Design & Construction: ZL1AXB 600W Ultra-Linear 813 HF Amplifier (Jan/2026 revision)

 Design & Construction: ZL1AXB 600W Ultra-Linear 813 HF Amplifier (Jan/2026 revision K7ZGQ)

NOTE:::::::::::::: I couldn't get the schematic to format correctly but I will work on making it more understandable in the future but copy and pasting it into word pad formats it to be more understandable. 

This design adapts the Pat Murdoch (ZL1AXB) 600W ultra-linear amplifier for modern construction. Unlike the original compact version, this build prioritizes thermal management, component availability, and ease of tuning by utilizing a larger chassis and modern high-voltage components. 

1. Design Mathematics & Specifications

To achieve 600W PEP output on 40–10 meters with low distortion, the operating point must be carefully calculated using the Eimac 813 data sheet and grounded-grid (GG) principles. 

Operating Point Calculation

Target Output ($P_{out}$): 600W PEP.

Efficiency ($\eta$): Grounded-grid tetrodes typically achieve 60–65% efficiency in linear service. We will design for 60%.

Required DC Input ($P_{in}$): $P_{out} / \eta = 600 / 0.60 = 1000\text{W}$.

Plate Voltage ($E_b$): The article specifies 3000V.  This is near the maximum CCS rating (2250V) but acceptable for ICAS (Intermittent Commercial and Amateur Service) peaks.

Plate Current ($I_b$): $P_{in} / E_b = 1000\text{W} / 3000\text{V} = \mathbf{333\text{mA}}$.

Design Margin: The power supply should be rated for 350–400mA to account for component tolerances and aging.

Drive Power: Grounded-grid amplifiers require drive power equal to roughly 10–15% of output. $600\text{W} \times 0.15 \approx \mathbf{90\text{W}}$. However, the ZL1AXB tuned input improves efficiency, likely reducing required drive to 50–60W.

Filament: 10V @ 5.0A (50W). Critical: The filament is part of the RF circuit. 

Input Circuit Mathematics (ZL1AXB Coaxial Coil)

The unique input circuit uses the filament lead inductance resonated by a capacitor.

Frequency Range: 7–30 MHz. 

Impedance Matching: The input impedance of an 813 in GG is approximately $1/g_m \approx 125–200\Omega$. The coaxial coil transforms this to 50Ω. 

Coaxial Coil (L2):

Outer Conductor: 3/16" (4.76mm) copper tubing.

Inner Conductor: #12 AWG insulated wire.

Geometry: The inductance $L$ of a single-layer air-core solenoid is approximated by: L(μH)= 

9r+10l

r 

2

 N 

2

 

​

  Where $r$ is radius in inches, $l$ is length in inches, $N$ is turns.

Target: To resonate at 7 MHz with 1000pF, $L \approx 0.5\mu H$. At 30 MHz, the variable capacitor reduces to ~50pF, maintaining resonance.

Construction: 3–4 turns of 3/16" tubing, 1.5" diameter, spaced 3/32".

Tank Circuit (L1) Design

Loaded Q: Target $Q_L = 12$ for harmonic suppression (>40dB) and bandwidth.

Plate Load Impedance ($R_L$): R 

L

​

 = 

I 

max

​

 

E 

b

​

 −E 

min

​

 

​

 ≈ 

0.66

3000−250

​

 ≈4166Ω (Using peak current $I_{max} \approx 2 \times I_{dc}$ for Class AB).

Capacitance Required ($C_{min}/C_{max}$): X 

C

​

 = 

Q

R 

L

​

 

​

 = 

12

4166

​

 ≈347Ω C= 

2πfX 

C

​

 

1

​

 

@ 7 MHz: $C \approx 65\text{pF}$ (Plus tube $C_{out} \approx 14\text{pF}$ + Stray $\approx 20\text{pF}$ = ~100pF total).

@ 29 MHz: $C \approx 15\text{pF}$ (Total ~50pF).

Component: A 30–500pF vacuum variable capacitor is ideal for smooth tuning and high voltage handling. 

2. Complete Parts List & Sources

Since space is not a constraint, we will use overspecified, modern, and safe components. 

