The Tech 21 Private Stock Formula 21 Vacuum Tube Bass Pre-Amp is a tube-driven preamp for bass, built as a Private Stock experiment: founder Andrew Barta wanted to hear how the SansAmp Bass Driver would sound if he made it with real tubes1. The result, in Tech 21's words, is "a product that is unique on its own with old-school analog, vintage sound"1. Because it is a preamp rather than a complete amplifier, it gives you "a lightweight tube-driven pre-amp and the flexibility to choose the power amp and wattage that best suits your" rig1.
Tech 21's product page lists the Formula 21 as SOLD OUT, and the company stated it was available direct from Tech 21 only. Private Stock pieces are limited runs, so this is not a pedal you will find on a normal store wall.
A tube take on the SansAmp Bass Driver
The SansAmp Bass Driver line is Tech 21's analog approach to tube amp tone. The standard Bass Driver DI is "much more than just a direct box," capable of dialing up big vintage tube tones, and its Blend control mixes the direct bass signal with the SansAmp Tube Amplifier Emulation circuitry, which is particularly useful for basses with piezo pickups2. The Programmable version carries the same "100% analog SansAmp Tube Amplifier Emulation technology" across the whole signal chain, from pre-amp to power amp simulation3. The line has kept moving: the SansAmp Programmable Bass Driver ELITE descends from the original SansAmp, which was, as Bass Magazine put it, "a personal mission to encapsulate tube amplifier sounds in a compact, portable pedal"4. Guitar World's buying guide, reviewing the Bass Driver DI V2, called it "a little dated compared to competition" while noting it is built to last with classic tube-style tones5.
The Formula 21 is the odd one out in that family. Instead of emulating tubes, it uses them: "All tube circuitry with 12AX7 low noise, hand selected tubes"1. Barta's starting question, per Tech 21, was that "he was curious how the SansAmp Bass Driver would sound if he made it with tubes"1. The Private Stock division itself exists for projects like this: products personally designed by Andrew Barta, inspired by equipment he originally built for himself6.
Controls
The controls follow the concept of the SansAmp Bass Driver, but being tube-based, the 3-band EQ uses passive controls, cutting up to approximately 6dB rather than boosting1.
| Control | What it does |
|---|---|
| Drive | Determines the amount of drive; gives a saturated pre-amp tube sound with warm distortion in higher settings1 |
| Character switch | Proprietary formula filter that warms up the bass, giving more bottom and more top end; disengaged, a more neutral, Jaco-style sound1 |
| Bright switch | Gives a broader range of treble and works in conjunction with the Drive control; as Drive increases, Bright becomes less active1 |
| Presence | The only active tone control, in the 2.5kHz range, capable of boosting approx. 6dB1 |
| 3-band EQ | Passive controls cutting up to approx. 6dB, following the SansAmp Bass Driver concept1 |
The Character switch is the fastest way to change the pedal's basic voice: engaged, it warms things up with more bottom and top; disengaged, you get the flatter, Jaco-style response1. The interaction between Bright and Drive is worth knowing when dialing in: pushing Drive automatically tames the Bright switch's treble lift1.
Inputs, outputs and build
The input is 1 megOhm. For active basses, Tech 21 says you can compensate the sensitivity by decreasing the Drive setting to eliminate unwanted distortion1. On the output side you get a 1/4-inch output for an amp and an XLR Output, switchable line/instrument level to run direct to a mixing console, plus a ground lift for hum problems1.
- 1 megOhm input; for active basses, decrease Drive to compensate sensitivity1
- 1/4-inch output1
- XLR output, switchable line/instrument level, for going direct to the console1
- Ground lift1
- Solid mahogany cabinet enclosure with a cherry finish1
- Approx. weight: 12 lbs1
At roughly 12 pounds in a mahogany cabinet, this is a different animal from the compact SansAmp pedals; it is built more like a piece of studio or rack gear than a board pedal1.
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Questions
Is the Tech 21 Formula 21 an all-tube preamp?
Yes. Tech 21 describes it as all tube circuitry using 12AX7 low noise, hand selected tubes. The whole preamp is tube driven, which is what separates it from the standard SansAmp Bass Driver, an analog solid-state design that emulates tube amplifiers. Because it is a preamp rather than a full amp head, you pair it with the power amp and wattage of your choice.
What does the Character switch do on the Formula 21?
The Character switch engages a proprietary formula filter that Tech 21 says warms up the bass, giving more bottom and more top end. With the switch disengaged, you get a more neutral sound that the maker describes as Jaco-style. It is a quick way to move between a scooped, full modern voice and a flatter, more traditional one.
Does the Formula 21 have an XLR output for going direct to a mixer?
Yes. It has an XLR output with switchable line or instrument level, so it can run direct to a mixing console. There is also a ground lift feature for dealing with hum in noisy venue or studio situations, and a separate 1/4-inch output for feeding an amplifier on stage.
Can I use an active bass with the Formula 21?
Yes. The input is 1 megOhm, and Tech 21 notes that for active basses you can compensate for the extra sensitivity by decreasing the Drive setting, which eliminates unwanted distortion. Passive basses can run the Drive higher for a saturated pre-amp tube sound with warm distortion.
Is the Tech 21 Private Stock Formula 21 still available?
The product page listed it as SOLD OUT. Tech 21 noted it was available direct from Tech 21 only, so it never went through regular retail channels. Private Stock pieces are limited runs, so if you are looking for one, the secondhand market is the likely route.
Who designed the Formula 21 bass preamp?
Andrew Barta, Tech 21's founder, custom-designed the Formula 21. He built it because he was curious how the SansAmp Bass Driver would sound if he made it with tubes. The unit is hand-built, tested and tweaked at Tech 21's factory in the United States.
Sources (6)
- Private Stock, Tech 21 NYC, 2019-09-11
- Bass Driver DI, Tech 21 NYC, 2024-03-13
- Bass Driver Programmable, Tech 21 NYC, 2025-12-05
- Tech 21 Brings the SansAmp Programmable Bass Driver into the Digital Domain with the ELITE, Bass Magazine, 2026-01-20
- Best bass preamp pedals: consistently great tone, anywhere, Guitar World, 2023-05-23
- Tech 21's Private Stock Division Introduces SansAmp Liverpoo, Guitar Interactive Magazine, 2012-05-10
