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An Innovative 2-kW LinearTube Amplifier

Saulo Quaggio, PY2KO

Can a tube full-legal-limit amplifier be small and light?

This prototype was built for testing anew concept in linear amplifiers,which could be named AB/F class. Itis not actually a new operating class, but hascharacteristics of the well known class ABand class F applied to the same amplifier. Itis composed of a bunch of old and new tech-nologies, resulting in a very small and lightamplifier able to operate in AM, CW and SSBat legal limit power.

The idea was to get linear performancefrom a non-linear high efficiency amplifier.To achieve it I adopted an envelope elimina-tion and restoration approach: The incomingsignal is demodulated and the original enve-lope amplified by a high efficiency, highspeed switching supply that modulates thefeeding of output stage power.

The incoming RF signal is also appliedto the tubes control grids. At low power lev-els the grid polarization circuit keeps theamplifier at class AB. As the power getshigher, the tubes are driven into saturationand the output power is controlled by the ris-ing voltage of the modulating power supply.

Why tubes and not semiconductors? Hereare some reasons:Relative fragility of power FETs in highpower RF circuits. Tubes are much more

resistant to mistuning and unpredictableconditions that may occur during devel-opment.Possibility of applying this project to morepowerful commercial amplifiers.Smaller variable capacitors at tuning outputcircuits, due to higher impedance levels.Tubes are fun! Its amazing to see four smallTV tubes effortlessly pumping out RF overthe legal limit. Old PL509 TV sweep tubeswere built to operate as switches, so theyperform very well in this saturated class,achieving high anode efficiency.

Refer to the block diagram of Figure 1.

Figure 1 Amplifier block diagram.

Figure 2 Basic waveforms.

(Continued on page 24.)

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Figure 3 RF amplifier schematic diagram.

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Figure 4 Power supply schematic diagram.

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Figure 5 Power supply control schematic diagram.

Figure 6 Grid bias schematic diagram.

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Figure 8 Thermal protection schematic diagram.

You can see the small size by comparing the amplifier to a Bird model 43 wattmeter.

(Continued from page 19.)

The amplifier is a harmonically controlledpush-pull arrangement operating in a modeknown as inverse class F. In the frequencydomain, the symmetric resonant load presentsan open circuit to odd harmonics, a short cir-cuit to even harmonics and is resistive to thefundamental frequency.

Another way to explain how it works inthe time domain is to consider the tubes asswitches grounding alternatively the twosides of a resonant load consisting of a resis-tor, a capacitor and an inductor in parallel.Across each switch is a half sinusoid voltagepulse when opened, and zero volts whenclosed. The two halves combine at the loadin a full sine wave. Two RF chokes feed dcpower to the circuit. Note that the load mustbe isolated from ground.

Basic Waveforms

Refer to the basic waveforms shown inFigure 2. Theoretically, this mode allows100% efficiency, since there is no voltage andcurrent simultaneous at the switches; but be-cause of parasitic capacitances, resistivelosses and minimum anode voltage, the ac-tual plate efficiency runs around 85%.

To ensure correct coupling, a differentialadjustable pi circuit lowers the impedancefrom 1800 at the tube side to 200 , and atthe same time provides the resonance needed.A 200 transmission line unun isolates

the pi circuit from ground, and a 200 to 50 balun also converts from differential to com-mon mode.

Circuit Description

Refer to the schematic diagrams in Fig-ures 3 through 9 during the followingdiscussion. A side benefit of the push-pulloutput arrangement is that since there are vir-tually no even harmonics generated, a loadedQ of 5 is enough to filter out the 3rd and 5th

harmonics, permitting smaller bobbins,capacitors and commutating relays than al-lowed by conventional unbalanced pi circuits.

A high frequency diplexer isolates theparalleled tubes against VHF oscillations.These oscillations actually occurred, makingthe internal tube plate connections glow red!

The incoming RF signal is detected bya discriminator circuit centered at5.4 MHz that actuates two high-current re-lays, which short the bobbins, halving the pi

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inductance when operating on 40 m.This configuration allows linear class-AB

push-pull operation until the amplifierreaches roughly 100 W. From this level upthe tubes start to saturate, and the currentthrough the control grids forces the polariza-tion level more negatively, out from class-AB. The amplifier changes smoothly fromclass-AB to class-F as the plate voltage risesfollowing the input signal envelope.