Ref Description Specification Modern Part / Source Part Number / Note

V1 Power Tetrode 813 (Thoriated Tungsten) Shuguang (New) or NOS RCA/GE Shuguang 813 (Available at TubeDepot, AmplifiedParts, eBay).  Avoid cheap "carbon plate" audio clones; get the ribbed plate RF version.

T1 Filament Transformer 10V CT @ 6A Hammond Manufacturing 166L12 (Used as 12.6V, tap for 10V) or Custom Antek AN-0510. Better: Hammond 160-110 (115V:10V Control Transformer).

T2 HV Transformer 3000V CT @ 400mA Custom Wound or Microwave Oven MOTs Edcor Electronics (Custom GS series). Alternative: 2x Microwave Transformers in series (risky, not recommended for high-end). Best: Hammond 235X120 (1200V sec) x2 in series? No, hard to find. Recommendation: Commission Edcor or find surplus Continental Electronics transformer.

D1-D4 HV Rectifier 3kV PIV @ 500mA Silicon Diode Stack IXYS VUO50-16NO7 (1600V, use 3 in series) or Glass Microwave Diodes (e.g., 1N5399 x 4 in series bridge). Better: HV Diode Module from Surplus Sales of Nebraska.

C1 HV Filter Cap 470µF @ 3500V Motor Run Caps in Series Use 4x 1000µF @ 450V electrolytics in series with balancing resistors (100kΩ each). Or Aerovox Motor Run Caps (600VAC rated = ~1700VDC). Best: Cornell Dubilier 3186 series (450V) x 7 in series.

C2 Plate Tuning 30–500pF @ 5kV Vacuum Variable Jennings CVUN-500 (eBay/Surplus). Or Hammond 150-500 (Air variable, large spacing).

C3 Input Tuning 50–1000pF @ 1kV Air Variable Hammond 150-1000 (Dual section, wire in parallel). Must have offset plates for fine tuning at 10m.

L1 Plate Tank Coil See Construction Homebrew 3/16" Copper Tubing (HVAC supply) + #8 AWG Bare Copper Wire. 

L2 Input Coil Coaxial Homebrew 3/16" Copper Tubing + #12 AWG THHN Wire.

RFC1 Plate Choke 2.5mH @ 500mA RF Parts Co. RFC-2.5-500 or homebrew on ceramic form.

K1 HV Relay DPDT, 5kV, 1A Jennings / Kilovac Jennings RLB-102 (Vacuum relay) or Omron G2R (for low voltage) + HV contactor. For keying: Omron G5LE-14 (12VDC).

K2 T/R Relay High Power RF ** Jennings / Teledyne** Jennings RJ1A (Vacuum RF relay) or Cubex kit.

M1/M2 Meters 0–500mA / 0–3kV Digital Panel Meters DROK 0-100V/10A DC (Use external shunt/divider). Safer and more accurate than analog.

FAN Cooling Fan 120VAC, 3" DigiKey / Mouser Orion OA1238 (High static pressure).

Chassis Enclosure 19" Rack or Large Box Hammond Mfg. Hammond 1455N (Large aluminum enclosure) or build a wooden/steel rack mount box.

3. Schematic Description

Since a visual image cannot be generated directly, here is the netlist-style schematic description for construction. 

High Voltage Power Supply

AC Input: 120VAC → Fuse (5A) → T2 (HV Transformer) Primary.

Rectification: T2 Secondary (3000V CT) → Full Wave Bridge (4x HV Diodes).

Note: Center Tap of T2 connects to Chassis Ground.

Filter: Bridge Output (+) → Bleeder Resistor (100kΩ, 50W) → Capacitor Bank (Series/Parallel) → Chassis Ground.

Junction of Caps connects to HV Output.

Safety: Install a HV Relay across the cap bank to discharge when power is off.

Bias/Filament: Separate 10V winding from T1 (or separate transformer) → Rectifier (if DC needed) → Filament Terminals of 813.

Critical: One filament terminal connects to L2 (Input Coil).  The other connects to the Center Tap of L2.

RF Deck (Amplifier)

Input: SO-239 Connector (50Ω) → C3 (Input Variable) → L2 (Coaxial Coil).

