Issue 36 Signal

Tricks of the Trade

Dave Porter G4OYX and Alan Beech G1GXB

Fifty years ago, in mid-1965, the UK offshore radio operations were nearing their peak with some of the earlier lower-powered setups being consolidated into more professional operations and a number of projects just about to come on the air. Three of the stations in the planning and construction phase at this time were Radio Scotland, Radio 270 and Radio 390; these all employed 10 kW versions of the RCA Ampliphase transmitter. The principle of outphasing (Ampliphase) to generate amplitude modulated carriers was described in the previous ToTT [1]. This article will develop our understanding of the RCA Ampliphase system and will examine the progress of the RCA design over the years following. In addition, some recently released documents detail what the BBC had researched and discovered concerning the RCA Ampliphase units. Detailed development history To provide for linearity correction and drive regulation, the RF drive to the PAs was progressively reduced by means The first RCA Ampliphase transmitter, the BTA-50G, was of a simple resistor connected between the PA grids, as released onto the market in 1955 but the outphasing the phase moved towards 180°. With 180° difference, a principle was already 20 years old at that time. The reasonable amount of drive power was lost in the resistor technique was first described by Henry Chireix in due to the current flow but, as the difference reduced, November 1935 [2] and just nine months before WH there was less current flow and therefore less power lost. Doherty patented his amplifier design. Information on early At zero degrees difference there would be no current flow outphased transmitters is now very scarce; however, by in the resistor and no power lost although, of course, the 1948, station KFBK in Sacramento, California had minimum phase difference of the system is 90°. The successfully home-built a 50 kW version. This station was original 50 kW transmitter had a “rather complex vacuum operated by McClatchy Broadcasting, which also operated tube RF voltage regulator” as did RCA's version when it KOH in Reno, Nevada and a 5 kW unit was subsequently was released several years later. built for that site. The May 1951 issue of Electronics magazine carried a three-page article, describing in great McClatchy Broadcasting also operated KMJ Fresno and detail the design and operation of the five kilowatt KBEE Modesto. It is possible that outphased transmitters transmitter, including a complete circuit diagram (Figure may have been used in at least one of these stations, 1). This design used a single modulator stage, followed by although no firm information on that is currently available. a tripler to provide the required phase shift at the carrier It is believed that, having pioneered outphasing frequency. However, it appears that the original KFBK modulation, McClatchy persuaded RCA to build these on system used three cascaded stages, as per the BTA-50G a commercial basis and the Ampliphase was born. The and BTA-50H. The PA stages at KOH were a pair of exact details of this ‘handover’ are also unknown to the 3CX2500F3s, driven from a pair of QY4-250As. author (G1BXG) at the present time.

