1. The DX-3 in the Emtron Product Line — Top of the Single-Tube Range

The Emtron DX-3 (April 2005 operating manual) is the most powerful single-tube HF linear amplifier in the Emtron production line, exceeding the DX-2SP’s 2,000 W by a full kilowatt of continuous carrier power. Its 3,000 W carrier / 4,000 W PEP output is achieved with a single GU78B (4CX3000A) tetrode in the same desktop cabinet form factor as the DX-2 series. The DX-3’s internal view (Figure in Section 1.1 of the manual) shows the familiar Emtron layout — blower, QSK module, temperature sensor, RF sensor, RF choke, AMPC board, soft-start terminal block, HV power supply, safety microswitch, tank coil pair, and front-panel controls — but the scale of each component is noticeably larger than in the DX-2.²

Like all DX-3 amplifiers, the DX-3 includes QSK as standard equipment — not an option as in the DX-2 family. The manual states: “QSK is standard in all DX-3 amplifiers.” The EBS standing current is 0.75 A, higher than in the DX-2 / DX-2A (typically 0.5 A) reflecting the GU78B’s higher transconductance and different Class AB1 operating point.

2. DX-3 vs DX-2 Family — Five Engineering Differences

3. DX-3 Technical Specifications

4. GU78B Tetrode (4CX3000A) — Service Notes

The GU78B is a Soviet-origin ceramic-metal tetrode with a 2,500 W plate dissipation rating, primarily designed for military SSB and distributed-amplifier service up to 250 MHz. Its Western equivalent is the Eimac 4CX3000A. The GU78B is a significantly larger tube than the GU84B used in the DX-2 series: 111 mm maximum diameter versus the GU84B’s smaller body, and the GU78B uses an indirectly-heated oxide-coated cathode with a 4-minute (240-second) minimum warm-up time — one minute longer than the DX-2’s 3-minute requirement.³ The tube’s higher transconductance (40–80 mA/V, versus the GU84B’s lower value) enables the DX-3’s higher gain and power output with the same basic circuit architecture.

4.1 GU78B Bias Target & AMPC Adjustment

The DX-3’s bias target is set by the AMPC board POT3 (BIAS). Following the DX-3 manual Section 17.1.2 BIAS ADJUSTMENT procedure, the target is established during initial power-up with the EBS jumper removed. On a new or serviced DX-3, allow the GU78B minimum 4 minutes of heater warm-up before applying plate voltage; applying plate voltage to a cold GU78B risks cathode damage and significantly shortened tube life. The DX-3’s EBS standing current is 0.75 A (versus 0.5 A typical in DX-2 family), which must be confirmed after any bias adjustment.

4.2 GU78B Sourcing & NOS Gettering

The GU78B is available NOS (New Old Stock) from Eastern European military surplus suppliers and specialist tube dealers. NOS tubes should be gettered with heater-only operation (no plate or screen voltage) for a minimum of 12–16 hours before first applying plate voltage; the larger size of the GU78B relative to the GU84B means residual outgassing from the glass-to-ceramic seals may be marginally higher. Current-production GU78B or 4CX3000A from Penta Laboratories does not require gettering.

5. Dual-Fan Cooling — The DX-3’s Unique Architecture

The DX-3 is the only model in the single-tube Emtron range that uses a two-fan cooling architecture. The DX-3 internal view diagram (manual Section 1.1) shows both the turbine blower and the temperature sensor fan position. The manual describes: the turbine blower is positioned below the chassis, pushing air upward through the GU78B’s cooling fins from below; a computer-type fan is positioned above the tube, sucking the hot exhaust air away from the tube body. Both fans operate at two speeds, controlled by the temperature sensor system.

This dual-fan arrangement is necessary because the GU78B’s 2,500 W plate dissipation at 3,000 W output generates thermal energy that a single turbine blower cannot exhaust fast enough for sustained operation at full power. The DX-3 manual states its 3,000 W carrier power output is supported for continuous commercial-type duty cycles. Any DX-3 that has had the upper computer-type fan removed, disconnected, or its duct obstructed will operate in a degraded thermal condition that shortens GU78B life and risks thermal cut-out activation during sustained operation.