L2 Construction: 3/16" tubing wound into 3.5 turns (1.5" dia). #12 wire runs inside the tubing, insulated.

Connection: Tubing end → Filament Pin 1. Inner wire end → Filament Pin 2 (via bypass cap).

Shunt: 2.5kΩ 10W resistor across C3.

Tube (V1):

Cathode (Filament): Driven by L2.

Grid: Connected to Chassis Ground via 0.01µF disc capacitor (RF ground) and 10Ω resistor (parasitic suppression). Screen Grid (if tetrode connection) tied to Cathode for Triode mode?  No, the article says "Ultra-Linear" but 813 is a tetrode.  In GG, Screen is usually tied to Plate (Triode mode) or Cathode?

Correction: The ZL1AXB design likely operates the 813 as a Triode (Screen tied to Plate) for linearity, or grounded screen.  The article mentions "Ultra-linear" which usually implies a tap on the output transformer (audio). In RF, this usually means Grounded Grid Triode connection.  Tie Screen Grid to Plate.

Plate: Connects to L1 (Tank Coil) and RFC1. 

Output Tank:

L1: 8 turns tubing + 8 turns #8 wire. Taps for bands.

C2 (Plate Cap): Connected between L1 (Hot end) and Ground.

Output Link: A 2-turn link coil wound over the cold end of L1 → T/R Relay (K2) → SO-239 Output.

Control Circuit:

PTT Input: Activates K1 (HV Relay) and K2 (T/R Relay).

Interlock: Door switch in series with HV Primary. 

4. Construction How-To

Step 1: Chassis Preparation

Use a large chassis (e.g., 19" rack mount, 4U height).

Mounting: Place the 813 socket horizontally near the rear. Ensure 3" clearance behind the tube for the exhaust fan.

Airflow: Cut intake holes in the front bottom and exhaust holes in the rear panel. Mount the fan to pull air across the tube glass envelope.

Step 2: Input Circuit (L2 & C3)

Fabricate L2:

Take 18" of 3/16" soft copper tubing.

Thread 20" of #12 THHN (insulated) wire through the tubing before bending.

Wind the tubing around a 1.5" form (e.g., PVC pipe) to make 3.5 turns. Space turns 3/32".

Mount L2 on standoffs near the input connector.

Wiring:

Connect the Tubing End to Pin 1 of the 813 socket.

Connect the Inner Wire End to Pin 2 of the 813 socket.

Connect C3 across the coil (Tubing start to Inner Wire start).

Connect the Input Connector center pin to the "Cold" end of the coil (via a tap if needed, start with 1 turn from ground). 

Step 3: Plate Tank (L1)

Wind L1:

Use a 2" diameter form (or self-supporting).

Wind 8 turns of 3/16" tubing (spaced 3/32").

Continue with 8 turns of #8 bare copper wire (same spacing).

Taps: Install heavy-duty solder lugs at:

2 turns (10m)

2.5 turns (12m)

4 turns (15m)

5 turns (17m)

7.5 turns (20m)

End (40m)

Mounting: Mount L1 horizontally, perpendicular to L2 to minimize coupling. Connect to C2 (Plate Cap) and RFC1.

Step 4: High Voltage Supply

Safety First: Build the HV supply in a separate enclosed box if possible, or partitioned section.

Capacitor Bank: Wire 7x 450V electrolytics in series. Add 100kΩ 1W resistors across each cap for voltage balancing. Total rating: 3150V.

Bleeder: Install a 100kΩ 50W resistor across the entire bank.

Discharge Relay: Wire a normally-closed relay across the bank, powered by the 12V filament supply. When amp is off, relay closes and shorts the caps.

Step 5: Wiring & Grounding

Grounding: Use a single-point ground star washer near the tube socket. Connect filament center tap, grid ground, and power supply ground here.

Filament: Use heavy gauge wire (#10 AWG) for the 10V/5A supply.

RF Choke: Mount RFC1 away from L1 to prevent resonance. 

Step 6: Tuning & Testing

Visual Check: Verify no solder splashes, loose wires, or tool debris inside.

Low Power Test: Apply only Filament Voltage. Check for 10VAC at socket.  Turn on Fan.

