Peter Chadwick wrote:
> If you figure that Eimac didn't sell chimneys for 4-400s because they
>weren't needed, you can reach the conclusion that they were meant to be
>blown from below.
That may have been the original intention, but other cooling methods may
work as well.
The "external anode" tubes have an interesting history as regards air
cooling. Eimac developed the SK-600 air system socket and chimney for
the 4X150A - their very first "external anode" tube - because the
glass-metal seals in the base would crack if they weren't blown directly
from below. The SK-600 and its later variants continued to be used and
recommended for the 4X250B (glass anode seal but ceramic base seals) and
later the 4CX250B, 350A etc. The air system sockets for the bigger
ceramic-metal tubes continued the recommendation for blowing from below
the chassis.
Meanwhile, some users of these tubes and sockets had developed
alternative air flow systems - in particular, the system popularized by
K2RIW. This blows the cold air into a sealed anode compartment, with the
tube chimney on the *top* of the anode cooler and leading directly
outside. The main air flow goes upward through the anode cooler and out
of the cavity, while 20-30% goes downward past the base seals and out
through a vent hole in the grid compartment. This has several advantages
over blowing upward, notably a much lower back pressure on the blower.
When blowing from below, the air flow resistance through the base is in
series with the flow resistance through the anode, so the back pressure
is quite high. In the K2RIW system, these two flow resistances are in
parallel, so the back pressure is much lower. The blower has to deliver
an extra 20-30% of volume flow to cool the base, but the reduced back
pressure means that the blower can supply *much* more air, so in most
cases you actually end up with more air through the anode cooler as
well.
The K2RIW system has one disadvantage: the base no longer receives the
full air flow, directly onto the seals. Instead it only receives part of
the total air flow, going downward past the seals. For that reason,
Eimac did not recommend the K2RIW system.
But consider this: the later Eimac pin-based tubes such as the 8877 and
the 3CX800 use a ceramic wafer socket that totally covers the base
seals... so maybe it isn't quite the engineering no-no that we were led
to believe.
And also consider the economics: if Eimac had decided that a wafer-style
socket would have been OK for the 4CX250B, they'd have had to scrap the
existing production line for the SK-600 series, and lay out money to
develop a new, cheaper and less profit-making alternative. Regardless of
the strictly technical issues, it's hard to imagine that was ever gonna
happen.
However, let's not be too hard on Eimac. That was all a long time ago,
still in the era when chief engineers wore bow-ties, smoked cigars and
made snap judgements that were never to be questioned again. Meanwhile,
over the past 30-40 years, amateurs and other professional engineers
have used the K2RIW system with great success, and it has become the
standard method of blowing VHF/UHF amplifiers with enclosed anode
cavities. Provided that enough air is bled downward past the base, the
tubes work just fine and the *measured* seal temperatures are well below
the allowable limits.
Coming back to the original topic, I'm certainly not suggesting the
K2RIW method for any glass-based tube - the allowable temperature rise
is too marginal. But don't automatically assume that the cooling methods
for which Eimac sells hardware are the *only* ones that will work. That
clearly isn't the case.
Rich has publicized the cross-flow cooling of the SB-200/220 amp, where
the bottom edge of the big fan blows air directly at the filament seals.
This isn't an easy retrofit for other amps, but today we have another
alternative - we can easily get very small muffin fans that will fit
edge-on inside the chassis, and can easily be positioned to blow
directly at the hottest problem parts. These small fans can work wonders
in all kinds of equipment by eliminating pockets of stagnant hot air.
--
73 from Ian GM3SEK
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