The input circuit is a wideband balancedcircuit, operating with no tuning from 3 to8 MHz with reasonable SWR. The inputtransformer is a 1:9 transmission-line typeloaded by an array of resistors paralleled tothe control grids.

The Switching Power SupplyThe power supply is an off-line, full-bridge,

phase-controlled, quasi-resonant converteroperating at 100 kHz, capable of 2500 W.Phase control means that each side of thebridge operates always with square waveforms,and the control circuit changes the relativephase between bridge sides from zero to 180,varying the output voltage from zero to1200 V.

The quasi-resonant approach was adoptednot only to increase efficiency but also to re-duce secondary ringing by slowing primaryturn-on slope. Unlike conventional switch-ers, this one has broad response bandwidth,necessary to follow the audio envelope of theincoming reference signal. Stability compen-sation versus ripple attenuation limits thebandwidth to 8 kHz, enough to follow AMand SSB input signal envelopes.

The output filter is a 2nd-order 15-kHzlow-pass Chebyshev, followed by a 200-kHzseries trap. The commutating ripple is attenu-ated 60 dB by this filter.

It is interesting to note that the small ca-pacitors at the output and the fast current lim-iting gives a soft characteristic to this powersupply: A sudden short circuit at full poweronly fires a small spark, very unlike a con-ventional high voltage supply of this size!

The 240 V ac power input is rectified byan SCR-diode bridge, provided theres a softstart circuit to limit inrush current and an in-put choke to improve the power factor. TheSCRs also act as the main power relay. Theinput rectifier could work as a doubler tooperate 120 V ac, if you set some jumpersand have heavy duty wiring in your shack.

I did not try to shield this big switcher forRFI: When not transmitting it is held totallyoff. This required a small 60 Hz transformerto supply tube heaters and control circuitpower, but it is worth the space occupied theres no receiving interference at all. Andwhile transmitting? Well, if you can keep theTV sets working during your 2 kW chatter-ing, interference from the switcher is no prob-lem.

Amplifier with power supply removed.

The RF deck top view: tubes, output bobbins, relays and balun stacked to outputtransformer. The vertical coil between tubes is a spring to keep them in place.

Demodulator, Limiter andControl Circuits

The input active differential RF detectorperforms better than simple diodes, present-ing very good linearity. Following the RF fil-ter, the limiting circuit keeps the power sup-ply above 250 V, to allow the tubes to oper-ate in class-AB at low power. It also limits

the maximum voltage, to protect the amplifierif the main ac supply goes high. Without it, theinstantaneous input power can reach3 kW, meaning danger to tubes and capacitors.

Another limiter function is to keep anodevoltage below 500 V during tuning, also toprotect components and to give a stable tun-ing reference current.

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RF deck bottom view: tubes, variable capacitors and dc feed chokes.

The conditioned envelope input is com-pared to a voltage output sample, and the er-ror signal is amplified, compensated and ap-plied to the power supply phase control viaan optocoupler. TR switching and sequenc-ing is performed according to the MODEswitch, whose positions are:

1 Off.2 Standby: Heaters and fans on, switch-

ing power supply off, RF input bypassed tooutput.

3 Tuning: Anode voltage limited to500 V, RF amplifier connected.

4 Operating: Maximum power en-abled, RF amplifier connected.

Tuning is performed by increasing drivepower until saturation is reached (no furtherincrease in plate current), at the same time set-ting a dip at 1.2 A adjusting alternatelyplate and load capacitors, as with any pi out-put amplifier.

The tuning procedure is the same for SSB,CW and AM. In AM the input carrier must beadjusted to read 1.2 A (approximately 500 WRF output). It must be done carefully becauseof the low power dissipation capabilities of thePL509. The efficiency drops quickly when itis out of tune, and if RF drive is applied care-

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Figure 9 Auxiliary supplyand soft-start schematicdiagram.

Left side view: power supply output transformer and filter, mode switch and tubes

lessly the thermal protection circuit can becalled on to save the tubes, turning power off.

Grid Polarization CircuitsThe switching supply provides unregu-

lated 200 V dc to the screen grid series regu-lator. The circuit also applies negative bias to