Figure 1. Circuit diagram of KOH homebrew Ampliphase transmitter, 1951

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From Class B to Ampliphase In 1955, solid-state rectifiers were in their infancy so all the main power supplies to the BTA-50G utilised mercury arc The BTA-50G, the first example of which was shipped to rectifiers. The high voltage was 16.2 kV provided by a total WINS in New York on 28th December 1955, inherited a of twelve 6894 rectifiers. These were arranged as pairs in number of features from its Class B modulated a conventional six-way, three-phase, configuration. Six predecessor, the BTA-50F. This included the 807 oscillator 8008 valves were used in another three-phase full-wave block and the air-cooled 5671 PA valves. Whereas the 50F configuration to provide the low/intermediate voltages of had four such valves, two in the modulator and two in the 2.5 kV and 5 kV. Each rectifier (or pair of rectifiers) PA, plus two installed spares, the Ampliphase had just one required its own filament transformer which had to be pair. This PA valve was over two feet in length, 16 inches insulated to withstand the 16 kV HT voltage. As mercury in diameter and weighed just over 200 lbs. To place one of vapour rectifiers will not strike at low temperatures (the these valves into service was considered a two-man job vapour condenses) a 400 W heating panel had to be fitted and a small trolley-mounted crane was provided to assist into the rectifier cabinet to prevent the valve temperature with this operation. The driver valves on the BTA-50G dropping below 20°C. were type 6076 and the pre-driver a pair of QY4-250s as used on both the KOH/KFBK transmitters and on the later Once the transmitter was up and running, a blower motor BTA-50H and BTA-50J systems. The BTA-50G driver unit had to circulate air through the cabinet in the normal is shown in Figure 2. manner to prevent the valves from overheating. A selenium metal rectifier was used to provide the –400 V bias supply. RCA made claims in 1955 about the compact size and lightweight concept of the BTA-50G which was approximately half the length of the BTA-50F and, with no modulation transformer or reactor, was a third of the weight. High power MF versions Although produced as a 50 kW transmitter for the domestic market, there is evidence that 100 kW and 250 kW versions were manufactured and shipped to Iran and Mexico. No further information has as yet come to light on these variants except for early pictures of the prototype variant units which show a modulator with most of the components laid in a single horizontal line and that the crystal oscillators were all fitted at the bottom of the cabinet in the space which was later used to house the drive regulator. There are reasons to suspect that this transmitter used a single phase modulator followed by a tripler, but it is not known if this modulator was purely a prototype or if it was shipped with early units. Later versions of the BTA-50G had the ‘conventional’ cascaded modulator, arranged in four vertical columns, with the drive regulator at the bottom of the cabinet and the oscillators to the side. Approximately 30 examples of the BTA-50G are thought to have been shipped. From mercury vapour to silicon rectifiers The BTA-50H, released in late 1960, inherited features from the BTA-50G, including the four column cascaded 19- valve modulator, drive regulator, general electrical and physical layout and that ubiquitous 807 oscillator block. However, the BTA-50H contained many ‘new’ features such as silicon rectifiers throughout, an updated valve line- up and facilities for remote control. The massive and heavy RCA-5671 PAs were replaced by a pair of lightweight Machlett Laboratories 6697s and the 6076 drivers were replaced by a pair of 4CX5000A's. The QY4-250 drivers were retained but now ran from a 1 kV supply and not the dropped 2.5 kV volt supply of the BTA-50G. The intermediate and high voltages remained the same at 5 kV and 16 kV, though the bias was increased to –950 V to suit the new PAs. RCA claimed that the use of silicon rectifiers Figure 2. Early BTA-50G driver unit with crystal allowed operation at temperatures as low as –20°C, oscillator at the bottom, the modulator with although no claims were made for how well the indicating meters centre, and the 6076 drivers at engineering staff would operate at that temperature! the top

