The 572B Transmitting Tube in Amateur Amplifier Service

The 572B is a work‑horse of the high‑voltage world, prized by amateur builders for its rugged construction, generous plate dissipation, and straightforward biasing. As a directly‑heated power triode, it inherits the classic 811A lineage while offering a higher plate‑current capability that makes it especially suitable for grounded‑grid configurations in HF linear amplifiers. In this article we explore why the 572B continues to be a go‑to choice for mid‑power amateur rigs, how to handle its filament and plate supplies, and what practical considerations keep it relevant in modern builds.

WHY THE 572B STAYS RELEVANT

Even as solid‑state devices dominate the market, the 572B holds a niche that blends nostalgia with performance. Its directly‑heated cathode eliminates the need for a separate heater‑to‑cathode bias network, simplifying the power supply. The tube’s plate dissipation of about 160 W per unit gives ample headroom for the typical 2‑2.5 kV plate supplies found in HF linear amplifiers. When two tubes are operated in a grounded‑grid pair, the combination can accept roughly 1000‑1200 W PEP input, delivering solid output while keeping the overall circuit relatively compact.

Grounded‑Grid Pair and Quad Configurations

Grounded‑grid operation flips the conventional grid‑bias paradigm: the grid is held at ground potential, the input is applied to the cathode, and the plate sees the high‑voltage supply. This topology offers several benefits for the 572B:

• Low input impedance, which matches well to typical driver stages.
• Excellent stability against oscillation, thanks to the inherent feedback path.
• Reduced inter‑modulation when multiple tubes are paralleled in a quad, making it a favorite for linear amplifiers that must handle a wide range of modulation depths.

Because each 572B can comfortably handle a few hundred watts of plate dissipation, a quad (four tubes) can be driven well beyond the 1 kW PEP range without pushing any individual tube into the danger zone.

POWER SUPPLY DESIGN CONSIDERATIONS

Designing a supply for the 572B requires attention to two distinct voltage domains: the filament (heater) circuit and the high‑voltage plate circuit. Both must be robust, low‑noise, and correctly referenced to avoid premature tube failure.

Filament Supply

The 572B uses a 6.3 V directly‑heated filament. Because the filament itself is the cathode, any ripple or voltage deviation appears directly on the cathode and can modulate the plate current. A well‑filtered, low‑impedance supply is therefore essential. Typical practice is to use a regulated 6.3 V AC transformer followed by a full‑wave rectifier and a large electrolytic filter, or a modern switch‑mode supply with tight voltage regulation and soft‑start to limit inrush current.

Plate Voltage and Current

The plate supply sits in the 2‑2.5 kV class. This range provides the necessary plate‑to‑cathode voltage to extract the full 160 W dissipation capability from each tube while keeping the anode current within safe limits. When wiring a pair or quad, the total plate current scales with the number of tubes, so the supply must be sized for the combined load plus a safety margin of at least 20 %.

Voltage regulation is less critical than ripple suppression; a well‑designed choke‑input filter followed by a high‑voltage capacitor bank will usually suffice. However, any sudden voltage spikes—such as those caused by switching transients—should be clamped with a fast‑acting varistor or spark gap to protect the tubes.

APPLICATIONS IN AMATEUR AMPLIFIERS

When choosing a tube for an amateur linear amplifier, the decision often balances power capability, availability, and ease of biasing. The 572B checks all three boxes for many builders.

Mid‑Power Linear Amplifiers (1‑2 kW)

For stations that need reliable output without the expense of a full‑scale 4 kW rig, a pair of 572Bs delivers a sweet spot of power and efficiency. The grounded‑grid pair can be driven from a modest driver stage, and the overall efficiency typically hovers around 55‑60 % at full output, which is respectable for a tube‑based design.

Portable and Mobile Installations

The 572B’s relatively compact size and moderate filament current make it a candidate for portable rigs where weight and power consumption matter. When paired with a lightweight high‑voltage transformer and a solid‑state filament supply, the entire amplifier can fit into a standard vehicle rack.

Legal Power Limits

In the United States, amateur operators are permitted up to 1500 W PEP output under FCC Part 97, though many bands and license classes impose lower limits. The 572B’s capability comfortably sits within this envelope, allowing builders to stay well under the legal maximum while still achieving strong signal levels. For CB operators, note that external linear amplifiers are not legal; the 572B is therefore a tube primarily for ham service.

QUICK SANITY CHECKLIST

• ✓ Verify filament supply is 6.3 V DC with ripple < 0.1 V.
• ✓ Confirm plate supply voltage is within the 2‑2.5 kV range.
• ✓ Ensure total plate current rating of the supply exceeds the combined tube dissipation by at least 20 %.
• ✓ Check that the grounded‑grid bias network provides a stable cathode drive voltage.
• ✓ Inspect all high‑voltage connections for secure, insulated terminations.
• ✓ Perform a low‑voltage warm‑up test before applying full plate voltage.

By respecting the 572B’s voltage requirements and leveraging its grounded‑grid strengths, builders can craft reliable, mid‑power amplifiers that stand the test of time.

Educational reference only. Always verify all values independently against your specific device datasheet and build documentation. Working on RF and high-voltage equipment carries real risk.

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