1. Henry Radio — Company History & Design Heritage

Henry Radio was established in Los Angeles as a full-service amateur radio dealer before moving into amplifier manufacturing. The company’s first HF linear amplifiers appeared in 1963 with the 2K (console) and 2KD (desk) models, both using a pair of 3-400Z triodes. Over the following four decades, Henry expanded the line from a modest kilowatt-class desk amp to the 8K Ultra with its 3CX3000A7 tube delivering 5,000 W PEP. By the time tube amplifier production closed in 2005, Henry had manufactured more than forty distinct model variants across six major series.

Henry’s amplifiers are distinguished by several design constants that persist across the entire product line. Every HF tube model uses grounded-grid (cathode-driven) triodes — a circuit configuration that provides inherent stability (no neutralisation required), simplicity of drive circuitry, and excellent linearity. Henry’s description in the 3K Classic family manual captures the philosophy: “All three models employ a grounded grid circuit to insure simplicity of design and years of reliable operation.”³ The second constant is the resonated filter choke power supply. Rather than using a purely capacitive HV filter, Henry resonates the filter choke at the mains frequency with a series capacitor. This produces exceptional voltage regulation and allows the amplifier to maintain near-constant plate voltage from standby to full RF output, a feature that distinguishes Henry power supplies from most competitors.

The company manufactured amplifiers in three physical configurations: console (floor-standing self-contained cabinet, typically larger); desk (desktop RF deck with separate matching HV supply in 2-piece models, or integrated single-piece desk design from the 1980s onwards); and the specialised remote-tune 2-piece design of the 3K Ultra and 8K Ultra where the tuning motors are controlled by a remote panel. The suffix “D” in model names denotes desk configuration; “HD” or “Classic X” denotes a heavy-duty power supply; and “Premier” indicates full 1.8–30 MHz coverage including 160 metres.

2. Complete HF Tube Amplifier Model Index

2.1 2K Series — Dual Tube (3-400Z / 3-500Z), 3.5–30 MHz

2.2 1K Series — Single Tube

2.3 3K Series — Heavy-Duty Supply Models

2.4 4K / 5K / 8K Series

2.5 Numeric Series (HF)

2.6 HF Solid-State Amplifiers

3. Common Circuit Architecture — Grounded-Grid Triode Design

Understanding the grounded-grid circuit is fundamental to servicing any Henry Radio amplifier. In a grounded-grid configuration, the control grid is connected directly to chassis ground (or to ground via a very low-impedance path). Drive RF is applied to the cathode (filament circuit) rather than to the grid, and the output is taken from the anode (plate). This is in contrast to tetrode amplifiers such as those in the Emtron range, which are grid-driven. Tom Rauch W8JI explains: “An amplifier with the input applied between the cathode and grid and the output between the anode and grid is called a grounded-grid amplifier … The grid connection to ground should always be as wide and short as possible, and use as many pins as possible. One indication of a good design and/or knowledgeable designer is how well the grid is grounded.”⁵

3.1 Grounded-Grid Advantages for Service

The grounded-grid topology provides three practical service benefits: no neutralisation is required (the grid acts as a Faraday shield between anode and cathode); the circuit is inherently simple with fewer active components than tetrode designs; and the cathode-to-ground bias path is a direct indicator of tube condition (a grid-cathode short in a 3-500Z immediately causes abnormal cathode current and can be diagnosed by the plate current rising without drive applied).

3.2 Resonated Filter Choke — Henry’s Unique Power Supply Design

Henry Radio’s HV power supplies use a resonated filter choke rather than a simple LC filter. A series capacitor resonates the plate supply choke at the mains frequency (50 or 60 Hz), producing near-perfect regulation and allowing the amplifier to maintain nearly constant plate voltage from standby to full RF output. This design is efficient and clean, but it creates a specific failure mode and a 50/60 Hz compatibility issue:

• Choke insulation failure: The filter choke winding insulation can break down over decades of thermal cycling, causing inter-winding shorts or shorts to the choke frame. A shorted choke frame is a safety hazard and causes erratic HV regulation. Isolating the choke frame from the chassis is a recommended restoration measure (ceramic standoffs). ⁶
• 50 Hz vs 60 Hz: Henry amplifiers designed for 60 Hz US mains cannot be directly operated on 50 Hz European mains without modification. The resonating capacitor must be changed to resonate the choke at 50 Hz; without this change, the HV rises excessively at standby and drops sharply under load. The troubleshooting guide for the 2K Classic family confirms: “Excessive high voltage … means the power supply is set up for 60 Hz operation but is operating at 50 Hz. Additional capacitance is required to resonate the choke at 50 Hz.” ⁷

