1. The DX-2SP in Context — Five Differentiating Features

Emtron’s marketing for the DX-2SP, distributed via Radioworld UK and the eHam.net product page, identifies the amplifier as follows: “Besides the tube and a very special high voltage, high current band switch, as well as a 4 KVA mains transformer, the DX-2SP is identical to our DX-2.” ¹ When the service literature is consulted alongside the marketing, five distinct differentiating features emerge:

1. 4 kVA mains transformer. The DX-2’s standard transformer is replaced by a 4 kVA unit capable of sustaining 2,000 W continuous output. This substantially increases the weight of the amplifier. The DX-3 manual states that the transformer installation “requires two people.” The same applies to the DX-2SP.
2. Heavy-duty “high voltage, high current” band switch. The 9-position ceramic band switch is a higher-specification part rated for the elevated RF tank voltages and higher circulating currents associated with 2,000 W operation.
3. Elevated plate supply voltage. Producing 2,000 W carrier from the same GU84B tube as the 1,500 W DX-2 requires a higher plate supply. Emtron’s own wording refers to the “high voltage” band switch in the context of the DX-2SP specifically, directly indicating a higher-than-DX-2 plate supply.
4. Horizontally-mounted AMPC control board. The Emtron DX-3 operating manual (April 2005) confirms: “Rotate clockwise for DX-1, DX-2 and anti-clockwise for DX-2SP and DX-3, which have horizontally installed boards.” ² This is the most critical service-differentiation point in the entire DX-2SP guide; getting this wrong will reduce protection sensitivity when the operator intends to increase it.
5. Dual-polarity regulated screen supply. The DX-2SP introduces a fully-regulated screen grid supply covering both positive and negative screen current conditions — a protection design unique to the SP family. The Emtron marketing states: “Fully voltage regulated for +ve and −ve screen currents, combined with very effective current limiting, not only totally protects the GU-84B tube, but also due to its wide dynamic range, keeps the tube static operation parameters absolutely stable.”³

2. DX-2SP Technical Specifications

3. DX-2SP vs DX-2 / DX-2A — Side-by-Side Comparison

4. Final Tube — GU84B (Early) and FU-728F (Later Production)

4.1 Early Production: GU84B (4CX2500A)

The original DX-2SP was built around the Svetlana GU84B, the same ceramic-metal tetrode used in the DX-2 and DX-2A. The GU84B was described in Emtron’s marketing as “produced for the military” and “physically and electrically exceptionally rugged”; its 2,500 W plate dissipation and rated operation up to 250 MHz make it a conservative choice at HF frequencies even at 2,000 W output levels.⁴ At the higher plate voltage used in the DX-2SP, the GU84B operates closer to its maximum plate dissipation than in the DX-2; thermal management and the two-speed blower are correspondingly more important. NOS GU84B tubes require the standard Emtron gettering procedure (12–16 hours heater-only operation before applying plate voltage). Current-production tubes from verified suppliers do not require gettering.

4.2 Later Production: FU-728F (Improved Chinese 4CX1500B)

A later production revision of the DX-2SP, DX-1SP, and DX-3SP substituted the FU-728F for the GU84B. A WorldwideDX forum post by an Emtron distributor confirmed: “The all new range of Emtron Amplifiers are now using the FU-728F tetrode and using the Eimac 4CX1500B base so you can still use the older 4CX1500B in the same amp. You have to change the voltage on the power supply but the tap is included also.” ⁵ The FU-728F is a Chinese military-grade tetrode with a 1,200 W plate dissipation rating (compared to the GU84B’s 2,500 W), a 9 V / 8.5–9 A heater, and the standard Eimac 4CX1500B octal socket. At the DX-2SP’s higher power levels, the FU-728F is used significantly above its conservative 1,200 W plate dissipation rating but within the tube’s actual operating envelope; DX-3SP marketing confirms the FU-728F “quite comfortably delivers 2000 W CW” and notes it is “in current factory production” ensuring supply availability.

5. The 4 kVA Mains Transformer — Installation & Service

The 4 kVA transformer is the single most physically significant difference between the DX-2SP and the DX-2 / DX-2A. It occupies the same chassis position as the standard DX-2 transformer but weighs considerably more; the DX-3 manual (for a comparable transformer installation) explicitly warns: “Two people are needed to install the transformer. The best is to put the DX-3 on a flat, smooth surface such as a bench or table top, covered by a small carpet or a folded blanket, so that it can be moved by sliding.” The DX-2SP transformer, rated at 4 kVA and providing the higher HV secondary for 2,000 W operation, should be treated identically.

