1. The DX-1SP in the Emtron Production Sequence & SP Designation

The “SP” suffix in Emtron model names designates the Special Performance variant within each power class: a design sharing the chassis, tank circuit architecture, and modular electronics of the standard model, but using a larger tube at higher voltage to deliver substantially greater output power from the same physical footprint. The SP designation was applied across the DX-1SP, DX-2SP, and DX-3SP models.¹

For the DX-1SP, the transition from GU74B/4CX800A to FU-728F represented the most significant upgrade in the DX-1 series history. The FU-728F is a Chinese military-specification tetrode designed as a higher-power variant of the 4CX1500B design. It runs a 9 V / 8.5–9 A filament (oxide-coated, unipotential cathode) vs the GU74B’s 12.6 V / 3.5–4 A filament, has 50% greater plate dissipation (1,200 W vs 800 W), and supports a higher anode voltage ceiling (3,000 V vs 2,600 V nominal for GU74B applications).

The DX-1SP was FCC-approved for the North American market, making it one of the few Emtron models with documented US regulatory compliance. Its weight of 20 kg and compact desktop footprint were specifically marketed for DXpedition use — an application where the combination of legal-limit-adjacent output power, portability, and indefinite duty cycle capability provided significant operating advantages.

2. FU-728F vs Earlier DX-1 Tubes — Critical Service Differences

Every aspect of tube service for the DX-1SP differs from the DX-1A and DX-1D because the FU-728F is a different tube in terms of voltage, current, filament supply, and sourcing. The table below summarises the key parameters that affect service decisions:

3. Pre-Service Assessment

3.1 Initial Diagnostic Checklist

• Verify the tube type installed: the FU-728F and 4CX1500B are physically similar but require different filament taps. An incorrect filament voltage will either under-drive the cathode (too low: insufficient emission) or burn out the cathode (too high). Check the transformer filament tap setting before powering up an amplifier of unknown history.
• READY LED not activating: C7 (22 μF timer capacitor, tantalum preferred) or U5 (LMC555) on the AMPC board. Same failure mode as DX-1A and DX-1D. See Section 5.
• Low plate voltage (substantially below 2,800 V): Soft-start module fault; check TDA1085C Vcc at pin 9 (nominal ~15.6 V). A low reading indicates TDA1085C failure. Also verify the 40 A TRIAC is fully conducting in steady state (bypass relay RL1 should close after the 3-second soft-start period).
• Excessive idle plate current: EBS jumper may be missing (solder side of AMPC board). Check first before any bias adjustment.
• OCC LED illuminating at low drive levels: POT8 (overdrive threshold) may need adjustment; or the RF sensor calibration requires verification. The OCC senses <50 μA of G1 current; an out-of-calibration threshold can cause premature activation.
• No output despite correct voltages: Screen supply fault; measure EG2 pin (blue wire, control board). At ~350 V, the FU-728F screen voltage is higher than the GU74B’s ~230 V. Verify the TIPL760A screen regulator is producing correct output.
• EBS jumper pre-check: Locate the EBS jumper on the solder side of the AMPC board before any disassembly. It is a known item to dislodge during cover removal. Default position: EBS ON (jumper fitted).

3.2 Required Test Equipment

• High-voltage DMM, minimum 4,000 V DC range (the DX-1SP HV supply approaches 3,000 V; a 3,000 V range DMM provides insufficient margin)
• Standard DMM for low-voltage and component measurements
• RF wattmeter 1.5–30 MHz, 2,000 W range (Bird 43 or equivalent; the DX-1SP can deliver 1,500 W PEP which will damage a 1,000 W-rated wattmeter if not attenuated)
• 50Ω dummy load rated 1,200 W continuous for CW or RTTY testing
• Adjustable DC current source 0–2 A for IPTRIP adjustment via TP2
• Insulated HV discharge probe (10 kΩ / 50 W for the higher stored energy; chassis clip lead)