3.3 Pi-Network and Input Circuit

Nearly all Henry HF models use a pi-network for the output tank circuit. Bandswitch positions select coil taps and add switched capacitance for lower bands (40m, 80m, 160m). The Tune (plate) and Load capacitors are typically variable air capacitors with 6:1 vernier drives. A few high-end models (4K Ultra, 3K Ultra, 8K Ultra) use continuously variable inductive tuning by motorised or manual slug-tuned coils, eliminating the bandswitch entirely.

The input circuit of most Henry models uses a separate tuned input network (one section per band on a bandswitch) to match the 50 Ω transceiver output to the cathode input impedance of the grounded-grid tubes. On grounded-grid amplifiers, the cathode driving impedance is complex and frequency-dependent; a properly tuned input circuit is important for low SWR into the transceiver and for preventing reflected RF drive from damaging the transceiver output stage.

4. Tube Types — Specifications & Service Notes

4.1 3-500Z — The Henry Signature Tube

The Eimac 3-500Z is the defining tube of the Henry product line, used in more model variants than any other type. It is a glass-envelope, directly-heated, thoriated-tungsten triode with 500 W anode dissipation per tube. As a grounded-grid zero-bias amplifier tube, it operates without any grid bias supply. The filament rating is 5.0 V nominal, with an absolute range of 4.75–5.25 V measured at the socket pins for maximum tube life.⁸

The 3-500Z’s filament helices clear the grid cage by only a few thousandths of an inch. Excessive filament inrush current on cold start can deform the filament helix, causing grid-filament shorts. Eimac’s recommendation: limit filament inrush to no more than twice the normal running current. This is why step-start (also called in-rush limiting) is important in Henry amplifiers; many models implement this with a series thermistor or relay-switched resistor that bypasses after a short delay. A Henry amplifier run without its step-start system functioning correctly will experience significantly shortened tube life.

4.2 8877 / 3CX1500A7 — The Indirectly Heated Ceramic Triode

The Eimac 8877 (NATO designation 3CX1500A7) is an indirectly heated ceramic-metal triode with 1,500 W anode dissipation. It requires a warm-up time of approximately 3–5 minutes before plate voltage should be applied; the 4K Ultra manual documents a 90-second standby/warm-up period indicated by a flashing STANDBY light. Grid current must not exceed 75–80 mA in the 4K Ultra and related models; exceeding this will damage the tube.⁹ The 8877’s filament voltage is critical at 5.0 V; unlike the 3-500Z, its indirectly heated cathode is more sensitive to both over- and under-voltage. A calibrated filament voltage test point is provided on the 4K Ultra rear panel for this purpose.

4.3 3CX1200A7 / 3CX1200D7 — The Modern Ceramic Triodes

The Eimac 3CX1200A7 and the related 3CX1200D7 are the tubes of choice in the later Henry Premier, Classic X, and 3K Ultra models. Both are forced-air cooled ceramic triodes with 1,200 W anode dissipation, operating in grounded-grid Class AB1 configuration. They offer high gain (approximately 13–14 dB per the 3K Classic family manual), making them easy to drive with modern 100 W solid-state transceivers without overdriving.

5. Commonly Replaced Components Across All Henry HF Models

6. Cabinet, Front Panel & Assembly Hardware

6.1 Console Configuration

Henry console amplifiers (2K, 2K2, 2K3, 2K4, 2K-4A, 2K Classic, 2K Classic X, 3K, 3KA, 3K Classic family, 4K, 4K2, 4K Ultra, 5K Classic, 8K Ultra) are floor-standing steel cabinets with a reinforced chassis inside. The cabinet finish is typically a textured grey or beige baked enamel, with the front panel anodised or silk-screened.

• Cabinet paint touch-up: Duplicolor Bumper Coating (grey texture) or equivalent aerosol texture paint. Clean area with IPA before application; apply in thin coats.
• Ventilation: Console cabinets must have the rear ventilation panel unobstructed; do not position the amplifier flush against a wall.
• HV shorting switch: Console models typically have a panel-mounted shorting switch that grounds the HV bus for safe access. This must be engaged before any internal work with the cover removed.
• Door/panel fasteners: Standard #10-32 machine screws; Philips or slotted head; use stainless or nickel-plated hardware for corrosion resistance on older units.

