1. DX-2 in the Emtron Product Line & Distinctions from the DX-1 Series

The DX-2 occupies the middle tier of the Emtron single-tube lineup: above the 750 W DX-1 series and below the DX-3 (3,000 W). The DX-2’s single GU84B tube is a substantial step up from the GU74B used in DX-1 models, reflecting the fundamental engineering challenge of delivering 1,500 W from a desktop amplifier: the GU74B can dissipate 800 W; the GU84B can dissipate 2,500 W, giving a 3:1 margin over the DX-1D at the same output power level.

The DX-2’s band coverage of seven bands (160–15 m) is a key service consideration. The omission of 12 m and 10 m was an engineering decision: at 1,500 W output into the higher HF bands, the GU84B’s larger inter-electrode capacitances and the physically large tank circuit required for 2,500 W tube operation create challenges at 21 MHz and above. The later DX-2SP solved this with an upgraded tank circuit, but the original DX-2 design was specifically rated for 15 m as the highest band.¹

2. Pre-Service Assessment

2.1 Mains Voltage Requirement

The DX-2 manual contains a specific warning not found in any DX-1 documentation: “The DX-2 should not be connected to a mains supply of less than 200 VAC.” The DX-2’s larger transformer and higher power requirements make it incompatible with 100–120 VAC operation without transformer modification — unlike the DX-1 series which supports 100/120 VAC with derating. The transformer supports 200, 220, 230, and 240 VAC configurations; verify the correct tap is connected before any power-on on a used DX-2.

2.2 Initial Diagnostic Checklist

• READY LED not activating: C7 (22 μF tantalum) timer capacitor failure is the primary suspect; same diagnosis as all DX-1 models.
• Low plate voltage: TDA1085C soft-start controller failure (measure Vcc at pin 9; expect ~15.6 V); DX-2 soft-start is a 5-second ramp (vs DX-1D’s 3 seconds).
• Blower operating at only one speed: temperature sensor or two-speed motor control circuit fault. Verify both temperature sensors are intact and positioned correctly (near horizontal).
• Excessive idle plate current: EBS jumper loss on solder side of AMPC board; check before any bias adjustment.
• No output despite correct plate voltage: Screen supply failure; measure at EG2 (blue wire). The DX-2 adds an additional SCREEN VOLTAGE adjustment (POT2) not present in DX-1D; verify screen voltage is within specification before tube investigation.
• Ig2+ or Ig2− bargraph showing constantly high reading: Screen current limit circuit fault on AMPC board; verify POT1/IG2LIMIT setting.
• Power output dropping steadily over time: GU84B cathode emission fatigue. The manual notes tube replacement is very unlikely, but long-service tubes do age. The GU84B has a designed life of 1,500 hours.

2.3 Required Test Equipment

• High-voltage DMM, minimum 4,000 V DC range
• Standard DMM for low-voltage, resistance, and diode measurements
• RF wattmeter, 1.5–22 MHz, 2,000 W range (Bird 43 or equivalent)
• 50 Ω dummy load rated 1,500 W continuous
• Adjustable DC current source, 0–2 A, for IPTRIP adjustment via TP2
• Insulated HV discharge probe (10 kΩ / 25 W; chassis clip lead)

3. GU84B (4CX2500A) Tetrode — Service & Heater Circuit

3.1 GU84B Tube Characteristics

The GU84B is a Svetlana (St. Petersburg, Russia) ceramic-metal forced-air-cooled tetrode designed primarily for submarine and military radio transmitters. Emtron described it as “constructed primarily for military communication, this tube is physically and electrically exceptionally rugged.” Its 2,500 W plate dissipation and exceptional mechanical robustness made it ideal for the DX-2’s 1,500 W output requirements, operating with a comfortable margin at approximately 60% of maximum plate dissipation in normal Class AB1 service.

The GU84B’s most distinctive service characteristic, compared to the GU74B in DX-1 models, is its 27 V heater (nominal), drawing 3.7 A. This is an unusual heater voltage that requires a dedicated secondary winding on the DX-2 transformer. An incorrect heater voltage will either damage the cathode (too high) or reduce emission (too low) with potential cathode contamination over time from under-temperature operation.²

3.2 NOS GU84B Conditioning (Gettering)

Replacement GU84B tubes are available primarily as new-old-stock (NOS) from Russian military surplus, from Svetlana factory stocks held in European warehouses. Like the GU74B, a NOS GU84B requires gettering before plate voltage is applied: a minimum of 8–15 hours with heater voltage only (no G1, no G2, no plate) to drive out adsorbed gas molecules. The GU84B’s larger physical mass and greater anode surface area means the gettering process may take longer than for the GU74B; conservative practice is to run the heater at full rated voltage for 12–16 hours before progressive application of plate voltage.