4. High Voltage Power Supply — Eight-Capacitor Bank

4.1 HV Supply Architecture

The DX-1SP power supply is a major upgrade from earlier DX-1 models, reflecting the higher plate voltage and current requirements of the FU-728F at 1,200 W CW output. The HV filter section uses eight 470 μF/500 V electrolytic capacitors connected in series. This configuration provides a total filter capacitance of 58 μF with a combined DC working voltage of 4,000 V — significantly more than the series capacitor strings in the DX-1D and providing adequate margin above the operating plate voltage.²

Emtron’s DX-1SP marketing documentation specifies: “The power supply contains a SOFT START INRUSH PROTECTION, a top grade heavy duty transformer, a full bridge rectifier and eight high voltage filter capacitors of 470 μF/500 volt rating. This gives a total filter capacitance in the high voltage section of 58 μF and a very conservative voltage rating of 4000 volts DC.”

The same 1 Ω precision sense resistor in the plate current return and the same resistive voltage divider for plate voltage metering are used as in earlier models, but the divider resistor values will be scaled for the higher operating voltage. The plate voltage metering (Vp) reading displayed on the AMPC board must be verified against a known HV measurement after any power supply service.

5. FU-728F Tetrode — Installation, Sourcing & Screen Protection

5.1 FU-728F Tube Architecture

The FU-728F is a coaxial-structure ceramic-metal forced-air-cooled tetrode designed primarily for 1.6 kW SSB final amplifier service and Class A/B linear amplification up to 110 MHz. It is the Chinese military equivalent of the Eimac 4CX1500B, sharing the same base configuration (which allows direct 4CX1500B substitution in the DX-1SP with only the filament tap change). The FU-728F is characterised by an oxide-coated indirectly heated cathode, a coaxial structure providing very low transfer capacitance (0.03 pF), and a minimum 180-second warmup requirement.³

Unlike the GU74B/4CX800A used in earlier DX-1 models, the FU-728F is available as a current-production component from multiple Chinese manufacturers and from distributors including Penta Laboratories (matched pairs burned in at full power for 48 hours before shipment), OM-Power (matched pairs), and DX Engineering. No gettering/conditioning procedure is required for new-production tubes, though a cautious operator will still benefit from a 3-minute filament-only warmup at reduced voltage on first installation before applying plate voltage.

5.2 Dual-Polarity Screen Current Protection

A significant engineering upgrade in the DX-1SP over earlier DX-1 models is the dual-polarity screen grid power supply. The GU74B in the DX-1A and DX-1D has a screen supply that protects against excessive positive screen current. The FU-728F and the DX-1SP’s higher-voltage screen supply introduces the possibility of negative screen current during certain operating conditions (particularly overdrive or mistuning), which can damage the tube if uncontrolled. Emtron’s DX-1SP marketing literature specifically highlights: “The screen grid power supply is fully voltage regulated for +ve and −ve screen currents. Very effective current limiting, not only totally protects the tube, but also due to its wide dynamic range, it keeps the tube static operating parameters absolutely stable.”⁴

In service, this means the screen supply circuit on the AMPC board is more complex than in a simple single-polarity design, and requires more careful diagnosis if the screen voltage is incorrect. The screen supply test point (pin EG2, blue wire, on the AMPC board) must show the correct ~350 V when the amplifier is in READY/OPR mode. Deviations from this voltage in either direction indicate a fault in the TIPL760A regulator circuit, its associated driving transistors, or the MOV protection devices in the screen circuit.

6. AMPC Control Board — Known Failures, EBS & SP-Specific Adjustments

6.1 AMPC Board in the DX-1SP Context

The DX-1SP uses the same AMPC control board family as all other post-1996 Emtron amplifiers, installed vertically (like the DX-1D and DX-2). The Version 7 board from Dan at emtrondv.com is compatible. The same known failures (C7 timer capacitor, U5 LMC555) apply. The critical orientation note from the DX-1D guide also applies here: the DX-1SP has a vertically-mounted AMPC board, which means clockwise rotation of POT3 (BIAS) reduces plate current — the same direction as in the DX-1D.

