6-tube 572B HF amplifier

Building a 6-tube 572B HF amplifier is a significant homebrew project typically aimed at achieving 1.5 kW to 2 kW PEP output. While commercial amplifiers like the Ameritron AL-82 use four tubes (3-500Z or similar), a 6-tube 572B design is less common commercially but feasible for high-power linear operation.

Here are the critical design parameters and steps for constructing such an amplifier:

1. Core Design Specifications

Tube Configuration: Six 572B tubes in parallel in a grounded-grid configuration.

Operating Class: Class AB2 or B linear (SSB/CW).

Expected Output: Approximately 1800 W PEP (assuming ~300 W per tube under ideal conditions).

Plate Voltage: Typically 2000 V to 2500 V DC. The 572B is rated for 2750 V maximum, but 2200–2400 V is a common sweet spot for longevity and power.

Plate Current: Total static current ~270 mA (45 mA/tube); peak SSB current ~1.5 A (250 mA/tube).

Filament Supply: Critical. Requires 6.3 V AC at 24 Amps (4 A per tube × 6). Heavy gauge wiring and a massive transformer are essential to prevent voltage drop, which kills thoriated-tungsten filaments.

2. Key Circuit Blocks

Power Supply

You will need a heavy-duty high-voltage transformer capable of delivering ~2200–2500 V at 1.5–2 A (for SSB duty cycle).

Rectification: Full-wave bridge using high-voltage, high-current diodes (e.g., 10 kV, 1 A stacks) or vacuum tube rectifiers.

Filtering: A capacitor-input filter (approx. 20–40 µF at 3000 V rating) with equalizing resistors.

Bleeder Resistor: Essential for safety to discharge HV capacitors when off.

RF Deck and Tank Circuit

Input Matching: A broadband transformer or switched pi-network to match the low cathode impedance (~4–6 ohms for 6 tubes) to 50 ohms.

Output Tank: A standard Pi-network (or Pi-L for better harmonic suppression) capable of handling 2 kW.

Plate Capacitor: High voltage, high capacitance (e.g., 250–500 pF, 5000 V rating).

Load Capacitor: Similar rating, often higher capacitance (e.g., 500–1000 pF).

Plate Choke: Must be resonant-free across 160m–10m (or 6m). A "silver bullet" or wound choke designed for high power is required.

Band Switching: Heavy-duty ceramic wafer switches with short leads to minimize inductance.

Cooling and Mechanical

Airflow: The 572B relies on radiation and forced air. Six tubes generate immense heat (~1 kW dissipation at full load). You need a high-CFM blower (squirrel cage) pushing air from base to plate cap.

Chassis: Large, well-ventilated metal enclosure. Tubes are often mounted horizontally or vertically with ample spacing.

3. Construction Resources & Schematics

No single "off-the-shelf" schematic for a 6-tube 572B exists in standard manuals (most are 2 or 4 tubes). You must adapt existing designs:

Adaptation Strategy: Take a proven 2-tube schematic (like the Ameritron AL-572 or Heathkit SB-200) and scale the power supply, filament transformer, and tank components for 3x the tube count.

Reference Designs:

Look for "Homebrew 1.5 kW Linear Amplifier" plans in amateur radio journals (e.g., QST, CQ Amateur Radio).

The W8JI website (Tom Rauch) offers extensive technical articles on designing high-power amplifiers, including tank circuit calculations and stability tips.

Search for "4-572B amplifier schematic" (e.g., Ameritron AL-82 or Dentron GLA-1000B) and add a third pair of tubes, ensuring the filament and HV supplies are upgraded.

572B 6 tube amplifier schematic homebrew

building 572B tube amplifier

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⚠️ Safety Warning

This project involves lethal voltages (2500+ V) and high currents.

Always use safety interlocks on cabinet doors.

Install bleeder resistors on all HV capacitors.

Ensure proper grounding and use a fuse or circuit breaker rated for the input current.

Verify filament voltage under load; low filament voltage drastically shortens tube life.

Building a 6-tube 572B amplifier requires sourcing specialized high-voltage components that are rarely found in standard catalogs like Mouser or DigiKey. While those distributors excel at low-voltage electronics, the specific HV capacitors, plate chokes, and high-current filament transformers needed for a 2 kW RF amplifier are typically sourced from ham radio specialty suppliers (e.g., RF Parts, Max-Gain Systems, Surplus Sales) or industrial surplus.

Below is a comprehensive Parts List categorized by function, with estimated costs and primary sources based on current availability.

1. Active Components (Tubes & Rectifiers)

The core of the amplifier. You need six matched tubes to ensure even current distribution.

RF Power Triodes (572B): 6 units.

Specs: 572B/T160L, Thoriated Tungsten Filament.

Source: RF Parts, DX Engineering, Ham Radio Outlet.

Cost: ~$40–$125 each. Matched sets (pairs/trios) are preferred.

Est. Total: $300 – $750.

HV Rectifier Diodes: 4–8 units (for full-wave bridge).

Specs: 10 kV PIV, 1 A (e.g., HVM10, 10A10 stacks).

Source: Island Amplifier, eBay, Surplus Sales.

Note: Often bought as pre-built stacks or kits to avoid series balancing issues.

Est. Total: $20 – $60.

572B vacuum tube matched set

2. Power Supply Components

This is the heaviest and most expensive section. Mouser/DigiKey can supply some capacitors, but the transformer is custom/surplus.

HV Power Transformer: 1 unit.

Specs: 2200–2500 VAC secondary @ 1.5–2 A (SSB duty). ~5–6 kVA.

Source: Surplus (e.g., Surplus Sales of Nebraska), eBay, or custom wind from Antek (multiple units in series/parallel, though difficult for HV).

