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Five and Ten
Ampliphase Ten

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RCA Plate Modulated Transmitters BTA5/10-G/H

The MV Ross Revenge, home of Radio Caroline from 1983 onwards is fitted with two smaller transmitters, as well as the Ampliphase fifty. These were also manufactured by RCA, Types BTA-5G and BTA-10H. I am unsure as to the exact manufacturing date of these, but 1954-1955 would be roughly correct.

Filaments glowing - December 2000

Despite being two different powers and different models the two are almost identical. The principal difference between the five and ten is an extra RF PA tube in the ten, plus an upgraded driver stage in the modulator. The modulator output stage is similar as is the final voltage. The plate and modulation transformers are also upgraded. The "G" and "H" suffixes are also similar, with just a few minor component and circuit changes to the later "H" models. Thus there are four main models in this family - plus another variant which came to light recently known as the BTA10K (see below)

An unusual design feature is in the high voltage supply which use "Thyratron" rectifier tubes - these are a thermionic version of the nowadays familiar thyristor or silicon controlled rectifer. These tubes are fired by a rotary pulse transformer which can be adjusted to alter the phase relationship between the high voltage AC and firing pulses. This allows the high voltage, and thus the output power to be adjusted from zero to the maximum of 4750 volts. As the tube is basically a mercury arc rectifiers, they are reputed to radiate a blue/purple neon glow when operating, which fluctuates according to the programme modulation. Unfortunately, I never had the opportunity to witness this for myself.

The transmitters are also well known, and remembered, for the "Accordian" style sliding doors which were prone to frequent jamming and other problems. The door mechanism incorporates both an electrical and a shorting mechanical interlock to prevent the high voltage being activated unless the door is fully closed. A version with conventional hinged doors, which appears to be otherwise electrically identical was marketed in Canada as the BTA-10K.

The layout from the handbook

The four cabinets, left to right, house the audio stages and modulator; RF PA stages; Oscillator and RF driver; and rectifier/control. The plate xmfr is in a separate steel cabinet normally to the RHS. For the ten variant, the modulation transformer also moves to an external steel cabinet.

Before getting into technicalities, it is worth taking a look at the history of these units onboard the Ross Revenge. When the ship first went on air in 1983, the ten was a standby for the fifty and was tuned to 963Khz. The five had been extensively stripped before it was obtained by Caroline, and was non-operational. It was put onboard as a source of spare parts. In December 1984, when the fifty on 963 was leased to Radio Monique for the Dutch programmes, the ten was retuned to 576Khz, (558 as from November 1985) to broadcast the English language programmes. As a single service I believe the fifty was operated at approx 35Kw, but when diplexed the powers were 27Kw and 7Kw.

In February 1985, an identical ten kilowatt was obtained from South Coast Radio in Cork (through DJ/engineer Johnny Lewis), and this was stripped down on deck and used to upgrade parts of the ten and then to rebuild the five, which was in turn used as a standby for the revenue earning Dutch service. This five was later modified for shortwave use, and in 1988/89 it broadcast the programmes of World Mission Radio on 6215Khz. Following the raid of August 1989, where the fifty was greatly dismantled, but not seriously damaged, the ten was extensively smashed, but the five was left in a repairable condition. This was subsequently rebuilt by Peter Chicago, retuned to 558 and put back on air within six weeks. At that time the power was only about 700 watts, but throughout the spring of 1990, this was brought upto about 7.5 KW. Not bad for a smashed five KW transmitter!

It appears that the ten was previously operated by the now defunct KAKA radio (1110khz), in Arkansas - although I have been informed that this station did not go on air until the early 1980's, in which case it is seems unlikely they disposed of the transmitter after just a couple of years. The five possibly came from WGMS in Washington DC on 570Khz. This station now broadcasts as FM only, but has an interesting history page. This transmitter was taken out of service in early 1976, it's whereabouts from then until 1982 is unknown. During its 20 or so years at WGMS, it survived a fire which destroyed the studios, a plane crashing into the mast, and was listened to by Pres. JF Kennedy. I have no information about the ten from South Coast. Can you help fill the gap? -if so please contact me at the address at the bottom of the page.

Pictures on this page are of the 5 kilowatt ex-shortwave transmitter. The Ten is extensively damaged, and during 1990 many of its remaining parts were stripped in order to keep the five running. See here for a rather sad picture of the remains of the ten.

The 807 Oscillator block.

The 833A triode driver stage.
The crystal oscillator block is the same 807 based unit as used on the ampliphase and many other transmitters of the time, its design dates back to the 1940's. There are two oscillators to ensure maximum reliability, with an unusual changeover mechanism. The two outputs are simply connected in parallel via a pair of 2k2 resistors, and a simple changeover switch grounds the cathode circuit of whichever unit is required. The standby cathode is left to float! The oscillator is followed by a pair of parallelled 807 buffers operating in a broadband circuit. The buffer stage drives a tuned grid circuit on an 833A triode driver stage, whose plate/anode is coupled into the PA stages. The output stages are type 5762 triodes - two are operated in parallel for the five, and three for the ten. The 833 is powered from the low voltage supply of 1700 volts, via a tapping on the modulation transformer. Applying low level modulation to the driver stage improves the distortion and efficiency - the grid drive to the PA's is controlled by the modulation, ensuring the drive is adequate on modulation peaks, and is not excessive on troughs.

Audio is applied via the input isolation transformer to the grids of a pair of 807's. The whole audio chain is operated as a symmetrical balanced chain. The 807's drive into the grids of a pair of 828 pentodes, which, on the five, drive a pair of 813 pentodes. On the ten, four 813's are used, with two in parallel on each arm of the chain. The cathodes of the 813's are used to drive the grids of the modulator output stages, a push-pull pair of 5762 triodes - the same as the PA's. The anode load for these is the modulation transformer primary, which is fed from the high voltage 4750 line on its centre tap.

The modulator cabinet. The output stages are on the lower shelf where they are air-cooled via ductwork. Above these the 807 input stages in the centre of the row, and the 828's, glowing brightly on the outside of the row. Above the metering panel are the 813 driver stages. Note that two are fitted, but there are spaces for another two in the ten kilowatt model.

A simple version of plate modulation would include the secondary of the modulation transformer in series with the high voltage supply to the PA plate. Although this works in simple designs, the secondary winding will need to carry the full DC current of the PA's without saturation or other unwanted effects. The form of plate modulation used on these transmitters is of the "Hiesing" variation, whereby the modulation transformer is actually in parallel with the DC supply to the PA, but separated from the power source by a modulation choke. To prevent the transformer forming a DC short across the supply, an AC coupling capacitor is included.
See here for a diagram showing Hiesing and Plate Modulation.

A day/night power cutback can be installed. This consists of an additional rotary pulse transformer for the Thyratron rectifiers, and the necessary switching relays. The two pulse transformers are adjusted for day and night power respectively. A switched attenuator pad reduces the audio input level for the lower power mode.

Hundreds, if not thousands of this generation of transmitter were made by RCA, and hundreds of them are still out there, giving sterling service after nearly 50 years. Although many have been retired to standby positions there are stations who still use these as their main on air transmitter.

Thanks to Duffy Egan, CE of WSKO Providence for the above serial number plate.

And that's where we leave it for now folks. Keep watching this space for more updates.

As there is a copy of the handbook for the BTA5/10-U series of transmitters on board, a future page on this web site will describe these transmitters, which had an unusual 3rd harmonic resonator to "square" the PA drive and improve efficiency. Was this the first "Class D" amplifier?

Always Under Construction - This page created : January 2001 with occasional updates.