6. RF Deck — GU78B RF Module & 700 µH Plate Choke

6.1 RF Module Schematic Notes — “R1 Changed to 680 Ω”

The DX-3 RF module schematic (Section 15.8 of the DX-3 manual, ManualsLib page 30) carries an important service note at its heading: “R1 changed to 680 Ω.” This indicates a documented production change to the RF module circuit during the DX-3’s production life. R1 is a resistor in the RF module whose original value was changed to 680 Ω during production; the note is a service advisory that any DX-3 entering service should have R1 at 680 Ω. If a DX-3 was manufactured before this change and has not been updated, or if R1 has been replaced with the original value during a past repair, the RF module is not in the current specification.⁴

6.2 700 µH Plate Choke

The DX-3 RF module schematic (15.8) documents a 700 µH plate choke. This is a notably higher inductance than the plate choke in the DX-1 and DX-2 series, which is specified at approximately 700 µH in the DX-3 manual. Dan at emtrondv.com stocks the plate choke (1” OD × 6” long); when ordering for a DX-3, verify the correct inductance specification. The choke must provide adequate impedance at 160 m (1.8 MHz) while not presenting a series resonance on any of the nine operating bands. A damaged or incorrectly valued plate choke in the DX-3 is more consequential than in the DX-2 due to the higher plate voltage and operating current.

7. AMPC Control Board — Horizontal Orientation, Counter-Clockwise Pots

The DX-3’s AMPC control board is mounted horizontally, as confirmed by the DX-3 operating manual (April 2005), Appendix 3: Adjustments: “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.” ⁵ The DX-3 and DX-2SP are the only single-tube Emtron amplifiers with this horizontal orientation. The pot direction is identical to the DX-2SP: counter-clockwise increases protection sensitivity (tighter trip threshold).

8. QSK Version 3 Module — Two Relay Options

The DX-3 uses the QSKv3 module, documented in the DX-3 manual as schematic 15.7 (QSKv3 DX-3, document number QSKV3DX3, dated Monday 7 January 2002). The QSKv3 differs from the DX-2’s QSKv2 in one specifically documented circuit difference: the DX-3 QSK schematic identifies two relay options: the Jennings TJ1A-26S (used in all earlier Emtron QSK modules) or the P1D-1V (a different vacuum relay type). The QSKv3 schematic shows both relay types as alternatives.⁶

The P1D-1V is a single-pole double-throw (SPDT) vacuum relay. If a DX-3 has been fitted with a P1D-1V rather than the Jennings TJ1A-26S, obtain the P1D-1V coil voltage specification before ordering a replacement; the relay designations indicate different coil voltages may exist in the P1D family. The QSKv3 circuit uses the same coil drive components (R3 = 100/1 W, R1 = 33/2 W, C1 = 47 µF/50 V, C2 = 10 µF/50 V, 1N4004 diodes throughout).

9. HV Power Supply & Transformer

The DX-3’s transformer ships in a separate carton from the RF deck. The DX-3 manual Section 3.3 and Section 4.2 both address this. The transformer weighs 25 kg and the manual 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 of a folded blanket, so that it can be moved by sliding.” At 30 A from the 240 VAC mains during 3,000 W operation, the transformer primaries must be wired for the correct mains voltage (200, 220, 230, or 240 VAC) before first power-on; an incorrect primary tap risks overloading the HV secondary.

10. Safety Systems — Cover Interlock, Discharge & Safety Capacitors

10.1 Cover Safety Microswitch

The DX-3 cover safety microswitch is the primary electrical interlock. The DX-3 manual Appendix 3 notes: “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.” The microswitch must open (break mains continuity) when the cover is removed. Verify after every cover-related service action. Never return a DX-3 to operational service with a defeated or failed microswitch.

10.2 Safe Discharge Procedure

  STEP 1 ── STBY switch; POWER switch to OFF.
               │
  STEP 2 ── DISCONNECT ALL REAR-PANEL LEADS:
            Mains cord, RF IN, RF OUT, PTT/Key,
            ALC cable. Any other control cables.
               │
  STEP 3 ── Wait MINIMUM 5 MINUTES.
            DX-3 HV filter bank at elevated plate voltage.
            Larger bank than DX-2; allow full discharge time.
               │
  STEP 4 ── ANODE DISCHARGE TOOL:
            10kΩ/25W resistor on insulated clip lead.
            Clip one end to chassis.
            Contact firmly to GU78B anode cap.
            Hold for 15 seconds. Watch for spark = still live.
               │
  STEP 5 ── HV MEASUREMENT (5kV-rated DMM preferred):
            Confirm HV filter cap positive to chassis < 50V DC.
            Note: DX-3 plate voltage may exceed 3,000V DC;
            ensure meter is rated for actual voltage present.
               │
  STEP 6 ── SCREEN SUPPLY:
            Measure EG2 (blue wire) or either side of R18
            (in DX-3, R18 is the easier measurement point)
            to chassis → < 50V.
               │
  STEP 7 ── BIAS SUPPLY:
            Measure –Eg1 to chassis → within –10V.
               │
  STEP 8 ── RE-VERIFY HV: < 10V DC.
               │
  STEP 9 ── TEMPERATURE SENSORS:
            Verify both sensors above GU78B are near-horizontal.
               │
  STEP 10 ── SAFE. Work may begin.
             Keep one hand behind back while probing.
             Never defeat microswitch and apply mains
             while simultaneously probing HV circuits.

