The 833 (FU33) Vacuum Tube in High-Fidelity Amplifiers

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The 833 (FU33) Vacuum Tube in High-Fidelity Amplifiers

The 833 (FU33) Vacuum Tube in High-Fidelity Amplifiers

Sep 16, 2025 | 0 comments posted by Vincent Zhang

Published by IWISTAO

Introduction to the 833 (FU33) Tube

A typical 833A (FU33) directly heated triode (Soviet GU-48, equivalent to 833A) as the above. The tube is massive, with anode and grid terminals brought out at the top of the envelope.

Originally launched by RCA in the late 1930s, preliminary technical data for the 833 tube was already available by 1938 . The improved 833A was first intended for medium-power RF oscillation and amplification, especially as a transmitter power tube or modulation amplifier. In the 1960s, China also manufactured the FU-33 tube as a counterpart to the 833, primarily for broadcast station PA equipment . Thus, the 833 (FU33) has a strong transmitter heritage—never originally designed for audio amplification, but for communications.

Technical parameters

The 833A is a directly heated triode with moderate amplification factor, employing a thoriated tungsten filament. The filament supply requires 10V, 10A DC, consuming a hefty 100W just for heating . The anode voltage rating can reach several kilovolts, with a maximum plate dissipation of about 300W continuous (CCS), and up to 350W under ICAS conditions . Its μ is about 35, transconductance ~20 mA/V (estimated from typical operating points), and interelectrode capacitances are significant (plate-to-grid ~6.3 pF) . The 833 tube is enormous: envelope diameter ~117 mm, height ~219 mm , weight ~1 kg. Its filament leads are heavy posts (9.5 mm and 11.1 mm diameters), while anode and grid terminals exit via top caps . Cooling is via natural air convection, but at high power the plate glows dull to bright red .

The table below compares the 833 (FU33) with classic audio triodes:

Tube	μ (Amplification Factor)	Max Plate Dissipation	Filament Voltage / Current	Typical SE Output Power*
300B	3.5–4	40 W	5 V / 1.2 A	≈8 W
845	~5	75–100 W	10 V / 3.25 A	≈25 W
833 (FU33)	~35	300 W	10 V / 10 A	90–150 W (SE)

*Single-ended Class A power, approximate, depending on operating point and distortion requirements.

Clearly, the 833’s high voltage and massive power distinguish it from audio tubes like the 300B or 845. Its filament power alone exceeds the full dissipation of many smaller tubes, meaning the 833 demands special attention in power supply design, cooling, and component selection .

Sonic Characteristics and Comparison with Classic Tubes

Although originally an RF tube, the 833 has been embraced by audiophiles for its majestic and full-bodied sonic presentation. As a directly heated triode, it retains the typical “tube flavor”: warm, lush midrange and smooth harmonic distortion. Thanks to its enormous reserves, it provides effortless drive and explosive dynamics, easily handling large loudspeakers. For example, some audiophiles remarked that the FU33 (833) tube delivers “excellent tonal quality, strong spatial imaging, powerful Class A output, and superb control” . Reviewers also note its tonal balance across highs, mids, and lows, combining triode charm with authority and dynamic scale .

Compared to the 300B, often called the “King of Tubes,” the sonic orientation is different. The 300B excels in sweet, enchanting mids and fine detail, but with only ~8W SE output it struggles with insensitive speakers or large-scale music . By contrast, the 833 can deliver tens to hundreds of watts SE, providing expansive soundstage and powerful bass, making it ideal for symphonic or large-scale works. Yet the 300B still retains superiority in its seductive intimacy for vocals and chamber music.

Against 845 and 211 high-voltage triodes, the 833 also stands apart. The 845 (~25W SE) is often described as deep and weighty, while the 211 (~20W SE) tends to be brighter and more transparent . The 833, with its much higher μ (~35), typically operates near or into grid-current (positive grid) regions. This makes it harder to drive but, when done right, the sound combines sheer authority with refinement, maintaining composure even at high power . Some listeners report the 845 strains under extreme dynamics, while the 833 remains relaxed and powerful.