6.2 Overdrive Control Circuit (OCC) — DX-1SP Calibration

The OCC triggers an overdrive warning LED when less than 50 μA of G1 (control grid) current begins to flow — the point at which the FU-728F begins to depart from Class AB1 operation and grid conduction begins. If drive continues to increase beyond this point, the OCC activates a 2-second bypass protection period. This threshold is non-adjustable by operator control. The DX-1SP manual states: “This protection is not adjustable — you must not exceed the limit. If this happens too often, you obviously do not have a fine control over the transmit level of your transceiver.”⁵

7. Soft-Start Module — 40 A TRIAC & Safety Capacitors

The DX-1SP soft-start module uses the same TDA1085C gate controller and the same 3-second linear ramp-up architecture as the DX-1D, but with a 40 A TRIAC to handle the larger transformer primary current of the higher-power supply. The 3-second ramp charges all eight HV filter capacitors linearly, avoiding transformer inrush current and mechanical stress. The filament circuit also benefits from the gradual ramp: oxide-coated cathodes of the FU-728F type are sensitive to large thermal shock during cold start, and the gradual voltage ramp extends cathode service life.

8. Safety: Interlock, HV Discharge & AC Line

8.1 Cover Safety Microswitch

The DX-1SP cover safety microswitch interrupts the mains supply when the cover is removed. This is the primary protection against accidental energisation with the cover off. Verify the switch at every service: with the cover removed, the switch contacts must be open (no mains continuity). A failed-closed switch allows the amplifier to be powered up with the cover off, exposing lethal voltages on the eight-capacitor HV bank, the transformer secondary, and all internal circuit nodes. Replace a failed switch before returning to service.

8.2 Safe Discharge Procedure

  STEP 1 ── STBY switch to STBY; POWER to OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS.
            Mains cord, all antenna/RF cables, PTT/key, ALC.
               │
  STEP 3 ── Wait minimum 5 minutes.
            8 x 470µF/500V capacitors store significantly
            more energy than earlier DX-1 models.
               │
  STEP 4 ── ANODE DISCHARGE:
            Insulated probe with 10kΩ/50W series resistor (note
            higher wattage for greater stored energy) clipped
            to chassis. Apply to tube anode contact.
            Hold 15 seconds.
               │
  STEP 5 ── HV MEASUREMENT:
            4000V-rated DMM: probe (+) to HV filter capacitor
            bank positive rail, probe (-) to chassis.
            CONFIRM < 50V DC before proceeding.
               │
  STEP 6 ── SCREEN SUPPLY (EG2) CHECK:
            Measure EG2 pin (blue wire, AMPC board) to chassis.
            DX-1SP screen supply is ~350V (higher than DX-1D).
            Confirm < 50V before touching any screen circuit.
               │
  STEP 7 ── BIAS SUPPLY CHECK:
            Measure bias supply (negative; typically -80 to
            -110V for FU-728F) to chassis.
            Confirm < 10V absolute value.
               │
  STEP 8 ── RE-VERIFY HV BANK: < 10V.
            Note: Dielectric absorption causes voltage recovery.
            Re-verify before each session.
               │
  STEP 9 ── All supplies discharged. Safe to work internally.
            ─────────────────────────────────────────────────
            Reminder: 8-capacitor bank has more stored energy
            than typical single-stage HV supplies. Use a 50W
            discharge resistor; a 25W unit may overheat on the
            first discharge of a fully-charged bank.

Figure 1. DX-1SP safe discharge procedure; note the higher-wattage discharge probe required for the eight-capacitor bank.

8.3 RF Power Safety at 1,500 W PEP

At 1,500 W PEP, the DX-1SP produces RF power levels comparable to the legal limit in most jurisdictions. The RF peak voltage at the antenna connector at 1,500 W into 50 Ω is approximately 390 V peak — sufficient to cause deep RF burns even from momentary contact with a live coaxial conductor. All RF connections must be made in STBY mode. Never open a live coaxial connector, change antenna connections, or handle coaxial cables while the amplifier is in OPR mode.

