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 24 August 2015 Signal Issue 36 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 August 2015 25 Issue 36 Signal 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.