Cost: $300 – $800 (New custom units exceed $1,500).

HV Filter Capacitors: 2–4 units.

Specs: 20–40 µF total, 3000–4000 VDC rating. (Often wired in series/parallel).

Source: Alibaba (for new electrolytics like Pluspark), eBay (surplus Cornell Dubilier), Mouser (limited selection >2kV).

Cost: $50 – $150.

HV Bleeder Resistor: 1 assembly.

Specs: ~220kΩ – 470kΩ, 50–100 W total rating.

Source: Mouser/DigiKey (series chain of 5W metal oxide resistors).

Cost: $20.

3. RF Deck & Tank Circuit

Critical for tuning and efficiency. Variable capacitors must be vacuum types to handle high voltage without arcing.

Plate Capacitor (Variable): 1 unit.

Specs: 5–500 pF, 5000–7000 V peak.

Source: Max-Gain Systems, Surplus Sales. (e.g., Comet CV05C-500 series).

Cost: $350 – $500.

Load Capacitor (Variable): 1 unit.

Specs: 10–1000 pF, 3000–5000 V peak.

Source: Max-Gain Systems, eBay.

Cost: $200 – $400.

Plate Choke (RFC): 1 unit.

Specs: 2.5 mH – 5 mH, self-resonant free across 160m–10m, 2 kW rating.

Source: RF Parts (e.g., RFC-245 or equivalent "Silver Bullet" chokes).

Cost: $80 – $150.

Band Switch: 1 unit.

Specs: 6-position, ceramic wafer, high voltage rating (e.g., Centralab 2000 series).

Source: Surplus, eBay.

Cost: $40 – $80.

Input Matching Transformer: 1 unit.

Specs: Broadband 1:9 or 1:16 impedance ratio (cathode to 50Ω).

Source: Homebrew (FT-240-43 cores) or RF Parts.

Cost: $20 – $50.

high voltage vacuum variable capacitor 500pF 5kV

4. Filament & Control Supply

The 6.3V supply is critical; voltage drop kills tubes.

Filament Transformer: 1 unit (or parallel combination).

Specs: 6.3 VAC @ 24–30 Amps (4A/tube × 6 + margin). ~150–200 VA.

Source: Antek Products (e.g., AN-0206M is only 2A; you need massive custom or parallel industrial transformers), Surplus.

Note: Standard audio transformers (Hammond) are too small (max ~5A). You may need to parallel three 10A units or find a large industrial control transformer.

Cost: $100 – $300.

Relays: 2–3 units.

Specs: HV vacuum relay (plate), 12V/24V DPDT (antenna bypass).

Source: Mouser (Jennings, Kilovac), DX Engineering.

Cost: $50 – $100.

Cooling Fan: 1–2 units.

Specs: High CFM (100+), low static pressure, 120VAC.

Source: Mouser/DigiKey (e.g., NMB, Papst), DigiKey.

Cost: $30 – $60.

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5. Chassis & Hardware

Chassis: Large aluminum or steel enclosure (e.g., 19" rack mount or custom sheet metal).

Source: Hammond Manufacturing, eBay.

Cost: $100 – $200.

Tube Sockets: 6 units.

Specs: Ceramic 4-pin (for 572B).

Source: RF Parts, Antique Electronic Supply.

Cost: $15 – $25 each ($90–$150 total).

Meters: 2 units.

Specs: HV DC (0–3000V) and Plate Current (0–1A or 0–2A).

Source: eBay, Fair Radio Sales.

Cost: $40 – $80.

Estimated Total Cost

Category Estimated Cost (USD) Notes

Tubes (6x) $300 – $750 Major variable; matched sets cost more.

HV Transformer $300 – $800 Best found surplus; new is expensive.

Tank Components $600 – $1,000 Vacuum capacitors are costly.

Filament Supply $100 – $300 Requires massive current capacity.

Chassis/Misc $200 – $400 Sockets, fans, meters, hardware.

Total $1,500 – $3,250 Excludes tools and test equipment.

Critical Sourcing Notes

Mouser/DigiKey Limitations: Use these for resistors, low-voltage capacitors, relays, fans, and hardware. They generally do not stock 3000V+ electrolytics, 500pF 5kV variable capacitors, or 25A filament transformers.

Surplus is Key: For the HV transformer and vacuum capacitors, check Max-Gain Systems, Surplus Sales of Nebraska, and eBay.

Safety: Ensure all HV components have adequate creepage/clearance distances. Use safety interlocks on the chassis.

building 2kW tube amplifier power supply

make calculations based on available sources for all of the required parts values

Based on the 572B datasheet specifications and standard high-power amplifier design equations, here are the calculated component values for a 6-tube grounded-grid amplifier operating at 2200–2400 V.

1. Operating Parameters & Load Impedance

To achieve ~1.5–1.8 kW PEP output, the tubes are operated in parallel.

Plate Voltage ($E_p$): 2200 V (Conservative rating for longevity; max is 2750 V).

Plate Current ($I_p$): 250 mA/tube $\times$ 6 = 1.5 A (Peak SSB).

Idle Current: ~45 mA/tube $\times$ 6 = 270 mA.

Input Power: $2200 \text{ V} \times 1.5 \text{ A} \approx 3300 \text{ W}$ (PEP input).

Estimated Output Power: ~1800–2000 W PEP (assuming 60% efficiency).

Plate Load Impedance ($Z_p$): Using the formula $Z_p \approx \frac{E_p}{1.8 \times I_{max}}$ (Class AB2/B approximation) or derived from typical single-tube load ($\approx 3500,\Omega$) divided by parallel count: Z

≈

3500Ω

≈580Ω to 600Ω Design Target: 600 $\Omega$ effective plate load.