9. RF Deck — Tank Circuit, Band Switch & Plate Bypass

The RF deck of the DX-1SP shares the same dual-tank-coil architecture (40–160 m wound coil on ceramic bobbin; 10–30 m silver-plated copper tube coil) and 9-position ceramic band switch as the DX-1D. All RF section service considerations from the DX-1D guide apply — with the critical caveat that the higher output power (1,200 W vs 750 W) generates proportionally higher RF voltages across the tank components. The 1,000 pF/6 kV plate bypass capacitors (4× per the RF module schematic) are under higher stress at the DX-1SP’s higher operating voltage and power level; they should be replaced as a preventive measure on any DX-1SP that has operated for an extended period or has experienced a plate arc event.

10. Cabinet & Assembly Hardware

The DX-1SP cabinet uses the same 2 mm steel chassis with dark yellow chromate coating, 3 mm anodised aluminium front panel, and baked enamel texture finish as the DX-1D. The assembly hardware, RF connectors (SO-239 standard), earth terminal (wing-nut type), OPR/STBY switch, band switch knob, and 6:1 reduction drive mechanisms are identical in service to those documented in the DX-1D guide. The key DX-1SP distinction is the front-panel label silk-screening for the DX-1SP model designation and the different metering range indicators reflecting the higher power output.

11. Parts Sources & Reference Documents

• emtrondv.com — Dan (former Emtron technician) — emtrondv.com — Version 7 control board (limited qty); plate choke 1" OD / 6" long (stocked); free technical advice; refurbished DX amplifiers (pickup Sydney). Primary service resource for DX-1SP.
• FU-728F Tube — Current Production Sources
  Penta Laboratories via DX Engineering: dxengineering.com/parts/plb-fu-728f (48-hour burn-in; matched pairs available)
  OM-Power Shop: shop.om-power.com (matched pairs for OM-Power and Emtron applications)
  EECTECH: eectech.store (individual; 1-year warranty)
• Eimac 4CX1500B (alternative tube) — rfparts.com; Alpha RF Systems P/N VTX-X120. Remember: change filament transformer tap from 9 V to 6 V when substituting Eimac 4CX1500B in the DX-1SP.
• Emtron DX-1SP Product Description & FCC Approval — qrznow.com — marketing specification sheet; primary published reference for DX-1SP power supply details (8-cap HV bank, 40A TRIAC, dual-polarity screen protection).
• Emtron DX-1D Operating Manual (April 2003) — manualslib.com — The closest publicly available manual to the DX-1SP architecture; all schematic sections (control board, soft-start, QSK, RF module) are directly applicable.
• Emtron DX-1B FCC Filing (AMPC v.3 schematic) — fccid.io/Q8VDX1B — Complete AMPC v.3E control board schematic and component legend; fully applicable to DX-1SP AMPC board.
• Emtron DX-3SP Operating Manual (February 2009) — manualslib.com — Documents B+ fault resistor (25 Ω/50 W), MOV screen circuit protection, and FU-728F RF module circuit; the SP-series architecture reference.
• FU-728F Datasheet — ok1rr.com/tubes/FU-728F.pdf — Complete FU-728F electrical specifications: 1,200 W plate dissipation, 9 V / 8.5–9 A filament, 3,000 V max anode, 350 V G2, −78.5 V bias, 3,000 l/min cooling.
• Mouser / DigiKey — mouser.com / digikey.com — Class Y2 and X2 safety capacitors; LMC555; TIPL760A equivalents; 7805; BC547; BTA40-600B TRIAC; 8× 470 μF/500 V HV electrolytics; M4-12H PCB relays.
• SP5BTB DX-1B Field Service Log — qsl.net/sp5btb/dx1b.html — C7/U5 timer diagnosis; IPTRIP adjustment via TP2; EBS jumper field-loss incident; TDA1085C repair procedure. Directly applicable to DX-1SP AMPC and soft-start boards.