Carl--
Is this a good amp? Or does it need all sorts of "mods" to work properly?
Andy K5VM
K1WPO@aol.com wrote:
> General Technical Comments of Emtron DX-2a
> (http://www.emtron.com.au/AmpDX2D.html)
>
> I received one of the first DX-2a shipped to the U.S. a few months ago, and
> have been pleased with the performance,
> value and performance. Several friends have asked about
> the amplifier, so without a lot of technical measurements,
> etc, the following are my general, non-technical comments.
>
> Tube
> This amplifier uses a Svetlana 4CX1600B
> (http://www.svetlana.com/docs/tubeframe.html) running in class AB1.I prefer
> the use of a single, 1600 watt plate dissipation tube over two (or even three)
> tubes in parallel. This tube is the tetrode equivalent of an 8877, which has
> always been an expensive tube. The lower cost allows a better 2KW amplifier at
> significantly lower cost.
>
> P/S
> The power supply is robust, with a very large and well
> constructed transformer. The unit I received was an export (50Hz) model. Later
> versions should provide some weight savings when a 60Hz transformer is
> available. The rectifier diodes are typical series string as a F/W bridge, and
> use the large 6 ampere variety.
> B(-) fault diodes are used to GND as well as B(+) glitch
> resistors, all located on a very compact HV P/S module.
> The entire P/S is removable with two screws and six push on terminals, no
> soldering - a very neat, compact design.
> Operation is silent, with no transformer hum or lamination
> noise. Idle voltage (bleeder load only) is around 3,200Vdc
> (at 245Vac in) and full load at 1.0 ampere is about 2,900 Vdc.
>
> AC Input
> The six foot cord set is a little light by American
> standards, and has a vinyl sleeve. I looks like the
> equivalent of 15-16 AWG. Australia rates it at 25 amperes,
> which is adequate, but I prefer a real, American cordset.
> I replaced mine with a longer (8ft), more flexible, rubber> 14-3 SJOW cordset.
> The amp ships without an end plug, so
> You will need to supply what ever you're using at your shack for your 240Vac
> mains. Since there are no 120Vac components used internally, and the cordset
> is three-wire, the GND is tied directly to chassis at the input. The two HOT
> leads go first to 20 ampere fuses accessible at the rear panel. HOT then
> proceeds to the cabinet interlock, and front panel switch which carries only
> relay solenoid current. Both sides are then switched through a large, 277Vac
> 25-ampere contactor to a proprietary soft-start thyristor circuit, which
> generates a true AC ramp function. The soft start takes about 2 seconds to
> complete, and operates the entire amplifier. Everything from the meter lamps,
> to the tube filament (12.6V) and the HV p/s are ramped. The AC input is wired
> with heavy, short runs, and is done better than most AC input circuits I have
> seen. Attention seems to have been paid to minimizing AC input losses through
> adequately sized conductors and terminations.
>
> RF Deck
> The RF deck components are large by today's standards. All
> wiring components, straps, taps, coils, etc are silvered.
> The L1 tank coils are entirely wire-wound, toroids are not
> used for the lower bands. Coils for the upper HF bands are
> heavy, 1/4 inch silvered stock, and for the lower bands
> are silvered #8-10 wound and mounted on a heavy ceramic form.
> Ceramic/porcelain standoffs are used throughout, something you don't see too
> often in today's amplifiers.
> Workmanship around the tank wiring and taps is excellent. The size and quality
> of the components and workmanship are similar to the old ETO 77dx.The band
> switch is a five deck silvered ceramic switch. All nine amateur bands are
> supported. Three decks are close spaced, and are wired in parallel for current
> handling capability. The two additional decks are used to add C1 and C2
> padding capacitance for 160m. The padders are the largest I have ever seen,
> measuring 1.5 and 2 inches in diameter for C1 and C2, respectively. The output
> tank circuit is a Pi-L with the output L an untapped 4:1 coaxial UNUN. This
> transforms the image impedance of about 200 ohms to the 50 ohm output
> impedance.
> The RF choke is the largest I have seen on any amateur
> amplifier, measuring about 7/8" diameter by 7" long on a
> ceramic form. The plate blocking capacitor is neatly
> fitted directly to the end of the RF choke, and is a 2700pF, 15KV component.
> Plate tune and Load capacitors are large air variable, and driven with high
> quality ball-bearing reduction drives from the front panel.
> The 4CX1600B is housed in a huge ceramic chimney with a
> Wall thickness of a half-inch. A parasitic suppressor connects the anode to
> the plate choke/blocking capacitor. The tube is similar in shape and size to
> an 8877.
>
> RF output Wiring
> Teflon coax (RG-303) is used on all output wiring to the
> QSK board which uses a Jennings RJ-1a vacuum relay for the
> output. Full break-in operation is supported at full power
> on all bands. A shielded and enclosed RF sensor is used
> for the forward, reflected and SWR Fault detector.