2. Filament Supply Calculations (Critical)

The 572B requires strict voltage regulation to prevent emission loss or filament burnout.

Voltage: 6.3 V AC (Tolerance $\pm$0.3 V).

Current: 4.0 A/tube $\times$ 6 = 24.0 A minimum.

Transformer Rating: Must be 6.3 V @ 30 A (20% headroom recommended).

Power: $6.3 \text{ V} \times 30 \text{ A} = \mathbf{189 \text{ VA}}$ (Minimum).

Wiring Voltage Drop: To keep drop $< 0.15 \text{ V}$ (to maintain 6.15 V at socket): R

max

24 A

0.15 V

=0.00625Ω Implementation: Use extremely short, heavy-gauge wiring (e.g., 00 AWG or multiple parallel 4 AWG strands) directly from transformer to a central bus bar, then to sockets.

30 amp 6.3 volt filament transformer

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3. Pi-Network Tank Circuit Values

Calculated for transforming 600 $\Omega$ (Plate) to 50 $\Omega$ (Antenna) with a loaded Q = 12. Values vary by band; extremes (160m and 10m) dictate component ratings.

Band Freq (MHz) Plate Cap ($C_p$) Load Cap ($C_L$) Inductance ($L$)

160m 1.8 480 pF 1650 pF 58 µH

80m 3.8 220 pF 850 pF 26 µH

40m 7.1 115 pF 480 pF 13 µH

20m 14.2 55 pF 260 pF 6.5 µH

10m 28.5 25 pF 140 pF 3.2 µH

Component Specifications Required:

Plate Capacitor ($C_p$): Must cover 25–500 pF.

Voltage Rating: $E_{peak} \approx E_{DC} \times 3.5$ (high Q/swing).

$2200 \text{ V} \times 3.5 \approx \mathbf{7700 \text{ V}}$.

Select: 500 pF, 8–10 kV Vacuum Variable.

Load Capacitor ($C_L$): Must cover 150–1700 pF.

Voltage Rating: Lower than plate, but RF current is massive.

Select: 2000 pF, 3–5 kV Vacuum Variable (High Current type).

Inductor ($L$):

Must handle 1.5 A RF current without saturation or overheating.

Use a "Silver Bullet" style air-core choke or switched tapped coil. Total inductance range 3 µH to 60 µH.

4. Plate Choke (RFC) Design

The choke must present high impedance across 1.8–30 MHz without self-resonance.

Inductance: 2.5 mH to 3.0 mH.

Impedance Check: At 1.8 MHz, $X_L = 2\pi f L \approx 34,000,\Omega$ (Safe, $>10\times Z_p$).

Construction:

Universal Winding (pie-wound) or Random Wound on a ceramic form to minimize inter-turn capacitance.

Length: ~6–8 inches, diameter ~2 inches.

Wire Size: #18 to #20 AWG enamel (handles DC + RF skin effect).

Warning: Do not use a single-layer solenoid; it will self-resonate within the HF band, causing arcs and failure.

5. Input Matching Network

Grounded grid input impedance is very low ($Z_{in} \approx \frac{1}{g_m} + \frac{Z_{load}}{\mu}$).

Single Tube $Z_{in}$: $\approx 50,\Omega$ / $\mu$ (approx 170) $\rightarrow$ Very low, typically 4–6 $\Omega$ cathode impedance.

6 Tubes in Parallel: $Z_{in_total} \approx \frac{5,\Omega}{6} \approx \mathbf{0.8,\Omega}$.

Matching Strategy:

Use a Broadband Transformer (Ferrite cores, e.g., FT-240-43 or T-200-2).

Turns Ratio: To match $0.8,\Omega$ to $50,\Omega$ ($R_{ratio} = 62.5$).

Impedance ratio $\approx 64:1$ $\rightarrow$ Turns ratio $\sqrt{64} = \mathbf{8:1}$.

Example: 2 turns (Primary) to 16 turns (Secondary) on multiple large ferrite cores stacked for power handling.

6. Power Supply Filter & Bleeder

Filter Capacitance:

Rule of thumb: 1–2 µF per 100 mA.

$1.5 \text{ A} \times 2 \text{ µF/A} \approx \mathbf{30 \text{ µF}}$ total.

Configuration: Two 60 µF, 3000 V capacitors in series (Result: 30 µF, 6000 V rating). Use 100 kΩ equalizing resistors across each.

Bleeder Resistor:

Target discharge time < 1 minute. $R \le \frac{t}{5C}$.

$R \approx \mathbf{220 \text{ k}\Omega}$, 100 W rating (dissipates $\approx 24 \text{ W}$ continuously at 2200 V).

Construction: Series chain of twenty 11 kΩ, 5 W metal oxide resistors.

pi-network tank circuit calculations tube amplifier

6 tube 572B amplifier component values site:reddit.com

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Summary of Critical Parts List (Calculated)

Component Calculated Value Minimum Rating Required

HV Transformer 2200–2400 VAC 2.0 A (SSB), 5 kVA

Filament Transformer 6.3 VAC 30 A (190 VA)

Plate Capacitor 25–500 pF 8 kV Peak

Load Capacitor 150–2000 pF 3 kV, High RF Current

Plate Choke 2.5 mH Self-resonant > 30 MHz

HV Filter Caps 30 µF Total 3000 VDC (x2 in series)

Input Transformer 8:1 Turns Ratio #12 AWG wire, FT-240-43 cores

6-Tube 572B HF Amplifier Schematic Description

1. Input Circuit (Cathode Drive)

Topology: Broadband Transformer Match (No tuning required per band).

Connections:

RF Input (SO-239): Center pin connects to the primary of T1.

T1 (Input Transformer):

Core: Stack of 3x FT-240-43 or FT-240-61 ferrite toroids.