6.2 Desk Configuration — 2-Piece (Pre-1980) vs 1-Piece (Post-1980)

Early Henry desk models (2KD through 2KD4, 2K Ultra) used a 2-piece configuration with a separate HV power supply unit connected to the RF deck via an HV cable and control cable. From the 1KD-5 and 2KD5 onwards, Henry integrated the supply into a single-piece desktop cabinet. Both configurations share the same service principles; the 2-piece units require individual discharge of both units before internal access to either.

Front panel controls on all Henry HF models follow a consistent layout: Plate Current meter (Ip) on the left; multi-function meter (grid current, HV, output power, reflected power) on the right; Band switch, Tune (plate), and Load controls along the bottom; Standby/Operate and power switches on the right panel. Meter switch, SSB/CW switch, and ALC adjust are typically on the rear panel or right side panel.

6.3 RF Connectors & Cables

• RF Input: BNC female on most 2K/3K Classic models; SO-239 on some variants. Use quality RG-8X or equivalent 50 Ω coax. Drive power 50–100 W.
• RF Output: UHF SO-239 female on 2K/3K family; N-type on 5K Classic (per manual). Use RG-213 or RG-8A/U with VHF-rated PL-259; verify SWR ≤2:1 before applying full power.
• Relay jack: RCA phono connector; shorts to ground on transmit (positive keying); PTT control from transceiver relay contacts.
• ALC output: RCA phono connector; negative-going ALC; adjustable via rear-panel screwdriver potentiometer.
• HV cable (2-piece models): Rated for the full HV; treat as live whenever mains is connected. Inspect insulation before every reconnection.

7. Safety Systems — Interlocks, Discharge & Safe Working Practice

7.1 HV Shorting Switch

Most Henry console and desk models include a high-voltage shorting switch that connects the HV bus directly to chassis ground via a resistor (to safely discharge the filter capacitors). The HV shorting switch must be manually engaged before removing covers for internal inspection. Verify the switch is in the grounding position and that the HV bus reads <50 V DC on a calibrated DMM before touching any internal components.

7.2 Safe Discharge Procedure

  STEP 1 ── TUNE or LOAD switch to STANDBY position.
               │
  STEP 2 ── POWER switch to OFF.
               │
  STEP 3 ── UNPLUG MAINS CORD from wall.
            Wait 5 minutes minimum.
               │
  STEP 4 ── ENGAGE HV SHORTING SWITCH (if fitted).
            The internal bleeder/shorting resistor discharges HV caps.
               │
  STEP 5 ── INDEPENDENT VERIFICATION with 5,000V-rated DMM:
            Probe HV bus to chassis; confirm < 50V DC.
            Do NOT trust front panel meter alone
            (meter fuse may be blown — HV may still be present).
               │
  STEP 6 ── ANODE DISCHARGE:
            Use 10kΩ/25W insulated probe.
            Contact tube anode (external fin on 3-500Z) to chassis.
            For ceramic tubes (8877, 3CX1200A7 etc.):
            contact the anode connection on the chimney/plate lead.
               │
  STEP 7 ── RE-VERIFY all HV nodes < 10V DC.
               │
  STEP 8 ── SAFE. Work may begin.
            Keep one hand behind back while probing.

Figure 1. Henry Radio HF amplifier safe discharge procedure. Note: never rely on the front panel HV meter alone to verify HV absence; a blown meter fuse (8AG 1.5A, documented in the 4K Ultra manual) can prevent an accurate HV reading while HV remains present on the bus.

7.3 Safety Capacitors in Mains Circuits

Henry Radio amplifiers are primarily line-connected equipment without the elaborate EMC filter circuits of modern amplifiers. However, any Y2/X2 safety capacitors present in the primary mains filtering must be replaced with IEC 60384-14 certified types — never with standard ceramic disc capacitors. The mains primary circuits of Henry amplifiers are protected by panel-mounted fuses (verified at each power-on) and a circuit breaker combined with the power switch on most models. The primary circuit breaker combined with the ON/OFF switch is a Henry design feature: the power switch is simultaneously a circuit breaker that opens both the primary fuses and the control circuits.