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The cosmetic total ‘New Look’ and oscillator at half carrier frequency was built around a 6AK5 followed by a 5763 buffer. A 6BX7, a gutsy power double- solid-state modulator crate triode, acted as a phase splitter, utilising a lumped L/C A later variant of the BTA-50H, the BTA-50H1, was delay line to obtain the initial phase separation. Phase produced sometime in 1966, though the technical modulation and frequency doubling was performed by a differences between that model and the earlier unit are not 6EA8 triode/pentode with audio applied to the triode grid. clear at the moment. However, with regard to cosmetics, A 6CL6 followed as a buffer with an 829B as exciter output the unit benefitted from RCA's ‘New Look’ marketing and providing approximately 40 W of drive. was shipped in a two-tone grey and blue finish. When RCA later produced a solid-state modulator, transmitters could be field-upgraded by RCA engineers to become a BTA-50H1S, which included the later alterations to the ‘H1’ (presumably without the new colours) plus the solid-state modulator designated by the ‘S’. It is believed that the first unit upgraded this way was for WNBC in New York. Upgraded units were given a new serial number plate, showing the new designation, although it seems that they retained the original serial number. It appears that a considerable number of units had the solid-state option fitted as the ageing and drift of the nineteen valves in the modulator was a frequent cause of poor performance – ‘Amplifuzz’. The prototype of the solid-state modulator is rumoured to have been built by Radio Pakistan and the design sold back to RCA. The development of this unit must have been a considerable project for a broadcaster so it is possible that Radio Pakistan operated a number of these transmitters. A good many BTA-50H transmitters were produced and a fair number of these continued in standby service into the new Millenium. It also appears there are possibly a small handful of stations still using them as their main service unit. The BTA-50H was succeeded by the BTA-50J in 1970 and Figure 3. RCA publicity picture of the BTA-10J included the solid-state modulator as standard. This transmitter also had the ‘New Look’ and the new square RCA logo (introduced in 1966 following the appointment of Robert Sarnoff to head RCA, replacing his father David Sarnoff) and was shipped in single-tone blue but had solid doors without the inspection windows and new rectangular style meters. RCA proudly claimed "only six tubes in transmitting circuits" though another four formed part of the monitoring and line protection unit. Information on these units is scarce and even RCA's own information shows that only about twelve were shipped. The last BTA-50J left the factory in May 1977. Advances in valve technology by the 1970s meant that screen-grid and pulse- duration modulation were commercially viable and transmitters such as the Harris MW50 and Continental 317C1 started to dominate the market. RCA Broadcast catalogues of the late 1970s/early 1980s show then that the only AM transmitter available from RCA was a 1 kW model. Figure 4. The modulator pull-out drawer 10 kW and 5 kW Ampliphase RF drive was provided by a pair of QY4-400's in each The BTA-10J and BTA-5J Ampliphase units were available channel operating from an 1800 V supply. One was used by 1962. They were constructed into a single cabinet, as a straightforward Class C amplifier but the second, although it was fairly wide at 44 inches (1120mm). The which was electrically in parallel with the first, had its bias electronics were located in the centre of the cabinet with controlled by the drive regulator circuitry. Thus, at the metering and controls down both edges. The RF power modulation peaks, both QY4-400's provided drive to the stages were behind an upper hinged panel and the 3CX10000 output whereas, at lower levels, only a single modulator stages on a pull-out drawer (Figures 3 and 4). QY4-400 provided drive. This arrangement was similar to The RF/Modulator design concept was radically different the drive regulator and linearity correction operation in the from the existing BTA-50G and BTA-50H Ampliphase BHF-100 shortwave Ampliphase transmitter (see later). transmitters in that a single phase modulator stage was Drive regulation in the negative ‘out-of-phase’ area was by used followed by a frequency doubler (Figure 5). The means of a pair of 2.6 k resistors connected between the

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PA grids, just as the KOH home-built transmitter had in Over the years three variants of these were produced, the 1951. The final supply voltage of 7 kV was originally original ‘L’ series, the ‘L1’ and the ‘L2’. Differences were obtained from six mercury vapour rectifiers; an unusual minor. All models featured the BTE-20 solid-state exciter feature for 1962, considering the BTA-50H had silicon as standard. As this was designed to be adaptable to a rectifiers the previous year, although silicon diodes were number of Ampliphase transmitters; the ‘L’ models used in later models whilst the combining and output featured a solid-state ‘pulse amplifier’ board between the networks followed the conventional Ampliphase design exciter and the RF driver stages. This pulse amplifier also and the output transmission line could be specified for seems to have the added feature of taking the drive either 50 or 230 . regulator output from the exciter, and using it to pulse width modulate the phase modulated pulses from the exciter before they were applied to the driver grids. Thus, the amount of RF drive was controlled by the duty cycle (width) of the RF drive pulse so that no additional drive regulation or linearity control was required. The RF driver stage was an 8122 running at 2 kV, with two in parallel for the 10 kW model. The output stages were 3CX5000 or 3CX10000 at 5 kV or 7 kV. Extensive power supply switching in these models allowed the output to be switched from 10 to 5 to 1 kW and 5 to 2 to 1 kW. The handbook for the "L" models is very comprehensive, and substantially more detailed than for the earlier Figure 5. Block diagram of the BTA-10J transmitters. It contains several pages on the theory of operation and detailed mathematical analysis of various modulator stages in the process, particularly the output network. This Updated versions information was conspicuously missing from earlier handbooks and was no doubt added by RCA in response In the late 1960s/early 1970s the smaller Ampliphase to the amount of criticism the Ampliphase system had transmitters were updated. These were designated the 5L received from the industry. and 10L (Figure 6) and occupied a cabinet with three doors, approximately twice the width of the 10J at Offshore usage 70 inches (1780 mm). BTA-10J transmitters were used by a number of UK offshore stations during the 1960s, notably Radio Scotland from 31st December 1965 on-board the M/V Comet with a pair of BTA-10Js and a combiner, Radio 270 from 4th June 1966 on-board the M/V Oceaan VII and Radio 390 on the Red Sands Thames estuary fort from 25th September 1965. Radio London on the M/V Galaxy had a BTA-10J as a reserve unit and this was used for a test transmission in 1966 on 1079 kHz, 277 m. The ex-Radio 390 BTA-10J was later sold to the MEBO Company of Switzerland for use on-board the Radio Northsea International ship, M/V MEBO 2 (Figure 7) and the Radio 270 transmitter was acquired by the Dutch Capital Radio project on board the M/V King David. A BTA- 10L was subsequently introduced featuring the solid state modulator; this transmitter was also available in a 5 kW version, BTA-5L.