Primary: 2 turns of #12 AWG Teflon wire.

Secondary: 16 turns of #12 AWG Teflon wire (Bifilar or Trifilar wound for coupling).

Center Tap: The secondary center tap connects to Ground (via RF bypass caps).

Ends: Connect to the Cathode Bus (Pins 2 & 3 of all 6 sockets).

Filament Feed:

6.3V AC connects to the cathode bus via two 2.5 mH RF Chokes (one on each side of the transformer secondary) to isolate RF from the filament transformer.

Bypass: 0.1 µF, 100V ceramic caps from each side of the filament supply to chassis ground.

Grid Grounding:

Grid pins (Pin 1) of all 6 sockets connect directly to a low-inductance ground plane (copper strap) surrounding the socket cluster.

Optional Stability: 47–100 Ω, 2W non-inductive resistors from Grid (Pin 1) to Ground on each tube socket (very short leads).

2. RF Deck (Tube Cluster & Plate Circuit)

Tubes (V1–V6): Six 572B sockets wired in parallel.

Pin 1 (Grid): Grounded directly to chassis.

Pin 2 & 3 (Cathode/Filament): Connected to the Input Transformer secondary and Filament supply.

Pin 4 (Getter/Cathode): Connected to Cathode bus.

Plate Cap: All 6 tube plate caps connected together via a wide copper strap or bus bar to the Plate Tank Capacitor ($C_{plate}$) and Plate Choke.

Plate Choke (RFC):

Connected between the Plate Bus and the HV B+ supply point.

Value: 2.5 mH, universal wound, self-resonant > 30 MHz.

Tank Circuit (Pi-Network):

$C_{plate}$ (Tuning): Vacuum variable capacitor (5–500 pF, 8 kV). One side to Plate Bus, other side to Ground.

$L_{tank}$ (Inductor): Switched air-core inductor (3 µH to 60 µH).

Connected between Plate Bus and the junction of $C_{plate}$ and $C_{load}$.

Tapped for bands (160m, 80m, 40m, 20m, 15m, 10m) via a ceramic wafer switch.

$C_{load}$ (Loading): Vacuum variable capacitor (150–2000 pF, 3–5 kV).

Connected between the Tank Inductor output and Ground.

Output Link:

A 2–3 turn link coil wound over the cold end of $L_{tank}$ (near ground).

Connected to the Antenna Relay and Output SO-239.

RF Choke: 2.5 mH choke from the output line to ground (DC block protection).

3. High Voltage Power Supply

Transformer (T2): 2200–2400 VAC secondary, center-tapped (or dual secondary).

Rectifier: Full-wave bridge (4x 10 kV, 1 A diodes).

AC inputs to transformer secondary.

DC+ output to Filter Capacitor.

DC- to Chassis Ground.

Filter:

Two 60 µF, 3000 V electrolytics in series (Total 30 µF, 6000 V).

Equalizing Resistors: 100 kΩ, 5W across each capacitor.

Bleeder Resistor: 220 kΩ, 100W total (series chain of 5W resistors) across the entire HV+ and Ground.

HV Feed: HV+ connects to the center tap of the Plate Choke (via a HV RF choke if necessary for decoupling).

Metering:

HV Meter: High-resistance divider (e.g., 100 MΩ total) from HV+ to a 1 mA meter movement.

Plate Current Meter: Shunt resistor (0.1 Ω, 10W) in the Ground return of the HV supply (common cathode current measurement).

4. Control & Safety Circuit

Relay Control:

12V/24V DC from transceiver key-line drives a DPDT relay.

Relay 1 (Antenna): Switches antenna between Transceiver (Bypass) and Amplifier Output.

Relay 2 (HV): Enables HV contactor (safety interlock in series).

Interlocks: Microswitches on all cabinet doors wired in series with the HV contactor coil.

Fans: Thermostatically controlled blowers (intake at bottom, exhaust at top near plate caps).

Text-Based Schematic (Netlist Representation)

[RF INPUT] --(SO-239 Center)--> T1_PRIMARY_PIN1

T1_PRIMARY_PIN2 --(SO-239 Shell)--> GND

[T1: INPUT XFMR] (FT-240-43 x3)

Primary: 2 Turns (#12 AWG)

Secondary: 16 Turns (#12 AWG), Center-Tapped

T1_SEC_CT --> GND

T1_SEC_END_A --> RFC_FIL_A --> CATHODE_BUS

T1_SEC_END_B --> RFC_FIL_B --> CATHODE_BUS

RFC_FIL_A/B: 2.5mH RF Choke, 5A rating

[FILAMENT SUPPLY]

6.3V_AC_1 --> RFC_FIL_A

6.3V_AC_2 --> RFC_FIL_B

C_BYPASS: 0.1uF 100V from each 6.3V line to GND

[TUBES V1-V6] (572B)

Pin 1 (Grid) --> GND_PLANE (Shortest path)

Pin 2 (Cathode) --> CATHODE_BUS

Pin 3 (Filament) --> CATHODE_BUS

Pin 4 (Cathode) --> CATHODE_BUS

Plate Cap --> PLATE_BUS (Wide copper strap connecting all 6)

[PLATE TANK]

PLATE_BUS --> C_PLATE (5-500pF, 8kV Vacuum Var) --> GND

PLATE_BUS --> L_TANK (Switched Inductor 3-60uH) --> JUNCTION_X

JUNCTION_X --> C_LOAD (150-2000pF, 5kV Vacuum Var) --> GND

JUNCTION_X --> LINK_COIL (2 Turns) --> ANTENNA_RELAY_COMMON

[OUTPUT]

ANTENNA_RELAY_NO --> AMP_OUTPUT (SO-239)

