There are good reasons against using suction in an amplifier for
cooling. Any small holes, seams in the chassis will aid the egress
(or is is ingress?) of dust and dirt. By pressurizing the chassis
from an external blower, all dust accumulation can be handled at the
intake scroll of the blower, where a nice filter can do its job.
Pressurized systems are much cleaner inside than evacuated systems.
Most commercial broadcast transmitters have followed this practice,
although there are a few which haven't, and they are filthy inside.
I agree with Bob about the requirement to cool the seals, but 200 is
pretty darn hot for them. We had a bunch of RCA 7651 tetrodes running
at that temp (on the filament button contact) and they wouldn't last
more than 6 months in service. Using Tempilaq paints, I have tried to
get cooling to where I can be under 200 C, more like 150-175. Another
technique is to use a Luxtron fiberoptic thermometer to probe the
seals while it is operating. These don't come cheap however, and they
are very expensive to operate (>$500 per probe which are fragile
glass fibers).
At BE many years ago, we learned how to cool the filament stem of the
4CX20,000A/8990 tetrode without adding another blower. We just put a
small Teflon pipe pushed up under the tube, so that it surrounds the
center filament 'button' contact. This is the part which often
tarnishes black during operation, from the elevated temperatures.
This pipe then leaked out of the bottom of the amplifier to ambient.
It was a 'controlled' leak, so that a quantity of cool air (from the
pressure in the box) would blow out via the filament stem to the
outside. Completely separate from the anode air, which flowed up
through fins to the lower pressure upper cavity area where the plate
circuit was located. For the TH781 and TH628 (250 and 800 kW
dissipation tetrodes) I am designing with at present, have separate
blowers (LARGE) for the stem cooling. These are ~1 HP machines
themselves!
73
John
K5PRO
Mark the critical points that need to be met when cooling a tube is
to keep both the plate and cathode-heater seals below their maximum
ratings. Typically they are between 200 and 250 Degrees Celsius.
Sucking air from the top of the anode in most cases does not provide
sufficient airflow over the cathode connections to keep them below
their ratings. The water cooled variant of the 4CX10K requires 30
CFM to keep the cathode seals below their rating. You can get around
this problem directing a second source of forced air at the base.
For all of the extra trouble this entails it's more practcal to push
the main source of cooling air up through the base in the first
place.
Adios
Bob K1TA
Mark Beckwith <mark@concertart.com> wrote:
Could you wise old sages help me with simple question - I seem to recall
hearing that forced air cooling was not the same as, shall we say, sucked
air cooling. Do I remember right? Seems you could destroy your amp if you
tried to suck the air through the tube(s) tops as opposed to pushing it
through from the bottoms.
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