When we developed the Broadcast Electronics FM30 and 30A transmitter
in the early 1980s, we found that customers were calling to tell us
that the little knob on the bottom of the Eimac 4CX20,000A/8990
tetrode was very oxidized and looked overheated when their tubes were
replaced. We looked into the problem and found that we had a dead
spot of stagnant air just there in the little pocket around the inner
filament contact area - an area we called the filament 'button'.
Despite that we had ~ 5 hp Cincinnatti PB series blower, we missed
that one spot. The lower input compartment was pressurized from this
large fan, and air routed through both the socket and several holes
in the deck (with EMI attenuating louviers) to pressurize the upper
anode area. Like the scheme mentioned in an earlier posting
(attributed to K2RIW), we fitted the upper cavity transmission line
resonator tightly around the anode cooler, so that it ducted the hot
exhaust out of the top of the cabinet. An insulating sheet of
Rexolite blocked the air from leaking out of the cavity above the
tube, so that it all had to leave through the anode. [The general
layout can be seen in the late Jim Aurand's US Patent 4,363,000
issued Dec. 7, 1982. There may be a way to view this patentent at the
USPO free online. Back to my story though.]
The solution was found by our clever mechanical engineer, in that we
installed a small PTFE pipe from beneath the amplifier enclosure,
which stuck up into this area under the tetrode base. It bled a small
amount of pressured are out of the amplifier (which was under
pressure, remember) to the atmosphere through the pipe, and that air
flow was enough to cool the filament 'button' to lower its operating
temperature and give acceptable long life. The heating was just due
to the filament power alone. So in this design, we achieved proper
cooling of the filament base with reversed airflow to the outside.
By the way, we fully instrumented the filament 'button' with a
thermocouple and floated the meter at the filament voltage, while we
energized and deenergized the filament and blower. What we found was
astounding, that the heat buildup under there would really creep up
even after the cooling fan was shut down. I think we might have
extended the filament cooling timer after that too, but my memory
escapes me on that. We later modified the transmitter to have a
larger blower with only half speed, to cut the noise level. Our
product had been accused (rightly so) of sounding like a jet airplane
starting up. Yet, we never had overheating problems at even high
elevation transmitter sites at full 30 kW output at 108 MHz.
I just looked at a new 4CX15,000A on my shelf here and it has a
series of small holes which are around the circumference of the inner
filament contact, to make sure some air gets moved around at the
ceramic to metal seals there.
73
John
K5PRO
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