The primary side of the 4 kVA transformer has four taps for 200, 220, 230, and 240 VAC. Correct tap selection is mandatory; under-tapping (connecting to 200 V when the supply is 240 V) will raise the HV secondary proportionally and elevate the already-higher-than-DX-2 plate supply to potentially dangerous levels. When relocating a DX-2SP to a different country or mains voltage regime, confirm the correct primary tap position before any power-on, following the transformer connection diagram on page 2 of the DX-2 operating manual (procedure applies to DX-2SP). This change must be performed by a qualified technician with all power disconnected.

6. Heavy-Duty High-Voltage / High-Current Band Switch

The DX-2SP’s 9-position ceramic band switch is a higher-specification part than the equivalent component in the DX-2 and DX-2A. Emtron’s own product text describes it as “a very special high voltage, high current band switch” — the emphasis on both voltage and current ratings reflects the demands of 2,000 W operation: higher tank RF voltages (from the elevated plate supply) and higher circulating tank currents (from higher RF output power) than the DX-2’s 1,500 W design point. At 28.5 MHz (10m), the combination of high RF voltage and tight capacitor settings places maximum stress on the switch contacts and ceramic wafer insulation.

7. High Voltage Power Supply — Elevated Voltage, Larger Filter Bank

The DX-2SP HV power supply provides a higher plate voltage than the DX-2’s 2,500 V DC, sustained by the 4 kVA transformer. The HV filter bank uses eight 470 µF / 500 V electrolytic capacitors in a series-parallel arrangement, providing 58 µF total at a 4,000 V DC working voltage — the same architecture described in the DX-1SP marketing for the SP-family supply, which uses identical components and layout at 1,200 W. In the DX-2SP, this bank is under higher continuous stress due to the combination of higher voltage and higher supply current drawn at 2,000 W output.

8. Soft-Start Module — BTA40-600B TRIAC & Safety Capacitors

The DX-2SP soft-start module uses a BTA40-600B TRIAC (40 A, 600 V) in a phase-controlled 3-second linear ramp circuit, documented in the DX-3SP operating manual (February 2009), which includes the complete DX-2SP soft-start schematic as a sub-diagram within the DX-3SP block diagram section.⁶ The 3-second ramp charges the HV filter bank electrolytics slowly and linearly — Emtron’s marketing emphasises this eliminates both transformer inrush and capacitor inrush, meaning “stress in components, failure and ‘transformer bong’ are non-existent.”

9. AMPC Control Board — Horizontal Orientation & Reverse Pot Direction

The AMPC control board in the DX-2SP is mounted horizontally rather than vertically as in the DX-1, DX-2, and DX-2A. This single physical difference reverses the effective rotation direction of all sensitivity potentiometers relative to the component orientation seen by the technician. The Emtron DX-3 operating manual confirms definitively: “Rotate clockwise for DX-1, DX-2 and anti-clockwise for DX-2SP and DX-3, which have horizontally installed boards — access on the component side.” ²

10. Dual-Polarity Screen Supply — SP-Family Exclusive Design

The DX-2SP’s screen grid supply is the most technically advanced in the DX-2 family. Emtron’s description of the design is worth quoting precisely: “Yet another unique new design, exclusive to the EMTRON DX-2SP amplifier is its screen grid power supply. Fully voltage regulated for +ve and −ve screen currents, combined with very effective current limiting, not only totally protects the GU-84B tube, but also due to its wide dynamic range, it keeps the tube static operation parameters absolutely stable. The protection against excessive screen current is so good, that we have tested the operation with full screen voltage, while the plate voltage was absent, for extended time, without damage to the tube.” ³

In conventional tetrode amplifier screen supplies, the screen takes positive current during normal RF drive and can draw negative (reversed) current during specific transient conditions such as high SWR events, overdrive, or plate voltage loss. A single-polarity screen supply (as used in the DX-2 and DX-2A) may not handle reversed screen current gracefully; the DX-2SP’s dual-polarity regulation absorbs both current directions with equal stability, preventing screen voltage excursions during fault conditions that could otherwise damage the tube.