Figure 7. Pair of BTA-10J with combiner as Figure 6. The BTA-10L with covers removed installed on board the MV Comet, home of Radio Scotland 1965-1967

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The RCA BTH-100B 100 kW MF as a Belaskis modulator and the pentode section as multiplier. The entire frequency range was accommodated transmitter in four bands with a motorised control adjusting six A 100 kW standard MF Ampliphase transmitter was inductor slugs. Each PA driver stage comprised a pair of available from the time the BTA-50H went into production, 4CX10000 tetrodes with an unusual bias and drive the BTH-100B. This consisted of four BTA-50H PA regulator arrangement (Figure 10). At carrier power and cabinets with a single exciter/modulator cabinet (Figure below only one driver tube operated with the second 8); indeed, RCA documentation shows this was handled in biased beyond cut-off. As the required carrier level production as two separate BTA-50H models. One of increased, the second tube was brought into operation these, serial number 002, was used on-board the Radio thus increasing the drive power to the PA stages. The Northsea International ship, M/V MEBO 2 (Figure 9), from output of the transmitter was to a balanced line of 300 or 28th February 1970 anchored off the UK and Dutch coasts 600 . until 1974. It is probable that only a small number of these transmitters were made, all for the export market. The RNI unit, which was most likely bought second-hand at only seven or so years old, apparently suffered overheating problems when running on full power. Other BTH-100Bs may have been shipped to Finland and Nigeria.

Figure 10. HF100 Block Diagram “Sitting here all these years later, I can't help but think that Figure 8. Block diagram of the BTH-100B two valves, one for carrier, and one for peaks has similarities to a rival modulation system (G1BXG)”. transmitter In the ‘RCA Broadcast News’ article of August 1962, in which the transmitter was described, the concluding paragraph stated that a diplexing system was available to combine two transmitters to give 200 kW of carrier. A device RCA called a ‘computer module’ was used to tune the eight PA tubes for maximum efficiency in this configuration. From reports received, it seems these transmitters were manufactured in Italy by a sub-contract facility and not by RCA in Camden, NJ – more information on this arrangement is being sought (see later). It seems that RCA made an engineering oversight with the mechanics of the transmitter; being intended for an international market, the assembled units would have to be shipped by air freight to their destinations. However, the units were too bulky (Figure 11) for most planes of that period and even the largest commercial freight planes Figure 9. The 100kW Ampliphase (right hand could accommodate the units with just a few inches to side) transmitter on board MEBO 2. spare. Photo: Martin Stevens/Gerry Bishop HF Ampliphase A 100 kW shortwave Ampliphase transmitter, the BHF-100A, was manufactured in small quantities, the first of which shipped in 1963. The design was significantly different from the AM version due to the frequencies involved and the need to change frequency on a regular basis. Four 6697 PA tubes were used in a grounded grid arrangement with two in parallel on each side of the modulation chain. The modulator was single-stage followed by a tripler; it had to operate wideband, 1–9 MHz, with minimal tuning prior to tripling. A 6EA8 triode-pentode was used for the modulator/tripler, the triode functioning Figure 11. Front view of BHF-100A