ANTENNA_RELAY_NC --> BYPASS_INPUT (Relay input from exciter)

RF_CHKE_PROT: 2.5mH from ANTENNA_RELAY_COMMON to GND

[PLATE CHOKE & HV]

PLATE_BUS --> RFC_PLATE (2.5mH, 2kV rating) --> HV_B+

HV_B+ --> FILTER_CAPS (+)

FILTER_CAPS (-) --> GND (Via Current Shunt)

[HV POWER SUPPLY]

HV_XFMR_SEC (2200V-0-2200V)

End_A --> DIODE_A (Anode)

End_B --> DIODE_B (Anode)

Center_Tap --> GND

DIODE_A (Cathode) & DIODE_B (Cathode) --> HV_B+

(Alternative: Full Bridge if no center tap)

FILTER_CAPS: 2x 60uF 3000V in Series

Midpoint --> GND (Virtual center)

Equalizing Resistors: 100k 5W across each cap

Bleeder Resistor: 220k 100W across total HV_B+ to GND

[METERING]

Plate Current Shunt: 0.1 Ohm 10W in HV Return path to GND

Meter Movement across Shunt (with range switch)

HV Divider: 100M Ohm chain from HV_B+ to 1mA Meter to GND

[CONTROL]

Key_Line_In --> Relay_Coil --> GND

Relay_Contacts: Switches HV Contactor and Antenna Path

Safety_Interlock_Switches (Door) in series with HV Contactor Coil

Construction Tips for the Schematic

Grounding: The "Ground" for the grids (Pin 1) must be a solid copper plane or heavy strap. Do not daisy-chain grid grounds.

Plate Bus: Use a wide copper strap (1–2 inches wide) to connect the six plate caps to the tank circuit to minimize inductance and ensure equal current distribution.

Filament Wiring: The voltage drop from the transformer to the sockets must be measured under load. If it drops below 6.0 V, the tubes will be damaged. Use bus bars rather than wires for the final distribution to the sockets.

Choke Placement: The Plate Choke (RFC) should be mounted away from the tank inductor to prevent coupling, which causes parasitic oscillations.

================================================================================

6-TUBE 572B GROUNDED-GRID HF LINEAR AMPLIFIER

COMPLETE ASCII SCHEMATIC DIAGRAM

================================================================================

SECTION 1: RF INPUT & CATHODE DRIVE NETWORK (BROADBAND)

--------------------------------------------------------------------------------

+------------------+

| FILAMENT XFMR |

| 6.3V @ 30A |

| (T_FIL) |

+--------+---------+

| 6.3V AC

+-------------------------+-------------------------+

| | |

[RFC_F1] [RFC_F2] |

2.5mH/5A 2.5mH/5A |

| | |

+-------------+-------------+-----------+-------------+-----------+

| | | | | |

=== C_B1 === C_B2 | | | |

0.1uF/100V 0.1uF/100V | | | |

| | | | | |

GND GND | | | |

| | | |

[RF INPUT] | | | |

SO-239 (J1) | | | |

| | | | |

+----[C_IN]----+ | | | |

| 100pF | | | | |

| | | | | |

| | | | | |

GND | | | | |

| | | | |

| | | | | |

| | | | | |

+--------------+----------+ | | |

| | | |

(To Cathode Bus via RFC_F1) (To Cathode Bus via RFC_F2)

SECTION 2: TUBE CLUSTER (V1 - V6) & GRID GROUNDING

--------------------------------------------------------------------------------

CATHODE BUS (Heavy Copper Strap, Low Inductance)

=========================================================================

| | | | | |

+---+ +---+ +---+ +---+ +---+ +---+

|V1 | |V2 | |V3 | |V4 | |V5 | |V6 | (572B Tubes)

|572| |572| |572| |572| |572| |572|

+---+ +---+ +---+ +---+ +---+ +---+

| | | | | | | | | | | |

Pin 1| |Pin 2| |Pin 3| |Pin 4| | | | | |

(G) | | (K) | | (F) | | (K) | | | | | |

| | | | | | | | | | | |

| +-----+ +-----+ +-----+-----+-----+-----+-----> TO CATHODE BUS

+----[R_G1]----+

| 100Ω 2W | (Optional Stability Resistor per tube)

| |

GND_PLANE (Solid Copper Flashing under sockets)

NOTE: All Pin 1 (Grid) connections must go directly to GND_PLANE with shortest leads.

SECTION 3: PLATE TANK CIRCUIT (PI-NETWORK) & OUTPUT

--------------------------------------------------------------------------------

PLATE BUS (Wide Copper Strap)

=============================

| | | | | |

(Plate Caps of V1-V6 connected here)

+-----------------------+

| |

[RFC_PLATE] [C_PLATE]

2.5mH Plate Choke (Tuning Capacitor)

(Self-Resonant >30MHz) Vacuum Variable

| 5-500pF / 8kV

| |

HV_B+ GND

[L_TANK] (Switched Air-Core Inductor)

+-------[BAND SWITCH]-------+

| (Taps: 160, 80, 40, 20, 15, 10m)

+-------------+

[C_LOAD]

(Loading Capacitor)

Vacuum Variable 150-2000pF

[L_LINK]

(2-Turn Link Coil)

+----[RFC_PROT]---- GND

| 2.5mH Choke

[K1_RELAY] (Antenna Changeover)

(DPDT, High Power)

+-----> TO ANTENNA (J_OUT)

FROM EXCITER (BYPASS) -----------------------+

SECTION 4: HIGH VOLTAGE POWER SUPPLY (HV PSU)

--------------------------------------------------------------------------------

AC MAINS (220/240V)

[FUSE] (15A Slow Blow)

