Elekit TU-8900 — 300B/2A3 Single-Ended Tube Amp Kit

The TU-8900 is the only single-ended triode kit that natively supports both the 300B and the 2A3 — and switches between them automatically. Power it up with either tube installed and the amplifier detects which one it’s dealing with, correcting filament voltage and B-power supply without any manual adjustment. Two of the most revered output tubes in the world. One amplifier.

Automatic 300B / 2A3 tube detection

Most single-ended triode amplifiers are designed around one tube type and commit you to it. The TU-8900 is built differently: installed tubes are detected on power-up and the filament voltage and B-power are set automatically to match. Blue LEDs beside the volume knob indicate 300B; green indicates 2A3. Red means the protection circuit has detected overcurrent and shut down that channel — and the left/right position tells you which side is at fault.

The result is that tube rolling between 300B and 2A3 is a genuinely practical proposition, not just a theoretical one. The 300B brings rich midrange and extended bass; the 2A3 offers a more delicate expression and gentler overall character. Both are available without touching a single adjustment.

Single-ended, class-A — what 8 watts can do

Single-ended triode amplifiers produce modest power by the numbers: 8W per channel with 300B, 3.5W with 2A3 (at THD 10%, 8Ω). But the character of that power — immediate, uncoloured, vanishingly low in feedback artefacts — is what the directly-heated triode format has been famous for since the 1930s and what keeps drawing serious listeners back to it today.

The TU-8900 pairs each output tube with a 12AU7 (or 12BH7A) driver, an input sensitivity of 1Vrms (300B) or 650mVrms (2A3), and a frequency response that runs from 7Hz to 90kHz with NFB engaged.

Tube-friendly warm-up design

Directly-heated triodes like the 300B and 2A3 reach operating temperature faster than the 12AU7 driver. In most amplifier designs this means the output tubes are briefly overloaded at switch-on while the driver tube catches up. The TU-8900 addresses this by gradually raising the voltage on power-up, reducing the stress on the output tubes at every start cycle.

Using without a preamplifier — the NON-NFB setting

The TU-8900 is designed as a power amplifier and is typically used with a preamplifier in the chain. For those who want to connect directly to a source — a DAC, CD player, or streaming device — without a separate preamp, the NON-NFB setting provides an additional 8dB of output gain that makes direct connection viable. The setting is changed via a PCB jumper with no modifications required. Frequency response without NFB is 12Hz–45kHz (–3dB).

Headphone output

A 6.3mm headphone jack is included on the front panel. Headphone impedance matching and attenuation level are adjustable by repositioning a short pin inside the front panel — accessible without tools. The filament power supply uses a DC-DC converter with extremely low noise, which benefits headphone listening in particular.

Power supply design

The power transformer has independent windings for the left and right channels, minimising inter-channel interference at the source. Schottky barrier diodes and fast recovery diodes are used throughout the rectifier stages to reduce high-frequency noise spikes. The main power unit circuitry is modularised per channel.

Speaker output taps cover 4–6.3Ω and 8–16Ω, switchable via a rear-panel switch, with gold-plated binding terminals compatible with banana plugs.

Coupling capacitor upgrade

The stock build uses high-quality conductive polymer electrolytic and polypropylene film capacitors. The PCB has ample space for alternatives: the maximum coupling capacitor size that fits is 22mm diameter × 44mm length. The Lundahl OPT upgrade variants also include Mundorf silver/gold oil coupling capacitors.

Lundahl output transformer upgrades

The quality of the output transformer in a single-ended amplifier is arguably more critical than in any other topology — the air gap required to prevent core saturation creates a direct tension between low-frequency inductance and high-frequency bandwidth that the transformer designer must resolve.

Two Lundahl upgrade variants are available. The LL2785C AM uses amorphous core output transformers: amorphous core material overcomes the magnetic saturation behaviour of conventional silicon steel, extending dynamic range and resolution. The LL2785B uses Lundahl’s core output transformers. Both variants are paired with Mundorf silver/gold oil coupling capacitors, and both replace the standard output transformers that would otherwise ship with the kit. Sweden’s Lundahl is a specialist designer and manufacturer of high-performance transformers with a reputation across professional audio and high-end consumer equipment.

Kit variants and build options

Three configurations: TU-8900 standard kit, TU-8900 with LL2785B OPTs & Ag/Au caps, and TU-8900 with LL2785C AM OPTs & Ag/Au caps. Tubes are not included in any variant — 300B, 2A3, and 12AU7 (or 12BH7A) tubes are sourced separately, giving you full choice of brand and rolling flexibility from the start.

All kits ship for self-assembly. Build assistance is available — see the Build service for details.

Specifications