Figure 1. DX-3 safe discharge and verification procedure. Note R18 as the preferred EG2 screen voltage measurement point in the DX-3.

10.3 Safety Capacitors — X2 and Y2 in Soft-Start Module

The DX-3 soft-start schematic (Section 15.6) is identical to the DX-2’s and shows the same safety capacitor requirements. All three capacitor classes are confirmed in the DX-3 schematic.

11. Troubleshooting Reference — DX-3 Fault Table

The DX-3 manual (Section 12) provides a comprehensive troubleshooting guide. The following table summarises key fault symptoms from the manual with the most probable cause and first-action service response for each:

12. Cabinet, Front Panel & Assembly Hardware

The DX-3 cabinet is built to the same specifications as other DX models: 2 mm steel chassis, dark yellow chromate, 3 mm anodised aluminium front panel, baked enamel texture finish. Cabinet dimensions: 435 mm W × 190 mm H × 465 mm D (slightly deeper than the DX-2 at 465 mm vs 465 mm).

The DX-3 front panel displays all 9 band positions on the band switch knob legend. The Tune and Load controls, Power/Stby-Opr switches, and all bargraph indicators are on the front panel as in other DX models. The DX-3 does not have an analogue meter for plate current; the manual states the design is “greatly simplified by the absence of meter switching, mechanical display and front panel level setting controls” — all indication is via the LED bargraph array. This means a failed LM3914 bargraph IC on the display board leaves the operator with no plate current or power indication.

13. Parts Sources & Reference Documents

• Emtron DX-3 Operating Manual (April 2005) — manualslib.com — Complete schematics including RF module (15.8), QSK v3 (15.7), soft-start (15.6), HV supply (15.2), and control board (15.9). The primary service reference for all DX-3 work. Also available as full text at archive.org.
• emtrondv.com — Dan — emtrondv.com — Version 7 AMPC control board (DX-3 compatible); plate choke 1” OD × 6” long (stocked; verify 700 µH DX-3 specification); free technical advice; refurbished DX amplifiers (pickup Sydney). The definitive human resource for all DX-3 technical queries.
• GU78B Datasheet — ok1rr.com/tubes/gu78b.pdf — Heater 27 V / 3.4–4 A; plate dissipation 2,500 W; 3,200 V DC max anode; grid 2 max 350 V; cathode heating time 240 seconds; input capacitance 100–150 pF; transconductance 40–80 mA/V.
• GU78B / 4CX3000A Tube Sources — NOS GU78B: Eastern European and Ukrainian surplus dealers on eBay (hfvhfparts and similar verified high-feedback sellers); confirm “new old stock, untested” and allow 12–16 hours heater-only gettering. Current production 4CX3000A: Penta Laboratories (pentalabs.com); stockists via RF Parts Co. (rfparts.com).
• Safety Capacitors (Y2 and X2) — Mouser (mouser.com), DigiKey (digikey.com); Kemet, Vishay, or Wima certified Class Y2 (4.7 nF/250 VAC) and X2 (470 nF and 220 nF / 250 VAC). Verify IEC 60384-14 certification on each datasheet.
• Jennings TJ1A-26S / P1D-1V (QSK relays) — TJ1A-26S: Jennings Technology (jenningsrelays.com), RF Parts Co. P1D-1V: verify coil voltage from existing relay marking; specialty vacuum relay suppliers or surplus sources. Confirm coil voltage before ordering.
• LM3914, LMC555, TDA1085C, TIPL760A, M4-12H — All standard parts from Mouser or DigiKey; same specifications and replacement procedures as documented for DX-2 series.