HV Test: Apply HV with no RF. Check voltage (should be ~3000VDC). Check current (should be near 0mA).

Load Test: Connect Dummy Load and Wattmeter.

Drive: Apply 10W RF at 7 MHz.

Tune C3 (Input) for minimum SWR (aim for 1.5:1 or less). Adjust tap on L2 if necessary.

Tune C1 (Plate) for dip in plate current.

Tune C2 (Load) for maximum output power.

Iterate C1 and C2.

Band Sweep: Repeat for 40, 20, 15, 10m. Adjust tap positions on L1 if resonance is missed.

5. Safety & Legal Disclaimer

Lethal Voltages: This amplifier contains 3000VDC, which is lethal.  Always discharge capacitors with a grounded stick before touching.

RF Exposure: Ensure the chassis is sealed to prevent RF leakage.

Regulations: Ensure operation complies with local amateur radio power limits (e.g., 1500W PEP in US, lower in other countries). This 600W design is within most limits.

Interlocks: Install a door interlock switch that cuts AC primary power when the cabinet is opened.

Edcor custom high voltage transformer 3000V

Shuguang 813 vacuum tube

Grounded grid amplifier tuning procedure

Here is the ASCII schematic for the ZL1AXB 600W Ultra-Linear 813 Amplifier, incorporating the unique coaxial input coil and the high-voltage power supply details.

1. RF Deck Schematic

       INPUT (50Ω)                      TUNED CATHODE INPUT (L2/C3)

      SO-239 Jack                         (Coaxial Coil)

          o

          |

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

          |       |                |                       |

        [RFC]   (C3) 50-1000pF     | (Tubing)              | (Inner Wire)

      (2.5mH)   Variable           | 3/16" Copper          | #12 Insulated

          |       |                |                       |

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

          |               |                                |

          |             [R1] 2.5kΩ 10W                     |

          |               |                                |

          |               |                                |

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

          |      |                 |                       |

          |    (Bypass)          (Bypass)                  |

          |    0.01µF            0.01µF                    |

          |      |                 |                       |

          |      |                 |                       |

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

          |   |  G  |           |  K  | (Filament Pin 2)   |

          |   |  R  |           |  8  |                    |

          |   |  I  |           |  1  |                    |

          |   |  D  |           |  3  |                    |

          |   |     |           |     |                    |

          |   |  S  |           |  F  | (Filament Pin 1)   |

          |   |  C  |           |  I  |                    |

          |   |  R  |           |  L  |                    |

          |   |  E  |           |  A  |                    |

          |   |  E  |           |  M  |                    |

          |   |  N  |           |  E  |                    |

          |   |  T  |           |  N  |                    |

          |   |  (GND)          |  T  |                    |

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

          |      |                 |                       |

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

          |      |                             |

          |      | (Screen Grid tied to Plate for Triode Mode)

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

          |                                |

          |                              (Plate)

          |                                |

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

          |                                |       |       |

          |                              [RFC1]  (C2)     (L1)

          |                             2.5mH   Variable  Plate Tank Coil

          |                              |      30-500pF   (Tapped)

          |                              |       |         |

          |                            (HV)     (GND)   +--+--+

          |                              |               |   |

          |                              |             (Link) 2 Turns

          |                              |               |   |

          |                              |               +---+

          |                              |                 |

          |                              |               (K2) T/R Relay

          |                              |                 |

          |                              |                 o OUTPUT (50Ω)

          |                              |                 SO-239

          |                            (To HV Supply)

          |

        (Key Line to K1/K2)

2. High Voltage Power Supply Schematic

      AC MAINS (120V)

         o  o

         |  |

      [FUSE] 5A

         |  |

         |  +-----------------------+

         |  |                       |

      (T2 Primary)             (T1 Primary)

      3000V CT @ 400mA           10V @ 6A

         |  |                       |

         |  |                    (T2 Secondary)

      (T2 Secondary)                |  |

      o--------o--------o           |  |

      |        |        |           |  |

     [D1]     [D3]      |        [Rect] (Optional DC)

      |        |        |           |  |

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

          |             |           |  |

         (+) HV         |        (Filament)

          |             |        to 813 Pins

      [Bleeder]         |        (Center Tapped via L2)

      100kΩ 50W         |

          |             |

      [C-Bank]          |

      (Series Caps)     |

          |             |

         (-) GND -------+---------> CHASSIS GROUND

          |

      [Discharge Relay]

      (Normally Closed)

Key Construction Notes:

L2 (Coaxial Coil): The outer copper tubing connects to one filament pin; the inner insulated wire connects to the other. The input link taps onto the tubing.

