The RF-110A had slow T/R switching making it almost unusable for my purposes.   

With the QSK-Master, the T/R switching is  better than anything I have ever experienced before and with the small modification I performed to control the bias directly from the unit ( a 10V control voltage at 4 mA is switched to control the bias of both driver tubes and output tubes ), the amplifier is now biased to cutoff when it is not keyed.  

This ensures much less heat and less waste of power.

The amplifier is even cut off between CW characters in QSK operation.

TFM 3928 DxO

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The amplifier has 2 x 4CX1500B tubes in parallel in the output stage (3000W plate dissipation) biased for Class AB1 operation and runs an easy 1 kW output in any mode (key down forever…) on all bands.

It takes 100 W from the exciter through a 30dB attenuator which provides 100mW to a pair of 8122 tubes running in Class A (capable of around 1kW by themselves) which amplify the 100mW signal to 10W to drive the PA tubes.

The amplifier is cooled with a very noisy 4″ muffin fan rotating at 11,000 RPM – a real screamer –  necessitating installation in the adjacent soundproofed equipment room.

The T/R switching is rather slow and clunky by today´s standards – on the order of 200mS until full output is reached due to timing constants in the bias circuit which prevents hot switching of the relays, but by increasing PTT to generated RF delays in the equipment everything functions reasonably smoothly.

An ArraySolutions QSK Master unit is on order to use with the amplifier – once this unit is installed there will be no moving parts inside the amplifier.

The RF-110A generates 3.5kW of heat for 1kW of RF output – this makes it necessary to improve ventilation of the equipment room.  It consumes 1kW in Standby mode.

I would appreciate comments on the quality of my signals as I test the unit.

From top to bottom:   Harris AM-7224 1kW amplifer, Harris RF-110A 1kW amplifier

Harris AM-7224 1kW amplifer and two power supplies for the AM-7224 amplifiers.

In addition the station has a Harris AM-7223 500W amplifer, an Emtron DX-2SP and an Emtron DX-3 amplifier.

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The amplifier fits very neatly into a small 19″ rack which I had available and was modified for 160 Meters by TF3BM who also built the single phase 230V/4400W continous duty power supply.

Due to the noise of the cooling system, the amplifier will be installed remotely in the adjacent equipment room once I have tested it thoroughly.

The amplifier accepts 100mW input for 1000W output and is fed from the exciter through a 30dB attenuator.

It uses two 4CX1500B tubes and is simply loafing at full power.   Two 8122 tubes take the 100mW and amplify this to 10W for the output tubes.  The pair of 8122 tubes are capable of a kilowatt by themselves but are only running at 10W!

The manual may be found here.

Some nice photos of the amplifier may be found here.

Harris RF110-A

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The DX Engineering Receive Antenna Interface for Transceivers is the unit that made this possible.

The only problem was finding a voltage on the Harris radio that would sink to ground under all circumstances of keying.

The solution was found in the Coupler connection on the back of the radio.

I connected a 10k pullup resistor to pin H which carries 13.8VDC and connected the other end of the resistor to pin B (Coupler Key).   This pin goes low on transmit, and could equally well be used to key a small relay if that was desired, but I wanted electronic control of the DX Engineering box.   Pin B in turn went to the Transmit Ground connection on the interface.

In addition, I power the interface itself from pin I, which also carries 13.8V.

Works beautifully.

It was very fitting that the first station I worked with the new setup – operating position C – was JA8NFV.

The Harris radio has filters the size of a amateur transceiver, with performance to match, what an incredible pleasure.

Since both the Harris RF-350K transceiver and the Harris 7224 KW LPA (Linear Power Amplifier) have RF Sample outputs, I set up scope monitoring of the system triggered by the  CW keying.

Update: After using the Harris in battle I am a bit disappointed. The sequencing leaves a bit to be desired, this will be improved with a DX Engineering sequencer already ordered, and there seems to be a bit of filter blow-by on CW. I am hearing CW signals outside the filter passband. I will test another unit to see if this is a common problem, perhaps a bit of contact cleaner will clear this up. On SSB the Harris radios are unsurpassed.

Further analysis indicates that this problem might be caused by  impedance mismatch with the headphones I have been using.  With those headphones, volume is at almost maximum settings so this seems likely.

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The two amplifiers are located in a sound proof room adjacent to the ComCen and they are remote controlled by their associated Harris RF-350K/RT-1446 transceivers.

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W2APE shows an  tube conversion procedure for these amplifiers on his web site.

I expect no failures though as these units are idiot proof as W2APE points out.  This is definitely a needed feature as every contester knows.

My station now has 4 operating positions, each with 1 KW output.

A Harris AM-7224/URC   500W is ready as a spare in case of failure, and a Harris RF110A (AM-16675/URT) is in the pipeline…