[SWITCH] (Interlocked with Cabinet Doors)

+----+----+

| |

[T_HV] [T_CTRL] (Control Transformer for Relays/Fans)

2200V-0-2200V 12V/24V AC

| |

| [BRIDGE_CTRL]

| |

| DC Out --> Relay Coils / Fan Thermostat

[RECTIFIER_BRIDGE] (4x 10kV/1A Diode Strings)

+----[F_HV]----+ (1A HV Fuse)

| |

| [C_FILTER_1]

| 60uF / 3000V

| |

| [R_EQ1] (100k 5W Equalizing Resistor)

| |

| [C_FILTER_2]

| 60uF / 3000V

| |

| [R_EQ2] (100k 5W Equalizing Resistor)

| |

| GND (Center Point)

+----[R_BLEED]----+ (220kΩ Total / 100W Chain)

| |

HV_B+ GND

+----[METER_HV]----+ (High Resistance Divider 100MΩ -> 1mA Meter)

+----[RFC_PLATE]---+ (Goes to Plate Tank in Section 3)

CURRENT SHUNT (Plate Current Metering)

Located in HV Return Path (GND Side of Capacitors):

HV_GND ----[R_SHUNT 0.1Ω 10W]---- CHASSIS_GND

[METER_Ip] (0-2A DC Scale)

SECTION 5: CONTROL & SAFETY INTERLOCKS

--------------------------------------------------------------------------------

KEY LINE FROM EXCITER (12V/24V DC)

[SW_MANUAL] (Operate/Standby)

[INTERLOCK_1] (Front Door)

[INTERLOCK_2] (Rear Door)

[RELAY_COIL_K1] (Antenna Relay)

[RELAY_COIL_HV] (HV Contactor)

GND

FAN CONTROL:

[THERMOSTAT] (Set ~40°C) ---- [FAN_1] (Intake)

---- [FAN_2] (Exhaust)

Airflow: Bottom (Filament) --> Top (Plate Caps)

================================================================================

END OF SCHEMATIC

================================================================================

Legend & Symbols Used

[ ] : Component enclosure (Resistor, Capacitor, Transformer, Relay).

=== : Capacitor (Non-polarized or Vacuum Variable).

--- : Wire / Connection.

GND : Chassis Ground / Earth.

| : Vertical connection.

+ : Junction point (3 or more wires).

( ) : Descriptive text or value.

CT : Center Tap.

RFC : Radio Frequency Choke.

K1 : Relay designation.

V1-V6 : Tube designations.

Critical Construction Notes from Schematic

Cathode Bus: The connection from T1_SECONDARY to the tube cathodes (Pins 2, 3, 4) must be extremely low impedance. Use a solid copper bar, not wire.

Grid Grounding: Pin 1 of every tube must connect to the GND_PLANE immediately. Do not run long wires to ground.

HV Safety: The R_BLEED resistor chain is mandatory to discharge C_FILTER capacitors within 60 seconds of power-off.

Filament Chokes: RFC_F1 and RFC_F2 prevent RF from entering the filament transformer while allowing 6.3V AC to pass.

Plate Choke: RFC_PLATE must be mounted physically distant from L_TANK to prevent coupling and parasitic oscillation.

1. Active Components (Tubes & Rectifiers)

Qty Item Part Number / Specification Source / Notes

6 RF Power Triode 572B (or T160L) RF Parts, DX Engineering. Buy a matched set of 6. Do not mix batches.

4 HV Rectifier Diode 10 kV, 1 A (e.g., HVM10, 10A10) Island Amplifier, eBay. Buy as a pre-built Bridge Stack if possible to avoid balancing resistors.

1 HV Fuse 1 A, 10 kV (Slow Blow) Mouser (Littelfuse). Essential for protecting the transformer.

2. Transformers (Power Supply)

Qty Item Part Number / Specification Source / Notes

1 HV Plate Transformer Custom / SurplusSpec: 2200–2500 VAC center-tapped @ 1.5–2.0 A (3–4 kVA).Alternative: Two Hammond 373EX (650V @ 345mA) in series/parallel is insufficient current. Look for Hammond 374EX (850V @ 475mA) x3 or a single surplus Henry/Alpha unit. eBay, Surplus Sales. Search: "2500V 2A transformer". A Variac (10A) is highly recommended to adjust input voltage.

2–3 Filament Transformer Antek AN-5006 (50VA, 6V) OR Custom 6.3V @ 15A.Spec Total: 6.3 VAC @ 30A minimum.Strategy: Wire three 6.3V @ 12A units in parallel. Antek Products, eBay. Single 30A units are rare. Verify phasing before paralleling.

1 Control Transformer Hammond 166J6 (6.3V @ 6A) or similar.Spec: 12–24 VAC @ 2A for relays/fans. Mouser, DigiKey.

3. RF Tank Circuit (High Voltage)

Qty Item Part Number / Specification Source / Notes

1 Plate Capacitor (Tuning) Comet CV05C-500XIH/5 (Used) or Jennings CVDD-500-10S.Spec: 5–500 pF, 5–10 kV Peak. RF Parts, Max-Gain Systems. Must be Vacuum Variable.

1 Load Capacitor Jennings CVDD-500-7.5S or similar.Spec: 150–2000 pF, 3–5 kV Peak, High RF Current. Surplus Sales, eBay. Often requires two in parallel if single unit not found.

1 Plate Choke (RFC) Custom Wound or RFI-2500 equivalent.Spec: 2.5 mH, Universal/Pie wound, Self-Resonant > 30 MHz. RF Parts (Silver Bullet style). Do not use single-layer solenoid.