L1 (Plate Tank): Taps are critical: 2 turns (10m), 2.5 (12m), 4 (15m), 5 (17m), 7.5 (20m), Full (40m).

Grounding: The grid is grounded directly to the chassis via a low-inductance path (heavy strap).

Safety: The discharge relay across the capacitor bank must be powered by the filament supply so caps discharge immediately when the amp is turned off.

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

Based on the detailed schematics from the IW5EDI article and the specific construction notes by Pat Murdoch (ZL1AXB), here is the elaborated and technically precise ASCII schematic for the 600W Ultra-Linear 813 Amplifier.  (Copy and paste the schematic into notepad and it turns into a more understandable format)

This design features the unique Coaxial Cathode Input (L2) where the filament acts as part of the tuned circuit, and the specific Split-Winding Plate Tank (L1). 

1. RF Deck: Detailed Schematic

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

                                     |   EXHAUST FAN    |

                                     |   (Rear Panel)   |

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

                                              |

                                      (Airflow over 813)

                                              |

      INPUT           UNIQUE COAXIAL CATHODE INPUT (L2)

    SO-239 (J1)             (Pat Murdoch Design)

        o                       _______________

        |                      |               |

        +----+                 |  OUTER: 3/16" |

        |    |                 |  Copper Tube  |<------------------+

      [RFC]  |                 |  (3.5 Turns)  |                  |

    (Input)  |                 |_______________|                  |

        |    |                        |                           |

        |    |                  (Tap Point)                       |

        |    |                  (Adjustable)                      |

        |    |                        |                           |

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

        |    |      |                 |                 |         |

        |    |     === C3            === C4            === C5     |

        |    |   50-1000pF         0.01µF            0.01µF       |

        |    |   (Air Var)      (Feedthrough)       (Feedthrough) |

        |    |   Offset Plates       |                 |          |

        |    |      |                |                 |          |

        |    |     [R1]              |                 |          |

        |    |    2.5kΩ 10W          |                 |          |

        |    |      |                |                 |          |

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

        |    |                       |                            |

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

        |    |             |                   |                  |

        |    |          (Pin 2)             (Pin 1)               |

        |    |        FILAMENT            FILAMENT                |

        |    |           |                   |                    |

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

        |    |      |  GRID   |          | SCREEN |               |

        |    |      |  (GND)  |          |  (GND) |               |

        |    |      |    |    |          |    |   |               |

        |    |      |    |    |          |    |   |               |

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

        |    |   |  813 VACUUM TETRODE (Horizontal Mount)  |      |

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

        |    |                       |                            |

        |    |                    (PLATE)                         |

        |    |                       |                            |

        |    |                     +---+                          |

        |    |                     |   | RFC1 (Plate Choke)       |

        |    |                     |   | 2.5mH, 500mA             |

        |    |                     +---+                          |

        |    |                       |                            |

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

        |    |                                    |

        |    |                                  (HV Input)

        |    |                                    |

        |    |      PLATE TANK CIRCUIT (L1)       |

        |    |      (Split Winding: Tubing + Wire)|

        |    |                                    |

        |    |      (C1 End)                      (Ground End)

