1. The DX-3SP — The Only Dual-Tube Model in the Emtron Range

The Emtron DX-3SP (operating manual February 2009, Emtron Division of Emona Electronics Pty Ltd, 92–94 Wentworth Avenue, Sydney NSW 2010) is described in its own marketing as the “biggest bargain in top brand 4 KW CW output linear amplifiers” and “physically the smallest 4000 Watt desktop RF amplifier on the market today.” Its 4,000 W output is achieved through a configuration unique in the Emtron product line: two FU-728F tetrodes operating in parallel on a shared RF sub-chassis, providing a combined 3,000 W plate dissipation capability and driven from a 3,400 V DC plate supply.²

Emtron’s marketing notes that all specifications and features are common across the Emtron product range except output power. This is broadly correct for the shared modular systems (AMPC control board architecture, soft-start, display board LM3914 array, RF sensor, 9-position band switch, 6:1 reduction drives), but the parallel tube configuration introduces six circuit elements unique to the DX-3SP that require specific service understanding. These are documented in Section 5 of this guide.

2. DX-3SP Technical Specifications

3. DX-3SP vs DX-3 — Five Key Differences

4. FU-728F Tetrodes — Parallel Operation & Service

The FU-728F is a ceramic-metal tetrode produced in China as an improved military-grade version of the Eimac 4CX1500B. It uses the 4CX1500B octal socket, allowing direct interchangeability through a filament tap change on the transformer. Two FU-728F tubes in parallel provide 3,000 W combined plate dissipation. Emtron’s marketing describes the tubes as “manufactured in present production and available directly from the manufacturer, or from Emtron, exclusively for Emtron amplifier owners.”³

In a parallel tube configuration, matched tube characteristics are important for current sharing. Two tubes with significantly different transconductance or cathode emission will share plate and screen current unequally, causing one tube to operate near its dissipation limit while the other runs cool. The DX-3SP’s three-pot bias system (Section 6) addresses this directly: POT10 and POT9 allow each tube’s idle current to be individually trimmed against the global POT3 setting for matched parallel operation.

4.1 Replacing One FU-728F vs Replacing the Pair

When one FU-728F fails in a DX-3SP, the question of whether to replace just the failed tube or both tubes simultaneously arises. The best practice for parallel tube amplifiers is to replace both tubes together as a matched pair from the same production batch. A new FU-728F with full cathode emission paired with an older tube with reduced emission will share current unequally; the new tube will carry the majority of plate and screen current and will reach its dissipation limit before the amplifier reaches full output. If only one tube is available, replace the failed tube and re-adjust POT10/BS1 and POT9/BS2 to equalise the two tube currents as closely as possible before returning to full-power operation.

5. DX-3SP-Exclusive Internal Components — Safety Choke, Mains Isolation Relay & More

5.1 Safety Choke (Between the Two Tubes)

The safety choke is visible in the DX-3SP internal view diagram (manual Section 1.1, Figure 1) labelled between the TANK COIL 40–160m and the tube assembly. It appears in the RF module simplified schematic (15.2.3) in the anode/plate feed of the parallel tube arrangement. Its function is to provide RF isolation between the two parallel FU-728F anodes; in a parallel tetrode arrangement, the safety choke prevents RF circulating current from one tube’s anode from directly loading the other tube’s anode path in the event of asymmetric tube operation or failure.⁴

5.2 Mains Isolation Relay

The mains isolation relay appears in the DX-3SP internal view labelled between the BLOWER and ANT. RELAY (QSK) positions, in the primary AC wiring area. This relay provides a switching break in the primary AC circuit — a function beyond the standard Emtron POWER switch. Its exact switching sequence (whether it opens before or after the soft-start TRIAC fires) is documented in the DX-3SP primary AC wiring schematic. After any mains-side fault event (blown fuses, surge damage), inspect the mains isolation relay contacts for pitting or welding; a welded-closed mains isolation relay bypasses the intended primary AC switching function and eliminates one layer of electrical safety.

5.3 Coupling Capacitor in RF Path

The coupling capacitor is visible in the DX-3SP internal view (Figure 1) labelled between the SOFT START and RF SENSOR positions in the RF section. It provides DC blocking in the RF path between the RF sub-chassis (where the two FU-728F anode outputs emerge) and the main pi-network. At 3,400 V and 4,000 W, this capacitor must sustain both the DC plate voltage and the RF peak voltage from two parallel tubes. After any arc event in the RF deck, the coupling capacitor must be inspected and tested for dielectric breakdown.

5.4 3,300 pF / 10 kV RF Output Capacitor

The RF output circuit schematic (15.2.4) shows a 3,300 pF/10 kV capacitor in the RF output network, the highest voltage-rated component in any Emtron RF section. Its 10 kV rating is appropriate for the instantaneous RF peak voltage at the plate capacitor junction with 3,400 V DC supply and 4,000 W RF output. At the voltage stress levels the DX-3SP operates at, replacing this capacitor with a lower voltage-rated type (e.g., 6 kV) would be dangerous; RF peak voltages can briefly exceed 6 kV at the plate under mistuned conditions at 4,000 W. Replace only with a 10 kV-rated 3,300 pF ceramic disc from a verified supplier (Vishay, Vitramon, or equivalent); do not substitute with NP0 capacitors from general-purpose electronic suppliers without confirming the 10 kV voltage rating.