>
> Operation
> There is a 2-3 minute wait for warm up. The Ready LED does
> not flash during warm-up, which would be a simple feature
> to add, and one that is needed. As received, all LEDs are out during warm-up
> until the timer has expired, then the ready LED comes on. The factory presets
> for all nine bands were right on the mark. The amplifier puts out 1,750 -
> 1,875 watts on all bands with 60 watts of drive. With 50 watts of drive, the
> amplifier is at 1,500 watts out.
> If an incorrect antenna is chosen, the amplifier faults
> immediately to bypass. I was not able to get the amplifier
> to flash over in any circumstance with the wrong antenna
> selected. Anything over about 2.5:1 kicks in the
> protection circuit, a very nice feature. This feature will protect against
> potentially significant RF deck damage in the event an antenna comes down, is
> covered with ice, or is simply the wrong antenna for the band. Tuning is
> simple, and similar to tuning a triode amplifier. After pre-sets are chosen,
> apply 25 watts and peak the Load and tune. Slowly increase power, while
> peaking the output, and watching the Ig2 LED's for screen grid current.
> Slightly heavier loading keeps the Ig2 LEDs to one or two RED segments, which
> represents about 20-25mA.
> The amplifier will immediately fault to bypass of too much
> input drive power is used. Both the SWR and Overdrive
> faults are reset automatically in two seconds. If the fault
> remains, the amplifier will not re-key. Observation on a scope of RF output
> during QSK operation was perfect with both full and semi-break-in.
>
> Cooling
> A high quality blower is used, and it moves a lot of air.
> It appears to be a 220Vac 50Hz motor. At 60 Hz, it runs a bit too fast and has
> a bit more high frequency white noise
> Than I would prefer. It is run at about 120Vac in the normal mode, using a
> primary tap on the transformer as an auto transformer (no neutral current on
> the mains). There are temperature detectors in the tube chimney air flow that
> will switch to 220Vac operation at about 100 Deg C. I never heard the blower
> increase in speed. The tube runs very cool, with efficiencies in the 65% range
> on all bands, and cools very quickly after CW or tune operations.
> Air flow in this amplifier is excellent for the tube, but
> not as good for the other two items that need attention.
> First, the tank components should have some air
> Circulating by them, and the p/s diodes and bleeders should also be in the
> intake air path. There is some intake air that flows by the p/s, but none
> flows through the tank.
> In this amplifier the tank components are so large, not
> Much heat is generated in any of the tank components, except the #10 ceramic
> coil form, which does get warm. The P/S diodes are so large they also run
> cool, but the bleeders and B(+)glitch resistors do get warm.
> This is about the only area in the entire amp that I can
> complain about, but realistically, even with RTTY operation
> at 1,500 watts continuous, it would probably never be a
> problem.
>
> PC Boards
> Glass epoxy boards are used throughout, for the HV p/s,
> Control board, Display board(s) and QSK. The HV p/s,
> Control and Display boards are all replaceable with push pin connectors.
> Should a replacement or upgrade be necessary, use of connector housings
> (rather than individual pins)would greatly simplify that exchange.
>
> Metering
> I chose the DX-2a version because I am partial to analog
> meters. The digital version has the advantage of seeing
> all five parameters at once, with no switching.
> The meters uses are very nice, large bright white meters
> with easy to read back-lit scales. One is fixed for plate
> current, the other is switched for Forward Power,
> Reflected Power and Plate Voltage.
>
> Ip = 1.5A f/s
> Vp = 4KV f/s
> Pf = 2KW f/s
> Pr = 200W f/s
>
> There are several spare positions on the meter function
> switch. I would like to see filament voltage added, since
> it may vary with line voltage. Since this is a tetrode with a 350Vdc regulated
> screen supply, it might be nice to read
> that too. Screen Grid current is also metered with a 6 segment LED, reading
> from neg 10mA to pos 35mA. The Ig2 LEDs are a great tuning aid, and make it a
> very fast chore to peak the tune and load, while keeping the loading
> sufficient to keep Ig2 below 25mA. At 35mA all four segments light, at greater
> than 35mA they flash. This is one of the better tuning aids I have seen, and
> is very intuitive to use.
>
> Documentation
> The users manual is one of the most extensive of any
> amplifier I have used. There are diagrams for linearity,
> overdrive conditions, QSK timing, etc... Partial
> Schematics are provided and the operation section is well written and
> thorough. A pre-set chart is provided with factory settings included.
>
> Summary
> While most US amplifiers have concentrated on cost
> Reduction in recent years, it's a pleasant surprise to see quality improving
> at a very fair price. This amplifier is a great value for the money (about
> $3200 delivered with QSK). It is built with more expensive components than
> most US amps selling for more money. Workmanship is excellent, and
> Layout is modular and easily serviceable.
> The performance is flawless, and the protection circuits
> work well. You can not easily damage this amplifier, so
> the user is protected from simple and common mistakes.
>
> A few recommendations to future revisions:
> 1. Flash the Ready LED during warm-up;
> 2. Duct the intake air by the tank components and P/S;
> 3. Add a few more metering functions to the meter switch;
> 4. Use AWG 14-3 SWOJ cordset;
> 5. Add complete schematics and calibration/alignment
> procedures;
> 6. Add connector housings to the wiring harness for PC
> board connectors.
>
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