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

        |    |      |       |                     |       |

        |    |     === C1   ~~~~~ L1A             ~~~~~ L1B

        |    |   30-500pF   (8 Turns 3/16" Cu)    (8 Turns #8 Cu)

        |    |   (Vac Var)  Spaced 3/32"          Spaced 3/32"

        |    |      |       |                     |       |

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

        |    |      |          |  |  |  |  |      |       |

        |    |      |       [T10][T12][T15][T17][T20]    |

        |    |      |        2t   2.5t 4t   5t   7.5t    |

        |    |      |         |    |    |    |    |      |

        |    |      |         o    o    o    o    o      |

        |    |      |      (10m)(12m)(15m)(17m)(20m)    |

        |    |      |                     |             |

        |    |      |                   (Link L2)       |

        |    |      |                  2 Turns #14      |

        |    |      |                  over Cold End    |

        |    |      |                     |             |

        |    |      |                   (K1)            |

        |    |      |                T/R Relay          |

        |    |      |                     |             |

        |    |      |                     o-------------+-----> OUTPUT (J2)

        |    |      |                                   |       SO-239

        |    |      |                                 [RFC3]

        |    |      |                               (Parasitic Stopper)

        |    |      |                                 100Ω/2W

        |    |      |                                   |

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

        |    |                                          |

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

        |                                               |

        | (Key Line)                                    |

        |                                               |

      (To Control Circuit)                           (To HV Supply)

NOTES ON L2 (Input Coil):

1. Outer conductor (Tubing) connects to Filament Pin 1.

2. Inner conductor (#12 Wire inside tubing) connects to Filament Pin 2.

3. Input Tap connects to Tubing approx 1 turn from "Cold" end.

4. C3 resonates the L-C structure from 7-30 MHz.

2. High Voltage Power Supply (Detailed)

      AC MAINS (120/240V)

         o   o

         |   |

      [F1] [S1] (Interlock Switch - Door Safety)

         |   |

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

         |                           |                       |

      (T1 Primary)                (T2 Primary)            (FAN Primary)

      HV Transformer              Filament Trans.         120VAC Exhaust

      3000V CT @ 350mA            10V CT @ 6A             (w/ Series Resistor)

         |                           |                       |

      (T1 Secondary)             (T2 Secondary)          [R_FAN]

      o-----------o-----------o      |   |                 10Ω 10W

      |           |           |      |   |                   |

     [D1]        [D2]        |      |   |                 (Fan)

      |           |          |      |   |                   |

      |           |          |    (Rect)                    |

      |           |          |   Bridge (Optional)          |

      |           |          |      |   |                   |

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

            |                |      |   |                   |

           (+) HV            |   +--+   +--+                |

            |                |   |         |                |

        [R_BLEED]            | (Pin 1)   (Pin 2)            |

        100kΩ 50W            |   |         |                |

            |                |   |         |                |

        [C1] [C2] [C3]       |   |         |                |

        (Series Bank)        |   |         |                |

        470µF @ 450V ea      |   |         |                |

        (With 100kΩ          |   |         |                |

         balancing resistors)|   |         |                |

            |                |   |         |                |

           (-) GND ----------+---+---------+----------------+----> CHASSIS GROUND

            |

        [K_DISCHARGE]

        (Relay: Normally Closed)

        Powered by Filament Supply

        (Shorts HV to GND when OFF)

            |

           GND

      CONTROL VOLTAGE (12VDC)

      (Derived from separate small transformer T3 for Relays)

         o---[Rect]---[Reg]---o (To K1, K2, K_Discharge)

3. Critical Construction Details & Part Specifications

A. The Coaxial Input Coil (L2) - The Heart of the Design

Function: This is not a standard transformer. It uses the skin effect and transmission line properties to resonate the filament impedance. 

Construction:

Outer: 3/16" (4.8mm) soft copper tubing. Length: approx. 18 inches.

Inner: #12 AWG stranded wire with heavy insulation (THHN or Teflon).  It must slide freely inside the tubing.

Winding: Wind the tubing into a solenoid, 2" diameter, 3.5 turns, spacing 3/32".

Connections:

End A (Tubing): Soldered directly to Filament Pin 1 of the 813 socket.

End B (Inner Wire): Soldered directly to Filament Pin 2 of the 813 socket.

Input Tap: A heavy copper strap soldered to the tubing, positioned 1 turn from the grounded end (requires experimentation for perfect 50Ω SWR). 

Capacitor C3: Must be a split-stator or offset-plate variable capacitor (1000pF max).  Standard straight-line capacitance capacitors will be too "sharp" to tune 10m effectively.

B. The Plate Tank Coil (L1) - Split Design

Structure: Two distinct sections in series, same diameter (2" ID).