6. RF Deck — Parallel Tube Circuit & Tank Components

The DX-3SP RF module schematic (Section 15.2.3, labelled “RF Module Simplified”) shows the two FU-728F tubes and their surrounding circuit. The key circuit elements from this schematic are:

• 4× 1,000 pF / 6 kV plate bypass capacitors (same rating per position as DX-2 series)
• Plate choke (supplying 3,400 V DC to both tube anodes)
• Safety choke (between tube anode feeds)
• 2,200 pF coupling capacitor
• 3× 150 Ω / 13 W resistors and 330 Ω / 13 W resistor (input matching network)
• 500 pF and 1,100 pF input bypass capacitors
• 5× A106 clamping diodes at 140 V
• 10 nF / 1 kV screen bypass capacitor
• 1N4148 diodes at various nodes

The RF output circuit (15.2.4) additionally shows: L1 (RF Coil, silver-plated copper tube, 10–30 m), L2 (Torroid, 40–160 m), and the 3,300 pF/10 kV plate capacitor / output network capacitor alongside a set of band-switch capacitors (1,100 pF, 1,500 pF, 250 pF, 50 pF, 47 pF, 470 pF).

7. AMPC Control Board — Horizontal; Three Bias Pots; Counter-Clockwise Direction

The DX-3SP AMPC control board is horizontally mounted, the same orientation as the DX-2SP and DX-3, and confirmed in the DX-3 manual: “Rotate clockwise for DX-1, DX-2 and anti-clockwise for DX-2SP and DX-3, which have horizontally installed boards.” The DX-3SP falls into the same category as the DX-2SP and DX-3 for horizontal board orientation; the adjustment procedures in the DX-3SP manual Appendix 3 (adjustments) use the same counter-clockwise direction.

The defining service distinction on the DX-3SP AMPC board is the presence of three bias adjustment potentiometers. The DX-3SP manual states: “There are 3 bias adjustment potentiometers: POT3 (BIAS), affecting both tubes and POT10 (BS1) and POT9 (BS2) only adjusting the bias to the tube connected to the pins BS1 and BS2 respectively. Adjust initially POT10 and POT9 fully clockwise, then rotate back about 20 degrees (they are likely to be already in the required position).” ⁵

8. HV Power Supply & Three-Fan Cooling

8.1 HV Filter Bank — 58 µF at 3,400 V

The DX-3SP HV filter bank is the standard Emtron SP-family design: 8× 470 µF / 500 V electrolytics, 58 µF total at a conservative 4,000 V DC rating. This is the same bank configuration as the DX-2SP, DX-1SP, and subsequent SP models. At 3,400 V DC and the higher supply current drawn by two parallel tubes at 4,000 W, the bank is under higher electrical stress than in any single-tube model. Equalising resistors across each capacitor section are critical; at 3,400 V, any inequality in capacitor capacitance or ESR will create voltage sharing imbalance in the series string.

8.2 Three-Fan Cooling System

The DX-3SP has three fans: the turbine blower below the chassis (same as all DX models) plus two suction fans above the tube area. The DX-3SP manual Section 8.1 states: “Above the tubes there is one fan sucking air and reducing the back pressure to the blower.” However, the marketing materials from cqdx.ru describe: “The tubes are air cooled by a commercial grade forced air turbine blower system and two extra suction fans above the tube.”⁶ Both the manual and marketing confirm the presence of upper suction fans; the total fan count is confirmed at three (1 turbine + 2 upper) from the marketing materials. All three fans must be operational for sustained 4,000 W operation.

The temperature sensor setpoint is documented at 80°C (176°F) for the fan speed control. Both upper suction fans operate at two speeds under this sensor’s control. Verify all three fans are rotating and transitioning between speeds correctly; at 4,000 W continuous carrier, the combined heat load of two FU-728F tubes and the high-power RF deck makes cooling system integrity more critical than in any other Emtron model in this series.

9. Safety Systems — Interlocks, Discharge & AC Line

9.1 Cover Safety Microswitch

The DX-3SP cover safety microswitch is identical in function to all other DX models. The DX-3SP manual CAUTION section is explicit: do not remove the top cover with any leads still connected, especially the AC mains. At 3,400 V with two parallel tubes, the energy stored in the HV filter bank is the highest of any single-chassis Emtron model. After any cover-related service, verify the microswitch correctly interrupts mains supply when the cover is removed before returning to operational service.

