The Shift Line A+ Flex is a tube bass preamplifier from Shift Line, built as a fully analog circuit around a 12AX7 tube running at high voltage. Shift Line describes it as a "fully fledged tube preamp with high-voltage power," aimed at players who prefer the classic East Coast sound1. It sits in the preamp pedal category: it shapes and amplifies your bass's signal before it reaches a power amp, an audio interface or a DI destination, rather than adding overdrive, fuzz or other time-based effects.
What it is and how it works
The A+ Flex is a fully analog preamp based on the 12AX7 tube, a common dual-triode tube used in countless instrument preamps1. Shift Line's design gives the tube a high-voltage supply, which the maker presents as the basis of the preamp's headroom and character1.
The input stage is built to accept whatever you plug into it. IN is a high-impedance input for monophonic signals, with a 1MOhm input impedance on a 6.3mm mono unbalanced jack, and it accepts an active or passive bass as well as any other instrument or signal source1. A PAD switch controls the amplification ratio of that first stage: in the DOWN position the sensitivity is -15dB, and in the UP position the PAD is 0dB, so hot active basses can be tamed before they hit the tube1.
Outputs and signal routing
The Flex has three outputs: main, balanced and THRU. Shift Line says these allow you to set up multiple independent signal chains, for example feeding an amp from the main output while the balanced output goes to a front-of-house desk and the THRU output passes the signal onward1.
Power and what comes in the box
The A+ Flex needs 12V DC at 500mA with a negative tip on a 2.1/5.5mm plug, and Shift Line specifies a regulated power supply only1. The included PSU is rated 12V DC 1000mA and comes with four changeable plugs for different wall outlets1.
Battery power and +48V phantom power on the balanced line are both insufficient to run the preamp, so the A+ Flex cannot be powered from a battery or from console phantom power1.
The delivery set includes the A+ Flex preamplifier and the 12V DC 1000mA PSU with its four changeable plugs1. The PSU itself weighs 120g (0.26lb)1.
Enclosure and size
The preamp is housed in a small, lightweight, pedalboard-friendly aluminum enclosure with top-mounted jacks, which keeps cable runs tidy on a board1. It measures 101x125x55mm (3.9x4.9x2.2", LxWxH) and weighs 430g (0.95lb)1.
Warranty and service
Shift Line's warranty terms for the A+ Flex are split by component:
- Electronics: 3 years, starting with the date of purchase1
- Parts subject to load (potentiometers, buttons, switches, jacks) and the pre-installed 12AX7 tube: 1 year1
Warranty and post-warranty repairs are only conducted at Shift Line's service center1.
Availability
As of 2026, the A+ Flex is listed on Shift Line's product page at a price of $3751.
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Questions
Does the Shift Line A+ Flex run on a battery?
No. Shift Line states that phantom power of the balanced line (+48V) or battery power are insufficient for the preamp to operate on. It needs its own external supply: 12V DC at 500mA with a negative tip on a 2.1/5.5mm plug, and the maker says to use a regulated power supply only. A 12V DC 1000mA PSU is included in the box.
What tube does the Shift Line A+ Flex use?
The A+ Flex is based on a 12AX7 tube, the same dual-triode tube found in many guitar and bass preamps. Shift Line describes the circuit as a fully fledged tube preamp with high-voltage power, and the pre-installed 12AX7 carries a one-year warranty while the rest of the electronics are covered for three years.
Is the Shift Line A+ Flex fully analog?
Yes. Shift Line describes the A+ Flex as a fully analog preamp based on the 12AX7 tube. There is no digital signal processing or modeling in the circuit.
What power supply does the Shift Line A+ Flex need?
The A+ Flex requires 12V DC at 500mA, negative tip, with a 2.1/5.5mm barrel diameter, and Shift Line specifies a regulated power supply only. The included PSU is rated 12V DC 1000mA and ships with four changeable plugs. Battery and +48V phantom power will not run the preamp.
What does the PAD switch do on the A+ Flex?
The PAD switch controls the amplification ratio of the preamp's first stage. In the DOWN position the sensitivity is -15dB, which tames hot signals, and in the UP position the PAD is 0dB. It is useful for switching between active and passive basses without overdriving the input stage.
How long is the warranty on the Shift Line A+ Flex?
Shift Line covers the electronics for three years starting with the date of purchase. Parts subject to load, meaning potentiometers, buttons, switches and jacks, and the pre-installed 12AX7 tube are covered for one year. Warranty and post-warranty repairs are conducted at the company's service center.
Sources (1)
- A+ Flex, Shift Line, 2026