1 Band Switch Centralab PA215 series (Surplus) or Ceramic Wafer Switch.Spec: 6 Position, Ceramic, High Voltage standoff. eBay, Surplus. Search: "Centralab 2000 series" or "Ceramic wafer switch 6 position".

1 Tank Inductor Custom Air-Core.Spec: 3 µH to 60 µH (Tapped). Wire: #10 or #12 AWG bare copper. Homebrew. Wind on a 3–4 inch diameter form.

1 Input Transformer Custom.Spec: 2:16 Turns (1:8 ratio) on 3x FT-240-43 cores. Wire: #12 AWG Teflon. Mouser (Ferrite Cores), Homebrew.

4. Capacitors & Resistors (Power Supply)

Qty Item Part Number / Specification Source / Notes

2 HV Electrolytic Cornell Dubilier 36DX series or similar.Spec: 60 µF, 3000–3500 VDC. (Wired in series for 30µF/6000V). eBay, Alibaba. Mouser rarely stocks >2kV electrolytics.

2 Equalizing Resistor Mouser 594-100K5W (Ohmite/Oxide).Spec: 100 kΩ, 5 W (Metal Oxide). Mouser, DigiKey. One across each HV cap.

20 Bleeder Resistor Mouser 283-11K-RC.Spec: 11 kΩ, 5 W (Metal Oxide). (20 in series = 220kΩ, 100W). Mouser, DigiKey.

6 Grid Stopper (Optional) Mouser 291-100-RC.Spec: 100 Ω, 2 W (Carbon Comp/Non-inductive). Mouser. Install at each tube socket pin 1.

4 Filament Bypass Cap Mouser 80-C320C104M5U5TA.Spec: 0.1 µF, 100 V (Ceramic Disc). Mouser, DigiKey.

5. Mechanical & Hardware

Qty Item Part Number / Specification Source / Notes

6 Tube Socket New Sensor SK4A or U4A.Spec: 4-Pin Ceramic, Heavy Duty (for 572B/811A). RF Parts, Antique Electronic Supply.

1 Chassis Custom Sheet Metal or Hammond 1455 series (Large).Spec: Min 19" W x 10" H x 20" D. Aluminum or Steel. Hammond Mfg, Local Shop. Must support ~60 lbs.

2 Cooling Fan Mouser 517-3206KL-04W-B39 (NMB) or similar.Spec: 120 VAC, 100+ CFM, Low Noise. Mouser, DigiKey.

1 Thermostat Mouser 658-KST010B100.Spec: Close @ 40°C, Open @ 60°C. Mouser.

2 RF Choke (Filament) Custom or Mouser 546-RF1555-10.Spec: 2.5 mH, 5 A (Must not saturate). Homebrew on ferrite rod often required for high current.

1 Current Shunt Mouser 534-100-1.0.Spec: 0.1 Ω, 10 W (Wirewound). Mouser. For plate current metering.

2 Panel Meters Surplus or DigiKey.Spec: 0–3000 VDC (with divider) and 0–2 A DC. eBay, Fair Radio.

1 Antenna Relay Jennings RCV-1A-12 (Vacuum) or Kilovac.Spec: SPDT, 3 kV, 10 A. Mouser, DX Engineering.

1 HV Contactor Kilovac HC1 series.Spec: SPST-NO, 5 kV, 1 A. Mouser, eBay.

6. Connectors & Misc

Qty Item Part Number / Specification Source / Notes

2 SO-239 Connector Mouser 523-1251 (Amphenol).Spec: UHF Female. Mouser, DigiKey.

1 Power Entry Mouser 538-478-101-1132.Spec: IEC C14 with Fuse Holder (15A). Mouser.

1 Variac (Recommended) Staco 3PN136 or similar.Spec: 0–140 VAC, 10 A. eBay, Surplus. Critical for tuning HV.

Sourcing Strategy Summary

Mouser/DigiKey: Use for resistors, low-voltage caps, fans, connectors, hardware, and relays.

RF Parts / Max-Gain: Use for tubes, sockets, vacuum capacitors, and plate chokes.

eBay / Surplus: Use for HV Transformer, Filament Transformers, Band Switch, and Chassis.

Homebrew: Input Transformer, Tank Inductor, Filament RF Chokes.

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

1. RF Signal Path (The Amplified Signal)

The RF path follows a Grounded-Grid topology, where the input signal is applied to the cathode, the grid is grounded, and the amplified signal is taken from the plate.

Input Stage:

RF energy from the exciter enters via the Input SO-239.

It passes through a 100 pF DC blocking capacitor into the primary (2 turns) of the input broadband transformer (T1).

T1 (wound on three stacked FT-240-43 cores) steps up the voltage and transforms the impedance by a ratio of roughly 64:1 (8:1 turns ratio).

The secondary (16 turns) drives the Cathode Bus. The center tap of this secondary is grounded, providing a balanced drive to the parallel cathodes of all six tubes.

Tube Interaction (Amplification):

The RF voltage on the Cathodes (Pins 2, 3, 4) modulates the electron flow from cathode to plate.

The Grids (Pin 1) are held at RF Ground via a solid copper plane, acting as a shield between input and output.

Current flows through the tubes in phase with the input voltage (Grounded-Grid action), creating a high-power RF current at the Plate Caps.

Tank Circuit (Impedance Matching & Tuning):

RF current from the six plates combines on the Plate Bus and enters the Pi-Network.

Plate Capacitor ($C_{plate}$): Shunts excess RF current to ground, determining the resonant frequency and plate loading.

Tank Inductor ($L_{tank}$): Stores energy and, via the Band Switch, selects the appropriate inductance (3–60 µH) for the operating band.

Load Capacitor ($C_{load}$): Couples the tuned circuit to the output, matching the high plate impedance (~600 Ω) down to 50 Ω.