9.2 Safe Discharge Procedure — Two Tubes, Both Anodes

  STEP 1 ── OPR → STBY; POWER → OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS:
            Mains, RF IN, RF OUT, PTT/Key, ALC, all control cables.
               │
  STEP 3 ── Wait MINIMUM 5 MINUTES.
            DX-3SP HV bank at 3,400V DC; highest voltage single-chassis model.
            Allow full natural discharge time.
               │
  STEP 4 ── ANODE DISCHARGE — BOTH TUBES:
            10kΩ/25W insulated resistor probe, one end to chassis.
            Contact firmly to FU-728F #1 anode cap. Hold 15 seconds.
            THEN: Contact firmly to FU-728F #2 anode cap. Hold 15 seconds.
            Both anodes must be individually discharged.
            Parallel tubes do NOT guarantee cross-discharge through the safety choke.
               │
  STEP 5 ── HV MEASUREMENT (5kV-rated DMM):
            Confirm HV filter cap positive to chassis < 50V DC.
               │
  STEP 6 ── SCREEN SUPPLY:
            Measure EG2 (blue wire) to chassis → < 50V.
               │
  STEP 7 ── BIAS SUPPLY:
            Measure –Eg1 to chassis → within –10V.
               │
  STEP 8 ── RE-VERIFY HV: < 10V DC.
               │
  STEP 9 ── TEMPERATURE SENSORS:
            Both sensors above tube assembly must be near-horizontal.
            High voltage exists between sensors and BOTH tube anodes.
               │
  STEP 10 ── SAFE. Work may begin.
             Keep one hand behind back while probing.
             Never defeat microswitch while probing HV circuits.

Figure 1. DX-3SP safe discharge procedure. Note: both FU-728F anodes must be individually discharged; the safety choke between them does not guarantee cross-discharge between parallel anodes.

9.3 Safety Capacitors — X2 and Y2

The DX-3SP soft-start schematic (15.2.1) is documented in the DX-3SP manual and shows the same safety capacitor configuration as the DX-2SP. All safety capacitor replacement rules from the DX-2SP guide apply identically.

10. Cabinet, Front Panel & Assembly Hardware

The DX-3SP cabinet is the largest in the single-chassis Emtron range: 470 mm wide by 230 mm high by 470 mm deep — taller and deeper than the DX-2 and DX-3. The same baked enamel texture finish, 2 mm steel chassis, and 3 mm anodised aluminium front panel construction apply as in all DX models. The front panel retains the all-LED bargraph display (no analogue meter), band switch, Tune and Load controls, Power/STBY-OPR switches, and the standard five LED indicators.

The 400 V two-phase transformer option, confirmed in DX-3SP marketing (“A 400 Volt two-phase transformer is also available!!!”), requires a different primary wiring arrangement from the standard single-phase unit. A DX-3SP fitted with the 400 V two-phase transformer is not interchangeable with a standard 200–240 VAC mains supply; the primary connections must be verified before any power-on.

11. Parts Sources & Reference Documents

• Emtron DX-3SP Operating Manual (February 2009) — manualslib.com — Complete schematics: RF Module (15.2.3), RF Output Circuit (15.2.4), Soft-Start (15.2.1), HV Supply (15.2.2), QSK (15.2.5), Control Board (15.3), DX-2SP Block Diagram (15.1.1). The primary service reference for all DX-3SP work. Includes both the DX-2SP and DX-3SP block diagrams confirming both amplifier configurations.
• emtrondv.com — Dan — emtrondv.com — Version 7 AMPC control board (DX-3SP compatible); plate choke (stocked; specify DX-3SP); safety choke specification; free technical advice; refurbished Emtron amplifiers (Sydney pickup). Dan is the only known source for safety choke, coupling capacitor, and mains isolation relay specifications for the DX-3SP.
• DX-3SP Technical Specifications (Marketing) — cqdx.ru and cqdx.ru (second source) — Two-fan upper cooling confirmation; 35 A mains specification; 400 V two-phase transformer option; 50 kg weight.
• FU-728F Tube Sources (Current Production) — DX Engineering / Penta Labs (dxengineering.com); emtrondv.com (factory stock); RF Parts Co. (rfparts.com). Current production — no gettering required. Replace as a matched pair.
• 4CX1500B Eimac (FU-728F substitute) — RF Parts Co.; Penta Labs. Change filament tap: 9 V (FU-728F) → 6 V (4CX1500B); tap included on DX-3SP transformer. Both tubes must be the same type (do not mix FU-728F and 4CX1500B).
• Safety Capacitors (Y2 and X2) — Mouser (mouser.com), DigiKey (digikey.com). 4.7 nF/250 VAC Class Y2; 470 nF/250 VAC Class X2; 220 nF/275 VAC Class X2. Verify IEC 60384-14 certification.
• 3,300 pF / 10 kV RF Ceramic Capacitor — Vishay/Vitramon; ATC (American Technical Ceramics); CoilCraft; from specialist RF capacitor distributors. 10 kV voltage rating is mandatory; verify in datasheet before ordering. Do not source from general electronic components distributors without explicit datasheet confirmation of 10 kV rating.
• Jennings TJ1A-26S (QSK vacuum relay) — jenningsrelays.com; RF Parts Co. Standard QSK relay used in all DX-3SP units.