4. RF Deck — Ferrite Toroid L-Coil & Pi-Network

4.1 Ferrite Toroid “L” Coil — Unique to DX-2 and DX-4

The most distinctive RF circuit feature of the DX-2 (shared with the DX-4 but absent in the DX-1 series and DX-3) is the use of a ferrite toroid as the low-band “L” coil in the pi-L output network. The DX-2 manual states: “Next to the switch is the ‘Pi’ coil. The ‘L’ coil is wound on a ferrite torroid.” The pi-L network uses the Pi coil (silver-plated copper tube) for the primary tank inductor and the ferrite toroid L-coil as the low-impedance secondary element providing the final 50 Ω output impedance transformation at the lower frequency bands (approximately 40–160 m).³

The ferrite toroid coil is a high-Q, compact inductor that allows the pi-L network to maintain good loaded-Q even at 160 m where the required output inductance is very large. At higher frequencies (15–20 m), the L-coil may be effectively bypassed or its contribution minimised by the band switch configuration. Service considerations for the ferrite toroid are distinct from service of wound wire coils on ceramic bobbins (as used in the DX-1D) because ferrite cores are susceptible to cracking from thermal cycling and can be permanently demagnetised by temperature extremes or mechanical shock.

5. High Voltage Power Supply — 2,500 V Architecture

5.1 Power Supply Architecture

The DX-2 HV power supply follows the same architecture as the DX-1D — transformer, full-wave bridge rectifier, and series-string filter capacitor bank — but the larger GU84B tube and higher plate voltage require a more capable transformer and a larger HV filter bank. The DX-2 manual describes the transformer as “a high performance type, designed to reduce the size and weight” while being built “with a generous reserve of power, handling easily the DX-2 requirements in continuous operation.”

The plate voltage metering divider uses the same 1 Ω plate current sense resistor architecture as the DX-1D: 1 V at TP2 = 1 A plate current. The IPTRIP protection (POT6) and associated adjustment procedure are identical to the DX-1D guide. The screen voltage for the GU84B is significantly higher than for the GU74B; the AMPC board’s TIPL760A screen regulator must sustain the higher screen voltage demanded by the GU84B’s rated G2 operating point.

6. AMPC Control Board — DX-2-Specific Features & Adjustments

6.1 Screen Voltage Adjustment (POT2) — DX-2-Specific

The DX-2 AMPC control board includes a SCREEN VOLTAGE adjustment (POT2, marked SCREEN) that is not documented as a required adjustment in the DX-1D service procedures. This adjustment sets the regulated G2 screen voltage to the GU84B’s correct operating point. The GU84B datasheet specifies a maximum G2 voltage of 400 V; the DX-2 screen supply is set to the Emtron-specified operating voltage which falls within this maximum.

The screen voltage is measurable at pin EG2 (blue wire) on the AMPC board when the amplifier is in READY/OPR state. After any tube replacement or control board replacement, POT2 must be adjusted to the correct GU84B screen voltage. The DX-2 manual’s service section (Section 15.1.1.5) provides the full adjustment procedure using the EG2 test point.

7. Display Board — Ig2+/Ig2− Screen Current Metering

The DX-2 Display Board uses the same LM3914 bargraph IC architecture as the DX-1D, but adds two additional metering channels not present in the DX-1D display: Ig2+ (positive screen current) and Ig2− (negative screen current). These channels are driven from the X3-8 and X3-9 connector pins (confirmed from the DX-2 Display Board schematic). The ability to see negative screen current is directly relevant to the GU84B’s operating characteristics: at high output power, the GU84B can exhibit negative screen current (the screen grid captures some of the electron flow back toward the cathode), which is a normal operating condition for high-power tetrodes but must be monitored to ensure it remains within safe limits.⁴

Display Board service follows the same procedure as documented in the DX-1D guide: LM3914 IC replacement for failed bargraph channels (still in production from TI/ON Semi), standard bargraph LED module replacement, and Display Board calibration from the DX-2 manual Appendix 3. Calibration of the Ig2+ and Ig2− channels requires a calibrated milliammeter and the DX-2 adjustment procedure; these adjustments should be performed when the tube is replaced.

8. Soft-Start Module — 5-Second Ramp & Safety Capacitors

The DX-2 soft-start module uses the same TDA1085C gate controller and TRIAC architecture as the DX-1D, but with a longer 5-second ramp (vs the DX-1D’s 3 seconds) reflecting the larger transformer and higher filter capacitance of the DX-2 power supply. The manual notes: “The soft start system will take about 5 seconds to fully turn the power on.” The SWR LED briefly lights during this ramp-up as the supply reaches operating voltage.

The DX-2 soft-start schematic (from the DX-2 manual Appendix, referenced in the FCC filing) shows a more elaborate soft-start circuit than the DX-1B version, with additional filtering components (470 kΩ bleeder resistors, 220 μF/25 V electrolytics, 47 nF and 3.3 μF capacitors in the gate drive circuit) to provide a smoother phase ramp over the longer 5-second period. The safety capacitors (Class X2 and Y2) are the same specification as all DX-series amplifiers.

