Other than a humming box, that is. RCA broadcast, prior to the period in the seventies just before they went out of business, made some good-looking transmitters:
RCA BTA-10U AM transmitter
The Art Deco design was favored for a number of years, especially with the AM units:
RCA BTA-1AR transmitter, circa 1960
Some of these RCA transmitters are still in service as backups.
GE made the BT-25A, which was a 50 KW transmitter in Syracuse, NY for a few years. These units were very similar to the RCA BTA-50 transmitters.
GE BT-25-A looking from the control cabinet
Gates of Parker Gates, pre-Harris, also made some classic transmitters:
Gates BC1J AM transmitter
I remember the BC5P had a similar look, with more transmitter cabinets.
Bauer FB5000J AM transmitter
Fritz Bauer made a very solid AM transmitter. Good looking, too. We need more pictures of old transmitters and other hardware.
This transmitter is in service at WSBS, Great Barrington, MA as a standby. It was new in January 1975.
Gates BC250GY transmitter, WSBS Great Barrington, MA
This was running into the dummy load for testing, which we try to do periodically.
Gates BC250GY AM transmitter audio section
The audio section is a pair of 8008’s 810s running in parallel. This goes through a modulation transformer to the RF section.
Gates BC250GY RF section
The RF section consists of another pair of 8008’s 810’s running parallel. The plate voltage for these tubes is 1,250 VDC which is fairly tame, all things considered. The transmitter is dirt simple 250-watt carrier power, 125% positive peak capable. It is not the most efficient unit under the sun but it can still be repaired with off-the-shelf parts.
Gates BC250GY Schematic
This is a somewhat faded schematic. The schematic shows a single 833A as the final, however, this particular transmitter has a pair of 810’s for the final, as shown in the above picture. Ham radio operators love these things as they are easy to convert to 160 or 80 meters for AM phone use. The bigger brother to this unit is the Gates BC1G, which is also a pretty simple unit using 833A tubes in parallel with 3,500 VDC plate voltage.
They say the first thirty years are the hardest, perhaps it is true. This Harris MW1A transmitter turns 31 this year:
Harris MW1A AM transmitter, WINE, Brookfield, CT
Truth be told, these are not bad units. They have some quirks, however, the overall circuitry is simple, the design is simple enough and parts for repair are readily available. They require regular infusions of RF transistors, but those are easy to change and are inexpensive to buy off the shelf from places like Mouser or Allied.
It is on the air as the main transmitter for WINE-AM in Brookfield, CT. This is Harris’s first solid-state AM transmitter design, based on the work of Himmler Swanson. This is not a PDM transmitter, rather, each module has RF transistors and audio transistors. The output of all twelve modules is combined for a carrier output of 1,000 watts with +125% modulation. Harris calls this PSM (Progressive Series Modulation), which is sort of high-level modulation.
This is also the only transmitter that I know of where blown fuses can cause damage to the RF devices.
The RF output transistors and audio transistors are still available from Harris. Non-OEM parts for this include the 2N5038G for the RF transistors and the MJ15011 for the audio transistor. Inside the front of the transmitter is a row of incandescent light bulbs that glow increasingly as the various transistors go bad. At 1,000 watts carrier power, the ratio of PA volts to PA amps is 52.5/22.5 respectively. If that ratio is off by any measure, there is a problem.
The other thing with this transmitter is it is very sensitive to any kind of VSWR. Any change in the output impedance will quickly make itself apparent. My Harrisburg MW1A had two ATU settings, one for winter and one for summer. It was a slightly tall tower on 1230 KHz, thus any change in the ground system (e.g. snow cover) would upset the tower base impedance.
The other thing that goes bad is the large Rotron fan at the bottom of the cabinet. They go bad about every 10-15 years or so.
The owner has spent some money on this particular unit, rebuilding and replacing several modules with new transistors, etc. Will it last another thirty years? Depends on if the RF and audio devices remain in production.
I have been spending my days in Brookfield, Connecticut, dragging transmitters around and reconnecting them in various ways. The WRKI-FM WINE-AM transmitter site is finally moving into the “new” transmitter building at the base of the tower. Today, we moved WINE.
WINE was first signed on in 1963 on 940 KHz from a 170-degree non-directional tower on top of a pretty high hill. That same tower serves as the antenna support for WRKI, which signed on in 1957. The station runs 680 watts daytime, however since it is non-directional, it has some pretty serious power reductions at night. The post-sunset power drops in two steps, 450 watts for the first hour, then 189 watts for the second hour, followed by 4 watts nighttime.
The 4-watt nighttime signal goes about 2-4 miles before it becomes unlistenable. The Post Sun Set Authority (PSSA) allows the station to stay on the air with at least some coverage up to about 6:46 pm in the winter time and 10 pm in the summer, which is better than nothing.
The problem is, the Harris MW-1A transmitter goes down to 250 watts and no lower. In order to make the nighttime power, the station switches to a dissipation network to burn off 246 watts of RF, at 50% percent AC-RF efficiency, which just ends up being a waste of power. Further, the station engineers have been ignoring the PSSA because there are too many steps and it was easier to just switch to night power at sunset.
What we decided to do instead, was install a small low power night time transmitter, a Radio Systems TR-6000. The MW1A can then be set to use the low power level for the first step of the PSSA, then switch the dissipation network in for the second step of the PSSA, and finally switch in the night transmitter at the proper time.
Harris MW1A AM transmitter, WINE 940 KHz, Brookfield, Ct
This is the Harris transmitter, new Circa 1981, which was cleaned up and moved into the new transmitter building.
WINE Parallel dissipation network and dummy load
The dissipation network. This will have to be reconfigured for the proper power levels, once the night transmitter is installed. The dissipation network is on the right, a dummy load is on the left. The two large RF contactors switch the dissipation network in and out, or select which transmitter is feeding the antenna/dummy load. This is the really, really old school way of doing it. Most transmitters manufactured after 1990 or so can run at any power level, making a dissipation network unnecessary.
Before re-installing the dissipation network/dummy load, we lined the enclosure with copper mesh. I don’t want that thing interfering with any of the other equipment nearby, which would be the STL receivers, satellite receivers, or Town of Brookfield police dispatch radios.
Schematically, it looks like this:
WINE 940 KHz Brookfield, CT night time dissipation network
This is the picture behind the transmitters, which shows the coaxial cable feed through ports and the dissipation network on the wall.
WINE WRKI transmitter room, behind the transmitters