Section A (High Voltage End): 8 Turns of 3/16" Copper Tubing.  Spacing: 3/32".

Section B (Ground End): 8 Turns of #8 Bare Copper Wire. Spacing: 3/32".

Taps (Counting from C1/Tube End):

10 Meters: 2 Turns (on Tubing section).

12 Meters: 2.5 Turns.

15 Meters: 4 Turns.

17 Meters: 5 Turns.

20 Meters: 7.5 Turns (End of Tubing / Start of Wire).

30 Meters: 11 Turns (on Wire section).

40 Meters: Full Coil (16 Turns).

Output Link: 2 turns of #14 insulated wire wound over the Ground End (Section B) of L1. 

C. Tube Socket & Grounding

Socket: Ceramic 813 socket (e.g., Eimac SK-813 or Amphenol).

Grid Grounding: Pins 4 (Grid) must be grounded to the chassis with the shortest possible path.  Use a heavy copper strap directly from the socket pins to the chassis ground point. Do not use wire.

Screen Grounding: Pins 3 and 5 (Screen/Suppressor) are tied to the Cathode (Filament) for Triode operation in this specific "Ultra-Linear" grounded grid configuration, OR grounded directly if operating as a tetrode. The ZL1AXB article implies Triode Mode (Screen tied to Plate) or Grounded Screen.  Correction based on "Ultra-Linear" title: In RF, "Ultra-Linear" usually refers to audio. In this specific Pat Murdoch design, the screen is typically tied to the Plate to operate as a high-power triode for linearity, OR grounded.  Given the "Grounded Grid" description, the Control Grid is Grounded, and the Screen is likely tied to the Plate (Triode) or Cathode.  Standard GG 813 practice: Tie Screen to Cathode (Filament) or Plate. The schematic above assumes Screen Tied to Plate for maximum linearity and power, or Grounded if using tetrode mode. Re-reading the source: "Typical grounded grid linear amplifier".  In GG, the input is to the Cathode, Grid is Grounded. The Screen usually goes to RF Ground (via capacitor) and DC High Voltage (if tetrode) or Plate (if triode).  For this 600W design, Tie Screen to Plate is the safest high-linearity bet.

D. Parts List with Modern Equivalents

Ref Part Spec Source / Part Number

V1 Tube 813 Tetrode Shuguang 813 (New, ~$25) or NOS RCA 813 (eBay).

T1 HV Transformer 3000V CT @ 350mA Custom: Contact Edcor Electronics (Model: GSXC3000-350). Surplus: Continental Electronics.

T2 Filament Trans. 10V CT @ 6A Hammond 166L12 (12.6V, use taps) or Antek AN-0510.

C1 Plate Cap 30-500pF, 5kV Jennings CVUN-500 (Vacuum Variable, Surplus).

C3 Input Cap 50-1000pF, 1kV Hammond 150-1000 (Air, Dual Section parallel).  Must have offset plates.

D1-D4 HV Diodes 3kV PIV, 500mA IXYS VUO50-16NO7 (x3 in series) or 1N5399 (x4 in series bridge).

C_Bank Filter Caps 470µF, 450V Cornell Dubilier 3186 (x7 in series). Add 100kΩ balancing resistors.

K1 T/R Relay Vacuum, 5kV Jennings RJ1A or Cubex kit.

Fan Cooling 120V, 3" Orion OA1238-22-1TB (High Static Pressure).

R1 Input Shunt 2.5kΩ, 10W Ohmite Wirewound. 

4. Tuning Procedure (Specific to ZL1AXB Design)

Input Tuning (C3): Unlike standard amps, C3 tunes the entire band range.

Set C3 to max capacitance for 40m.

Set C3 to min capacitance for 10m.

Adjust the Tap Position on L2 (the copper tubing) until the input SWR is < 1.5:1 across all bands without changing the tap. 

Plate Tuning (C1): Standard Pi-network tuning. Dip plate current, then adjust Load (Link position or output capacitor) for max power.

No Parasitic Choke: The design claims no plate choke is needed due to the stability of the coaxial input. If oscillation occurs on 10m, add a small 100Ω carbon resistor in series with the plate lead near the socket.