11. Safety Systems — Interlocks, Discharge, & Safe Working Practice

11.1 Cover Safety Microswitch

The DX-2SP cover safety microswitch interrupts the mains supply when the top cover is removed. At the DX-2SP’s elevated plate voltage, this interlock is more critical than in the DX-2. The Emtron DX-3 manual warns (and the text applies equally to the DX-2SP): “Most of the following adjustments require the amplifier to be open and powered up. This also implies defeating the mains interlock safety switch, which is extremely dangerous since high voltage / high power DC and AC and RF voltages are exposed.” Defeat the safety microswitch only for the specific adjustment requiring it; restore it immediately upon completion. A defective microswitch must be replaced with an identical type before returning the amplifier to service.

11.2 RF Module Safety — Temperature Sensor Position

The two temperature sensors above the GU84B (or FU-728F) tube must remain in a near-horizontal position after any RF deck service. The Emtron DX-2 manual warns: “The sensors must stay in a position close to horizontal. Make sure you are not pushing them down, too close to the tube, as high voltage exists between them.” At the DX-2SP’s elevated plate voltage, the voltage between the sensor mounting points and the tube anode is higher than in the DX-2; this makes incorrect sensor positioning a more serious hazard.

11.3 Safe Discharge Procedure — DX-2SP (Elevated Voltage)

  STEP 1 ── OPR switch to STBY; POWER switch to OFF.
               │
  STEP 2 ── UNPLUG ALL REAR-PANEL LEADS immediately:
            Mains cord, antenna (RF IN / RF OUT), PTT/Key,
            ALC cable, cooling air duct if external.
               │
  STEP 3 ── Wait MINIMUM 10 MINUTES.
            DX-2SP HV filter bank (8× 470µF at elevated voltage)
            is larger and slower to discharge than DX-2.
               │
  STEP 4 ── ANODE DISCHARGE TOOL:
            10kΩ/25W resistor with insulated clip leads.
            Clip one end to chassis; contact other end
            firmly to GU84B (or FU-728F) anode cap.
            Hold for 15 seconds. Visible corona = still live.
               │
  STEP 5 ── PLATE VOLTAGE MEASUREMENT:
            Use 4,000V-rated DMM (Fluke 1000V + 10:1 probe
            is NOT adequate for DX-2SP elevated voltage).
            Confirm HV filter cap positive to chassis = <50V DC.
               │
  STEP 6 ── SCREEN SUPPLY (DUAL-POLARITY) MEASUREMENT:
            Measure EG2 (blue wire) to chassis → <50V.
            Also measure negative screen rail to chassis → >–50V.
            Both rails must be confirmed discharged.
               │
  STEP 7 ── BIAS SUPPLY MEASUREMENT:
            Measure –Eg1 (bias) to chassis → within –10V.
               │
  STEP 8 ── RE-VERIFY HV: confirm < 10V DC.
               │
  STEP 9 ── TEMPERATURE SENSOR CHECK:
            Verify both sensors above tube are near-horizontal.
            High voltage exists between sensors and tube anode.
               │
  STEP 10 ── SAFE: both rails < 10V DC. Work may begin.
            Keep one hand behind back while probing.
            Never defeat microswitch and apply mains simultaneously
            with internal HV probing in progress.

Figure 1. DX-2SP safe discharge and verification procedure — includes dual-polarity screen rail confirmation unique to SP-family amplifiers.

11.4 Mains Earth Connection

A wing-nut earth connection is provided at the rear of the DX-2SP. The DX-2 manual states: “Connect this first, before making any other connection to the amplifier.” At the DX-2SP’s higher power levels, RF and safety earth bonding is more critical than in the DX-2; use a low-impedance earth strap of at minimum 6 mm² cross-section and the shortest practical length. Earth bond must be confirmed with a milliohm meter; a high-resistance earth connection is particularly dangerous at this amplifier’s power and voltage level.

12. RF Sensor — Forward Power Calibration at 2,000 W

The RF sensor (bi-directional coupler in a small metal box at the RF output) must be calibrated at the DX-2SP’s 2,000 W reference. Both the DX-2 and DX-3 manuals include an identical calibration table: “1500 W for DX-2 / 2000 W for DX-2SP / 3000 W for DX-3. Adjust potentiometer RF F for correct display indication.”⁷ A Bird 43 (or equivalent calibrated directional wattmeter) is required for this adjustment. Perform calibration on 20m into a 50 Ω dummy load; confirm the RF sensor forward power bargraph matches the reference wattmeter reading at 2,000 W. An incorrectly calibrated RF sensor will allow the SWR protection to trigger at the wrong threshold, either failing to protect the tube at high SWR or interrupting output unnecessarily.