Typical Amplifier Circuits and Applications

Single-Ended Class A

Most 833 audiophile amplifiers use SE Class A topology, prized for its triode purity. Below is a typical schematic as an example:

Schematic of a single-ended Class A 833 triode amplifier . The input stage uses a small-signal triode (2C22), the driver stage a DHT 45, and the final stage the 833. The plate couples to the loudspeaker via a massive SE output transformer (5 kΩ, e.g. Hammond 1642SE, rated for 300 mA DC ). B+ supply is about 1000V, filtered by a π-network of capacitors and chokes . The 833 typically idles at 150–200 mA, dissipating ~150–200W at idle. Some designs operate at zero bias (Vg≈0, ~1kV plate), naturally drawing ~100–150 mA into Class A .

Driving the 833 is the main challenge. Its grid requires huge voltage swing (hundreds of volts) and can draw significant current in positive bias. Strategies include:

DHT drivers like 45, 2A3, or 300B .
Beam/pentodes in triode connection such as KT88 .
MOSFET source followers for near-zero output impedance and grid-current capability.

Output transformers must handle 2–5 kΩ primary impedance, high DC currents, and excellent insulation. For example, Hammond’s 1642SE (5kΩ, 300mA, ~50H primary inductance ) is one viable option. Custom transformers are common for wider bandwidth.

There is another example for tube 6L6/6CA7 driving 833.

Push-Pull Amplification

In RF service, push-pull 833 pairs in Class AB2 deliver hundreds of watts . In audio, though, PP 833 amps are rare, due to complexity, matching, and transformer challenges. Most audiophile implementations stick to SE for purity.

Design Challenges and Recommendations

High-Voltage Supply: B+ often ranges 1–2kV . Components must withstand this: capacitors in series with balancing resistors , well-insulated transformers/chokes . Delayed HV application after filament warm-up is recommended. Some manufacturers use separate power-supply chassis for safety and noise isolation .
Filament Supply and Heating: 10V/10A is demanding. DC supplies with large filter capacitance (even supercapacitors, e.g. 4.5F) are used . Startup surges exceed 15A . Filament + plate idle losses exceed 200W, so strong ventilation or forced-air cooling is mandatory . Plates are designed to glow dull red in operation . Heat-resistant chassis and safety covers are essential.
Driving and Transformers: See Section 3. Driver stage must deliver high-voltage swing with current. MOSFET buffers or triode drivers are popular. OPT must be large, gapped, and custom-wound in many cases.
Tube Supply and Safety: 833 tubes are rare. Some manufacturers, like Meixing, buy NOS FU33 stock and pre-age them . Alternatives include Russian GU-48 . Adjustable bias is recommended for tube variation. Safety is critical: always discharge capacitors, insulate HV nodes, and never operate without covers.

Case Studies and Practice

IWISTAO monoblock 833 amplifier. Each chassis uses one FU33 (833) output tube driven by a 300B transformer coupling, delivering 70W SE Class A . Each channel is housed in its own chassis for maximum isolation and stability. The front end employs **6J7 driver tubes feeding 300Bs**, which in turn couple through output transformers to drive the mighty 833. Operating at 1650V plate voltage and 140mA current in pure Class A, the 833 stage is free from global feedback, ensuring uncolored, naturally flowing sound.

IWISTAO 833 (FU33) 1 Piece Single-Ended Class A Tube Amplifier Split-Chassis 300B Transformer-Coupled to 833 Zero Global Feedback

WAVAC SH-833 (Japan): One of the most extreme SE triode amps ever: 150W Class A SE per monoblock . Uses WE437A + KT88 driver, innovative IITC inverted interstage coupling , and multiple isolated PSU chassis (4–6 boxes per channel) . Transformers are custom by E.I. Co. Noise floor >100 dB . Retail price in the tens of thousands of USD.
DIY Community: Builders worldwide share experiences , using GU-48 substitutes, parafeed OPTs, supercapacitor filament supplies, and MOSFET drivers. Despite challenges, properly executed 833 amps offer unmatched scale, dynamics, and authority. There is an example DIY project for tube 833 below.