This transmitter was built at WBRU 1987-1988 by Craig Healy, with assistance from Greg Galer, NN1P. It uses a Broadcast Electronics exciter, an Eimac 750 watt amplifier for a driver, and an Eimac 30 kilowatt amplifier for the final stage. It was built entirely in house in the WBRU tech room, tested, and moved to the transmitter site in East Providence, RI in April of 1989. A smaller 1.5 kilowatt auxiliary transmitter was built soon after. These photos were taken by Greg during construction.
Front view of transmitter showing the exciter and driver stages in the left|
rack, the final amplifier in the middle, and the power supply on the right.
Metering for the three phase AC is top left, below is the filament metering
The center rack has the final amplifier metering and control at the top. Meters
The rightmost cabinet is the plate voltage supply. It's a standard choke-input
The power supply section is built on a custom heavy steel chassis. It's more
than strong enough to hold the three 250lb. transformers, the 55lb. contactor,
the 100lb. choke, 25lb. capacitor and other parts. Three separate transformers
were custom-wound by Peter Dahl Co. in Texas. They have been trouble-free.
The power supply is capable of over 4 amps at 11,000vdc continuous.
This supply includes a large bleeder resistor. This does lower overall efficiency,
The rectifier stack is a unit manufactured by Peter Dahl Co. No failures.
The Plexiglass exhaust chimney is a bit hard to see in the photo, but it ducts
hot exhaust air from the final amplifier cavity outside the transmitter. Inside
the chimney are two resistors used as surge supressors in case of a catastrophic
failure. And, the voltage divider ladder for the final plate voltage meter.
These resistors are cooled by the air ducted through the chimney. Temperature
at the outlet on top seldom goes over 120 degrees F.
The chimney is easily removeable when the final tube needs to be changed.
Visible here are the blower and secondary air filters. These are heavy-duty
automotive air filters made for a Chevy truck. They provide a better cleaning
for whatever gets past the four furnace-type filters on each end of the transmitter.
In the many years this has been running continuously, only a minor bit of dust
has collected inside the final cavity. A manometer measures the pressure drop
across these secondary filters.
The blower itself is a 2hp unit from Grainger. Four bolts, an air duct, and a
On the right center is the filament transformer. It was custom-wound to limit
A 4 inch copper strip runs across the bottom and up both sides for grounding.
In the upper part of exciter/driver cabinet can be seen the backside of
various meters, the filament contactor and two variacs for filament voltage
control. As is common practice, the tubes are run about 5% lower filament
The back of the Bird Wattcher, and the top of the BE exciter are also visible.
This shot of the lower part of exciter/driver cabinet depicts the back of the
driver amplifier and fan. This was set to pull air through the driver, rather
than push. It was more a function of where the fan could be mounted and still
allow it to slide out on the rails.
Under that are the final bias supply, the final screen supply, and the driver
All three supplies are three-phase to take advantage of the better regulation