9. Safety: Interlock, HV Discharge & Mains

9.1 Mains Not Less Than 200 VAC

Unlike the DX-1 series (which supports 100–240 VAC), the DX-2 must be connected to a mains supply of not less than 200 VAC. This is a specific technical limitation documented in the DX-2 manual and should be verified for any DX-2 that is to be operated outside Australia or Europe. Operation below 200 VAC will reduce plate voltage below 2,500 V, decrease output power, and may stress the transformer secondary by requiring higher primary current to maintain power.

9.2 Safe Discharge Procedure

  STEP 1 ── STBY switch to STBY; POWER to OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS.
            Mains cord, antenna cables, PTT/key, ALC.
               │
  STEP 3 ── Wait minimum 5 minutes.
            GU84B has larger thermal mass than GU74B;
            allow additional cooling time for the tube.
               │
  STEP 4 ── ANODE DISCHARGE:
            Insulated probe with 10kΩ/25W series resistor
            clipped to chassis. Apply to GU84B anode contact.
            Hold 10 seconds.
               │
  STEP 5 ── HV MEASUREMENT:
            4000V-rated DMM: probe (+) to HV filter cap positive,
            probe (-) to chassis. CONFIRM < 50V DC.
               │
  STEP 6 ── SCREEN SUPPLY CHECK:
            Measure EG2 (blue wire, AMPC board) to chassis.
            GU84B screen voltage is higher than GU74B models.
            Confirm < 50V.
               │
  STEP 7 ── BIAS SUPPLY CHECK:
            Measure negative bias supply to chassis.
            Confirm < 10V absolute value.
               │
  STEP 8 ── TWO-SENSOR AREA PRECAUTION:
            After accessing the tube area, verify both
            temperature sensors above the tube are repositioned
            near-horizontal before next power-up.
            HIGH VOLTAGE EXISTS BETWEEN SENSORS AND TUBE ANODE.
               │
  STEP 9 ── Re-verify HV bank: < 10V.
               │
  STEP 10 ── Now safe to work internally.

Figure 1. DX-2 safe discharge procedure, including the two-sensor repositioning reminder unique to the DX-2.

9.3 Cover Safety Microswitch

The cover safety microswitch on the DX-2 interrupts the mains supply when the cover is removed. Verify it operates correctly at every service. The DX-2 manual confirms this interlock is present and is described in the same terms as all Emtron DX amplifiers: defeating it for live adjustments is extremely dangerous.

10. Cabinet & Assembly Hardware

The DX-2 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-1 series. The front panel is adapted for 7-band operation and includes the larger dual-speed blower ventilation requirements. Cabinet service follows the same procedures documented in the DX-1D guide, with the additional note that the DX-2 transformer is heavier than the DX-1D transformer; when repositioning the amplifier for bench service, use appropriate lifting technique for the increased weight.

11. Factory Initial Capacitor Settings Reference (7 Bands)

The DX-2 manual provides a factory-inscribed table of initial Plate and Load capacitor settings on page 2, unique to each serial number (measured during factory test and recorded by the technician). The bands covered and their approximate typical settings into a 50 Ω dummy load are noted below for reference; the factory-inscribed values on the specific unit’s title page are the authoritative reference. Note the absence of 12m and 10m from the DX-2 coverage.

Use only the factory-inscribed values from the serial-number-specific DX-2 title page as the primary reference. The above table provides directional guidance only.

12. Parts Sources & Reference Documents

• emtrondv.com — Dan (former Emtron technician) — emtrondv.com — Version 7 control board (limited; compatible); plate choke 1" OD / 6" long (stocked); free technical advice; refurbished DX amplifiers (pickup Sydney).
• Emtron DX-2 Operating Manual — manualslib.com — Full service manual including all schematics (HV supply, control board, soft-start, QSK, RF module, display board), adjustment procedures, and internal component layout.
• Emtron DX-2 FCC Filing (FCC ID QYNDX-2) — fccid.io/QYNDX-2 — US FCC filing confirming DX-2 compliance and providing supplementary schematic detail.
• GU84B Datasheet (OK1RR) — ok1rr.com/tubes/gu84b.pdf — Key parameters: 2,500 W plate dissipation; 27 V / 3.7 A heater; 2,200 V max CW anode voltage; 400 V max G2; 3-minute warm-up; 96 m³/h cooling.
• GU84B Tube Sources (NOS) — DL3JJ / QRO-Shop (qro-shop.com); RF Parts Co. (rfparts.com); Soviet-Tubes.com. Mandatory NOS gettering (12–16 hours heater-only) before plate voltage. Verify heater voltage (27 V) at tube socket before installation.
• Mouser / DigiKey — mouser.com / digikey.com — Class Y2 and X2 safety capacitors; LMC555; TIPL760A equivalents; 7805; BC547; 275 V MOV devices; 1,000 pF/6 kV disc ceramics; LM3914 bar-graph ICs.
• Jennings Technology (QSK vacuum relay) — jenningsrelays.com; RF Parts Co. — Jennings TJ1A-26S as used in all Emtron QSK modules.
• SP5BTB DX-1B Field Service Log — qsl.net/sp5btb/dx1b.html — C7/U5 diagnosis; TDA1085C repair; IPTRIP adjustment. All procedures apply to DX-2.