Output Stage:

The matched 50 Ω signal is picked up by a 2-turn link coil wound over the ground end of $L_{tank}$.

It passes through a protection choke and into the Antenna Relay (K1).

Transmit Mode: The relay connects the amplifier output to the Output SO-239 (Antenna).

Receive/Bypass Mode: The relay disconnects the amplifier and connects the exciter directly to the antenna.

grounded grid amplifier operation animation

2. Electrical Power Paths (DC & AC)

A. Filament Circuit (6.3V AC)

Source: The Filament Transformer (or parallel bank) provides 6.3 VAC @ 30A.

Isolation: The AC lines pass through two 2.5 mH RF Chokes ($RFC_{F1}, RFC_{F2}$). These chokes present high impedance to RF, preventing the RF signal from shorting out through the transformer windings, while allowing 60Hz AC to pass freely.

Bypass: 0.1 µF ceramic capacitors shunt any residual RF on the filament lines to ground.

Distribution: Power is delivered via heavy copper bus bars to Pins 2 and 3 of all six sockets. The voltage drop is kept under 0.15V to ensure tube longevity.

B. High Voltage Circuit (DC B+)

Generation:

Mains AC enters the HV Transformer, stepping up to ~2200–2500 VAC.

A Full-Wave Bridge Rectifier (4x 10kV diodes) converts AC to pulsating DC.

Filter Capacitors (two 60µF/3000V in series) smooth the ripple to pure DC.

A Bleeder Resistor (220kΩ) continuously drains charge for safety.

Distribution:

DC High Voltage flows through a 1A HV Fuse and the HV Contactor (safety switch).

It passes through the Plate Current Shunt (0.1Ω), where the voltage drop is measured by the Plate Current Meter.

The HV line connects to the center of the Plate Choke (RFC).

RF Isolation: The Plate Choke (2.5 mH) presents a high impedance to RF (blocking it from entering the power supply) while allowing DC to pass freely to the Plate Bus and tube plates.

C. Control Circuit (Low Voltage DC/AC)

Keying: A 12–24V signal from the exciter energizes the Control Relay.

Sequencing:

The relay closes the Antenna Relay (switching the antenna to the amp).

Simultaneously, it energizes the HV Contactor, applying High Voltage to the plates.

Safety: Door Interlock switches are wired in series with the HV Contactor coil. If a cabinet door is opened, the circuit breaks, instantly dropping the HV.

grounded grid amplifier circuit operation theory

Summary of Interaction

RF flows: Input $\rightarrow$ Transformer $\rightarrow$ Cathodes $\rightarrow$ Plates $\rightarrow$ Tank Circuit $\rightarrow$ Output.

DC flows: Mains $\rightarrow$ HV Transformer $\rightarrow$ Rectifier $\rightarrow$ Plate Choke $\rightarrow$ Plates $\rightarrow$ Cathodes $\rightarrow$ Ground (via Shunt).

AC (Filament) flows: Transformer $\rightarrow$ RF Chokes $\rightarrow$ Cathodes/Filaments $\rightarrow$ Return.

The Grounded-Grid design ensures that the input and output circuits share the cathode connection, providing inherent stability and eliminating the need for neutralization in most HF applications.

The plate requires high voltage (typically 2,000–2,500 V for a 572B amplifier) for three fundamental physical and electrical reasons:

1. Electron Acceleration and Kinetic Energy

The primary function of the plate voltage ($E_p$) is to create a powerful electrostatic field that attracts negatively charged electrons boiled off the cathode.

Velocity: The magnitude of this voltage determines the velocity at which electrons strike the plate. Higher voltage creates a stronger pull, accelerating electrons to higher speeds.

Power Output: RF Power is the product of Voltage and Current ($P = E \times I$). To achieve high power output (e.g., 1.5 kW) without exceeding the tube's maximum current rating (which would melt the cathode or grid), you must increase the voltage.

Example: To get 300 W from one 572B tube, you can run 1,000 V @ 300 mA or 2,000 V @ 150 mA. The 572B is designed for the latter; running it at low voltage/high current would exceed its emission limits and cause rapid failure.

2. Efficiency and Heat Dissipation

High voltage allows the amplifier to operate more efficiently by minimizing wasted heat.

Voltage Swing: In a Class AB or B amplifier, the RF signal swings the plate voltage from near zero up to nearly twice the supply voltage (due to the "flywheel" effect of the tank circuit).

With a 2,200 V supply, the peak RF voltage can reach ~4,400 V.

With a low 500 V supply, the peak is only ~1,000 V, severely limiting the possible output swing and power.

Plate Dissipation: When electrons hit the plate, their kinetic energy turns into heat. If the voltage is too low, a larger percentage of the input energy is dissipated as heat on the plate rather than being converted into useful RF energy. High voltage ensures that most of the electron's energy is converted into the RF field in the tank circuit before the electron hits the plate.

3. Linearity and Space Charge Control

Space Charge: A cloud of electrons (space charge) surrounds the cathode. A strong positive plate voltage is required to penetrate this cloud and pull electrons through the grid structure effectively.

Linearity: For linear amplification (SSB/CW), the tube must operate in the straight portion of its characteristic curves. High plate voltage ensures that the tube remains in this linear region even during high-current peaks, preventing distortion and "flat-topping" of the signal.

Summary for the 572B Design

In your 6-tube 572B project:

Low Voltage (<1,000 V): The tubes would require massive current to produce power, exceeding their emission capability and melting the plates.

High Voltage (~2,200 V): Allows the tubes to operate at a moderate, safe current (~250 mA/tube) while generating high power (~300 W/tube) with good efficiency and linearity.

K7ZGQ-ANTENNAS,P.F.O. Skunkworks

6-tube 572B HF amplifier

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