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BBC/MWT outphasing at Daventry in small finned tanks, were used for the 15 kV anode supply. No special precautions were taken to protect the The BBC and Marconi undertook experimental work with rectifier against overloads; the primary contactor outphasing technology in the late 1940s which culminated disconnected the supply within three cycles and no trouble in the trial of a 60 kW prototype transmitter at Daventry for was experienced in spite of the rectifier output being the 647 kHz Third Programme from 15th March 1950. This frequently short-circuited by the gravity-operated short was to implement the 1948 Copenhagen Frequency Plan circuiting device. This functioned before the micro-switch and saw the Third Programme move from 583 kHz at at the other end of the cabinet door broke the interlock Droitwich. The trial lasted until a higher-power transmitter circuit; we noted the burn marks on the earthing device. combination (150/200 kW, 2 x Marconi BD206) was The rectifiers were operated at half the maker’s voltage installed on 8th April 1951 and the MWT Ampliphase rating, and paralleled elements were not used. Each transmitter was used for a short while as standby. element had a parallel R and C across it to distribute the Although the 60 kW transmitter was noted for being inverse and transient voltages evenly. remarkably small and allegedly performed satisfactorily during the trial, the BBC was not in the market for new The performance of the transmitter was said to be very transmitters at that time and the set was returned to MWT. good, with only 5 dB of negative feedback being necessary to achieve less than 2% distortion at modulating The above paragraph was all the information acquired in frequencies up to 7.5 kc/s. The mains to aerial efficiency respect of that engineering activity of the MWT was about 55% and varied little between zero and 100% Ampliphase. However, the author (G4OYX) was made modulation. aware just a few months ago by a US broadcast colleague Dan Brown W1DAN of the document which can be Within two years they were able, of course, to hear for accessed via the link [3]. This document, recently themselves a BTA-50H as Radio London was on the air to declassified, is actually a BBC Research Department (RD) the southern UK, first on 1124 then 1133 and finally report of a visit made to the USA over the period 30th 1137.5 kHz. th November – 9 December 1963 by Mr H Page of BBC RD In addition, they reported on the new 10 kW Ampliphase and Mr DE Todd of the BBC Transmitter Planning and units as follows: Installation Department. Interestingly, they were accompanied by Mr HK Robin of the UK FCO/DWS site RCA have produced a competitive design of 10 kW MF Crowborough. The visit was organised under the umbrella unit (BTA-10J) which they hope to sell internationally for of the VoA Science Advisory Group and was centred at the about $16,000. It is very small in size, measuring about 3’ VoA Greenville C facility in North Carolina with scheduled 6” wide, 3’ 0” deep and 7’ 0” high and it has been made as visits to the brand-new Greenville A and Greenville B HF simple as possible throughout. transmitter sites as well as NBC’s New York Video Tape Whilst Todd, Page and Robin were impressed with the MF Centre, FCC Washington, RCA Camden and stations units, their comments on the 100 kW HF Ampliphase WNYC and WBAL. transmitter were somewhat different… MWT Ampliphase revelations… The eighth 100 kW HF Ampliphase transmitter had left the works the previous week. None were yet in service, due it At RCA Camden, Todd, Page and Robin noted that: The was said to delays with the buildings. At an anode voltage RCA Ampliphase transmitters differed from those of 12.5 kV the overall efficiency was 53% and the distortion originally made by MWT in having modulated drive applied under 3%. The incidental phase modulation had been to the output valves to assist in reducing the output to zero checked at less than 2%. The transmitter would operate at at 100% modulated negative peaks. Without this, normal power output into a (balanced) feeder of nominal Marconi’s had difficulty in producing performance figures, impedance 300 up to a VSWR of 1.5:1. Two men could as this meant achieving at all modulating frequencies good  wavechange it from 6 Mc/s to 21 Mc/s within five minutes. amplitude balance between the opposed vectors. This A description of the transmitter is given in the August 1962 modulated drive also assisted with the positive peaks by issue of RCA Broadcast News. From this description it ensuring that the valves had adequate drive at all parts of does not seem at all attractive having numerous sliding the modulation cycle. RCA also claim that the valves have contacts carrying high RF currents, many tuning motors an easier time with Ampliphase than with Class B anode and lead screws made of insulating material. It will almost modulation, as the anode voltage swing is almost constant certainly be a troublesome transmitter. and has a peak value considerably lower than that with anode modulation. The 100 kW HF predictions came true It will be recalled from [3] that problems with linearity and Only about twelve of these 100 kW models were built and their correction were discussed and the dynamic drive even RCA engineering staff regarded it as an unmitigated regulator chassis used by RCA was explained. From the disaster. One engineering prototype was built to prove the above it would appear that such an active corrector had concept but was sold by an over-enthusiastic sales not been employed by MWT. department before it had reached the state of being a Also, they were able to see the latest version of the RCA commercial product. As if this was not bad enough, the first 50 kW BTA-50H transmitter: units were shipped to Rawalpindi in Pakistan and Nigeria for customer installation with an RCA engineer attending We were shown in the laboratory, one of the latest model the commissioning process. This scenario made the 50 kW transmitters (BTA-50H) and were impressed by its inevitable field modifications even more impractical and simplicity and small size. Very special precautions had even worse, construction of the units was sub-contacted been taken on this transmitter to suppress RF harmonics out to an RCA subsidiary in Florence, Italy to reduce the to below –80 dB. Silicon rectifier elements, immersed in oil labour costs. Unfortunately, somewhere along the way,