7.4 Safety Around the 3-500Z Specifically

The Eimac 3-500Z data sheet warns: “The EIMAC 3-500Z is pumped to a very high vacuum, which is contained by a glass envelope. When handling a glass tube, remember that glass is a relatively fragile material, and accidental breakage can result at any time. Breakage will result in flying glass fragments, so safety glasses, heavy clothing, and leather gloves are recommended for protection.”¹⁰ Never handle a 3-500Z by the glass envelope with bare hands when the tube is hot; thermal shock can crack the envelope, causing sudden implosion. Always let tubes cool for at least 30 minutes after operation before handling.

8. Solid-State HF Amplifiers (SS500/SS750/SS1200HF)

The SS500HF, SS750HF, and SS1200HF represent Henry’s HF solid-state amplifier product line from 1996 to 2005. Unlike the tube amplifiers, these units operate from low-voltage DC supplies (13.8 V for the SS500HF and SS750HF; 24 V for the SS1200HF) and are designed for both mobile and base station use.

Service of the SS-series amplifiers is fundamentally different from tube models. There are no high voltages present (13.8 V or 24 V DC), no warm-up periods, and no tube replacements. The primary failure modes are: MOSFET or bipolar power transistor failures in the RF output stage (typically caused by operating into an antenna with SWR >2:1 or by reverse power from antenna faults); low-pass filter component failures (capacitors and inductors in the output harmonic filter); and bias circuitry faults causing improper quiescent current. Service requires RF power measurement equipment and a spectrum analyser to verify harmonic compliance after any output stage repair.

9. Parts Sources & Reference Documents

• Henry Radio Inc. (current company — RF amplifiers and service) — henryradio.com — Although the tube amplifier business closed in 2005, Henry Radio still operates as a service entity. The company historic amplifier page (henryradio.com/oldamplifiers.html) is the primary reference for the complete model/year/tube specification table.
• N3XKB Henry Radio Resource Site — n3xkb.com/henryinfo/ — Comprehensive Henry Radio amplifier documentation including 2K Classic service notes, schematic errors (D1 zener polarity, R115/R119 values), QSK modification guide for 2K4, and filament voltage discussion. Primary community reference for Henry Radio owners.
• Groups.io — Henry Radio Legacy Amps — groups.io/g/Henry-Radio-Legacy-Amps — Active community for Henry Radio amplifier owners; service assistance, parts sourcing, and restoration discussion.
• ManualsLib — Henry Radio Schematics and Manuals — manualslib.com; search “Henry Radio” — the 2K-Classic / 2K-Classic X / 2KD-Classic manual, 3K Classic Mk II / 3K Classic X Mk II / 5K Classic manual, 4K Ultra manual, and others are available as free downloads.
• Internet Archive — Henry Radio Manuals — archive.org — Scanned 2K-Classic / 2K-Classic X / 2KD-Classic manual; freely accessible.
• Eimac Tube Data (3-500Z, 8877, 3CX1200A7, etc.) — w7brs.com/3500z/ (3-500Z); OK1RR tube archive for other types. Critical for filament voltage limits, plate voltage maximums, and anode dissipation ratings.
• RF Parts Co. (tubes and components) — rfparts.com — Primary supplier for 3-500Z, 8877/3CX1500A7, 3CX1200A7, 3CX1200D7; HV bridge rectifiers; ceramic tube sockets (Eimac SK-410 for 3-500Z). Eimac SK-410 socket and chimney are the recommended mounting for the 3-500Z.
• Antique Radio Tubes / Fair Radio Sales / Ebay tube dealers — NOS 3-400Z tubes (pre-1965 models); 8873 (2K Ultra); 8874 (T2000, 2001); other legacy types. Verify emission before installation; rejected NOS tubes should be retested.
• W8JI Grounded-Grid Reference — w8ji.com/grounded_grid_amplifiers.htm — Tom Rauch W8JI’s definitive technical reference for grounded-grid triode amplifier design, grid stability, cathode driving impedance, and filament voltage management. Essential reading before any substantive Henry Radio service or modification.
• Cornell-Dubilier / Aerovox HV Capacitors — cde.com; Mouser / DigiKey. Modern replacement HV filter electrolytics for the power supply. Verify voltage and capacitance matches original specification; use 105°C rated types.