*** The Ampliphase Ancestry ***
The Smaller Ampliphases, BTA5J, BTA10J, BTA5L and BTA10L
The BTA-10J and 5J ampliphase were available by 1962. It was constructed into a single cabinet, although it was fairly wide at 44 inches (1120mm). The electronics were located in the centre of the cabinet with the metering and controls down both edges. The RF power stages were behind an upper hinged panel (which hinged upwards) and the modulator stages on a pull out drawer. The design concept was radically different than the existing 50G and 50H ampliphases in that a single phase modulator stage was used followed by a frequency doubler. The oscillator at 1/2 of the carrier frequency was built around a 6AK5 followed by a 5763 buffer. A 6BX7 acted as a phase splitter, utilising a lumped L/C delay line to obtain the initial phase separation. Phase modulation and frequency doubling was performed by a 6EA8 triode/pentode with audio applied to the triode grid. A 6CL6 followed as a buffer with an 829B as exciter output providing approx 40 watts of drive.
RF drive was provided by a pair of 4-400's in each channel operating from an 1800 volt supply. One was used as a straight forward classs C amplifier, but the second, which was electrically in parallel with the first, had its bias controlled by the drive regulator circuitry. Thus at modulation peaks both 4-400's provided drive to the 3CX10000 output, whereas at lower levels only a single 4-400 provided drive. This arrangement was similar to the drive regulator and linearity correction operation in the BHF-100 shortwave ampliphase. Drive regulation in the negative "out-of-phase" area was by means of a pair of 2K6 resistors connected between the PA grids, just as the KOH home built trasnmitter had in 1951! The final supply voltage of 7KV was originally obtained from six mercury rectifiers - (an unusual feature for 1962, considering the 50H has silicon rectifiers the year before) - though silicon diodes were used in later models, whilst the combining and output networks followed the conventional ampliphase design though the transmission line could be either 50 or 230 ohms.
In the late 60's/early 70's these smaller ampliphases were updated. These were designated the 5L and 10L and occupied a cabinet with three doors, approx twice the width of the 10J at 70 inches (1780mm). Over the years three variants of these were produced, the original "L" series, the "L1" and the "L2". Differences were fairly minor. All models featured the BTE-20 soild state exciter as standard. As this was designed to be adaptable to a number of ampliphase transmitters, the "L" models featured a solid state "pulse amplifier" board between the exciter and the RF driver stages. This pulse amplifier also seems to have the added feature of taking the drive 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 hence no additional drive regulation or linearity control was required. The RF driver stage was an 8122 running at 2KV, with two in parallel for the ten kilowatt model. The output stages were 3CX5000 or 3CX10,000 at 5KV or 7KV. Extensive power supply switching in these models allowed the output to be switched from 10/5/1Kw and 5/2/1Kw.
The handbook for the "L" models is very comprehensive, and substantially more detailed than the earlier transmitters. It contains several pages on the theory of operation, and detailed mathematical analysis of various stages in the process, particularly the output network. This information was conspicuously missing from earlier handbooks, and was no doubt added by RCA in response to the amount of criticism the ampliphase system had received from the industry.
I really appreciate all feedback to these pages. If you can shine any light on these transmitters, or spot an error in the above text, please let me know. I would also like some pictures of installed 5 and 10's if you wish to share them with the world. Thanks! My E-mail address is : "ampliphase [at] radio [dot] fm" or use the links to the feedback form. Thanks! |
