored. This was a clean tube preamp for bass guitar. It sounded natural in its presentation of my instrument's sound. By then it was about 5:00 am and I was toast. I couldn't face disassembling the V-1 again, and I didn't want to wait until after sleeping. So I desoldered the first 500pf cap from the incorrect 220k resistor and soldered it onto the correct one. Already lacking the proper soldering iron and tip, I made a mess this time, scorching components and melting a bit of wire insulation. Sad. I wrapped the damaged insulation in good electrical tape. Someday when I have skills and better tools, and am well rested, I will clean up that job. But for now, my modified V-1 Bottle Rocket works, meets my sonic objectives, and sounds great. Conclusion The Mesa Boogie V-1 Bottle Rocket is a "secret weapon" audio processor for bass guitar. Used stock, it is an excellent overdrive and a very good preamp, if with a narrow sonic focus. But with the addition of the 500pf cap, it becomes a credible tube preamp for clean preamplification of bass guitar. The result is not like Demeter, Universal Audio, Millennia Media, etc. It cannot compete with their capacity for slight coloration or high headroom. Instead, it is much more like the preamp in a vintage guitar tube amp. It is a bit wooly, quite sweet, noticeably compressed, and tough but relaxed-sounding. I am using my V-1 Bottle Rocket as if it were the flavorful preamp for my bass amp (an Acoustic Image Focus 1 Series III that by itself sounds sweet enough and very clean, but is rather devoid of personality.) An Aguilar TLC compressor (set at maybe 3:1 or 4:1, with the threshold in the middle of its range) is just before it, and all other effects feed the compressor (I don't subscribe to the "clean amp for effects" approach; I like an amp with saturation and a real voice, into which I feed whatever I have, including spatial effects). In addition to its general duties, the compressor spares the V-1 needing more headroom: I still get plenty of amplitude dynamics into and out of the V-1, which is itself very responsive within its limited dynamic range. Though by no means scooped-sounding, the modified V-1 Bottle Rocket does have very articulate, almost delicate highs, massive low end, and somewhat diminished upper midrange. Also, the treble pot can now be used for actual tone shaping: for real presence, it needs to be cranked; but rolling it off makes velvety tones that remind me more of the passive tone pot on a vintage bass than of the treble pot in a preamp of a bass or guitar tube amp.