August 2015 29 Issue 36 Signal the specification for brass hardware and fittings was lost offerings were TV cameras, videotape and domestic and the units were assembled with steel fixtures. This transmitters. There was little scope in the managerial world meant that an almost entire rebuild of the cabinets was for long protracted product development plans by required on site before their operation was anywhere near engineers. satisfactory. Other initial units went to Thailand with similar results and the fourth customer, Vatican Radio, had their Conclusion units extensively modified at the Camden, New Jersey With these stations just about to come on the air in headquarters of RCA before they were shipped. In all, 47 1965/1966 there only remains the final one which is major field modifications were implemented on the early Swingin’ Radio England and Britain Radio with two units, the end result being a fairly reliable and user-friendly transmitters on one ship. This will be described next time transmitter. As a result of this fiasco and huge loss of in ToTT and the fort-based Radio City will be examined as revenue and credibility, RCA management withdrew from some new engineering data has recently become shortwave manufacturing and the worldwide support it available. entailed. These transmitters were produced at a time when great changes were taking place within American industry, References the broadcast market, and especially within RCA. It is interesting to note that when the first prototypes were 1. D Porter G4OYX and A Beech G1BXG. Tricks of the made RCA was a turnkey broadcast supplier, Trade. Signal 2015, 35 (May), 21–23. manufacturing everything from microphones, consoles, 2. Henry Chireix in his paper High Power Outphasing tape and cartridge machines, studio-transmitter links, Modulation in the IRE proceedings, November 1935, compressors/line amplifiers, transmitters, antennas and no. 11, Volume 23. even studio woodwork and cabinets. Within a few years of 3. https://ia600703.us.archive.org/13/items/bbc-rd- these transmitters being dropped, RCA's sole broadcast reports-1963-03/1963_03.pdf

The Radio Northsea International (RNI) ship, M/V MEBO 2 anchored off the UK and Dutch coasts from 28th February 1970 until 1974. The transmitter room with its BTH-100B 100 kW ampliphase equipment is pictured in Figure 9. Photo: Martin Stevens/Gerry Bishop

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