HomeBRU FM-30K Transmitter
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
    and control for the tubes. An hour meter is included. A Bird Wattcher is
    used. Below that is the driver metering. Ep, Ip, Ig is metered. The BE FX-30
    exciter, and a Moseley 1600 RCU is below that. The driver amplifier and power
    meter is next, and three shelf-mounted power supplies are at the bottom.

    The center rack has the final amplifier metering and control at the top. Meters
    are for grid voltage and current, screen voltage and current, and power amplifier
    voltage and current. Buttons for on/off, raise/lower and O/L reset are used.

    The rightmost cabinet is the plate voltage supply. It's a standard choke-input
    three-phase supply. Three individual transformers are used for ease of service.
    The plate contactor is a 300 amp GE, and like most relays, was purchased from
    Grainger. All parts were off-the-shelf whenever possible.


    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,
    but improves regulation. It also bleeds the HV capacitor in under a second.

    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.
    Running at somewhat less than maximum power allows a final tube to last for
    over three years.


    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
    twist lock plug are what hold it in. It's rubber mounted for low vibration.

    On the right center is the filament transformer. It was custom-wound to limit
    inrush current to the cold tube filaments on startup. The conductors to the bolt
    connections on the cavity are made of laminated copper strip.

    A 4 inch copper strip runs across the bottom and up both sides for grounding.
    It's bonded to the power neutral, and the RF transmission line.


    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
    voltage.

    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
    plate supply. All are on slide-out rack chassis, and are connected with twist-
    lock plugs and easy disconnect barrier strips. This allows repairs to be done
    on a bench, rather than trying to stand on your head to reach.

    All three supplies are three-phase to take advantage of the better regulation
    and lower ripple. The screen supply is fed through a three-phase motorized
    variac to provide power control for the transmitter.