13. QSK Module — Optional; Jennings TJ1A-26S Vacuum Relay

The DX-2SP QSK module was priced at US$100 as a factory-installed option. The Jennings TJ1A-26S vacuum relay, documented in the DX-2 schematic (section 16.9, “QSK v2 DX-2”), provides the antenna T/R switching for CW and digital modes. The no-hot-switching sequencing (output relay switches first, then RF drive is applied) is the same in the DX-2SP as in all Emtron QSK-equipped models.

The Jennings TJ1A-26S is rated for the RF power levels in the DX-2SP. At 2,000 W, verify the relay’s coil voltage is correct (24–26 V DC per the part number) and that the RF contacts show no silver erosion. A TJ1A-26S with arc-eroded contacts will cause RF power loss and reflection anomalies at the 2,000 W level before the contacts fully fail. Replacement relays: Jennings Technology (jenningsrelays.com) and RF Parts Co.

14. Display Board — LM3914 Bargraph Array

The DX-2SP Display Board is identical in architecture to the DX-2: eight LM3914 dot-mode bargraph ICs providing Ip (plate current), Vp (plate voltage), forward power, reflected power, Ig2+ (positive screen current), and Ig2− (negative screen current) channels. The Ig2− channel is particularly important on the DX-2SP, reflecting the dual-polarity screen supply’s ability to absorb reversed screen current. A persistently elevated Ig2− bargraph at rest (without RF drive) indicates a problem in the negative screen rail regulation. The LM3914 ICs are standard parts available from Mouser and DigiKey; replace individually if a specific bargraph column fails to illuminate or shows wrong-direction indication.

15. Cabinet, Finish & Assembly Hardware

The DX-2SP cabinet is built to the same standard as all Emtron DX models: 2 mm steel chassis with dark yellow chromate coating, 3 mm anodised aluminium front panel, and baked enamel texture finish in dark grey. The DX-2SP marketing confirms: “The finish is of high quality durable baked enamel texture paint and silk screening of a quality not found on any other similar equipment.” Despite this shared construction, the DX-2SP cabinet is noticeably heavier than the DX-2 due to the 4 kVA transformer; the difference is apparent when lifting the amplifier.

16. Parts Sources & Reference Documents

• emtrondv.com — Dan (former Emtron technician) — emtrondv.com — Version 7 AMPC control board (DX-2SP compatible); plate choke 1" OD × 6" long (stocked); limited spare modules; free technical advice; refurbished DX amplifiers (Sydney pickup). The definitive human resource for all DX-2SP technical queries including horizontal-board adjustment direction confirmation and FU-728F substitution specifics.
• Emtron DX-2 Operating Manual — Service Procedures Reference — manualslib.com / usermanual.wiki — Contains all shared modular service procedures (AMPC adjustments, soft-start, display board, HV supply), the forward power calibration table (2,000 W for DX-2SP), and the RF sensor adjustment procedure.
• Emtron DX-3 Operating Manual — AMPC Horizontal Board Confirmation — manualslib.com — Definitive reference for the DX-2SP horizontal AMPC board and counter-clockwise pot direction. Supersedes the DX-2 manual for this specific adjustment parameter.
• Emtron DX-3SP Operating Manual — DX-2SP Soft-Start Schematic — manualslib.com — Section 15.1 (Block Diagrams) includes the DX-2SP block diagram and the soft-start schematic showing BTA40-600B TRIAC, 4.7 nF Y2 safety capacitors, and 30 A fuses. Essential schematic reference for soft-start module service.
• GU84B / FU-728F Tube Sources — GU84B (NOS): DL3JJ / QRO-Shop (qro-shop.com), RF Parts Co. (rfparts.com); FU-728F (current production): Emtron direct (contact via emtrondv.com), DX Engineering (dxengineering.com) stocks Penta Laboratories FU-728F.
• Safety Capacitor Sourcing (Y2 and X2) — Mouser (mouser.com) and DigiKey (digikey.com); search for Class Y2 4.7 nF/250 VAC and Class X2 470 nF/250 VAC by Kemet, Vishay, or Wima. Verify IEC 60384-14 certification on the datasheet before purchase.
• BTA40-600B TRIAC — STMicroelectronics TO-220AC; Mouser part 511-BTA40-600B or equivalent. Verify STMicroelectronics origin; counterfeit devices with BTA40 markings are prevalent from grey-market suppliers and may have significantly lower actual current ratings.
• Jennings TJ1A-26S (QSK vacuum relay) — jenningsrelays.com; RF Parts Co.; Surplus City Liquidators. The 24 V coil model is the TJ1A-26S; verify coil voltage before ordering.