On 3/16/2013 11:10 PM, Paul Hewitt wrote:
Greetings Ian
Besides the lower back pressure advantage of blowing into the anode
compartment, this method also cools the tank components. This helps
reduce themal tuning drift in very hi-Q tanks.
73, Paul
That is also true - the complete opposite of layouts like the Dentron
DTR-2000 where the 8877 blasts HOT air at the tank circuit.
One thing to remember about external anode tubes. They often have
cooling
requirements listed as so many cfm at a given back pressure,
I do not know of any way to achieve the required air flow at a reduced
back
pressure other than an exhaust fan reducing the exhaust pressure thus
making it a
little easier to get more cooling air through.
I believe Emtron and OM both use this approach on some models.
Back to physics. The only fundamental requirement for cooling the tube
is the air flow rate in cfm. Data sheets also quote the associated back
pressure, but that is not a fundamental requirement - it also depends on
the manufacturer's chosen air-system layout. In Eimac data sheets this
was always for blowing upward through the specified air system socket
and chimney, which places all the flow resistances in series and greatly
increases the back pressure.
Some history: the original Eimac air system socket was designed for the
4X150A, which had fragile glass seals. The recommended method of
blowing upward was entirely appropriate for the 4X150A because that tube
certainly did need the full blast of air directly onto the base wafer.
The 4X150A was very soon superseded by ceramic-metal tubes starting with
the 4CX250 - but Eimac still specified the same cooling method.
Even when faced with the evidence of the K2RIW and similar amplifiers,
Eimac continued to insist that only their own specified cooling method
would do, because the base seals of a metal-ceramic tube required just
as much air flow as the anode. Until it suited them to change, that is.
Enter the 8877 and the 3CX800, with a wafer socket that completely
covers the base seals. In other words, Eimac's rejection of the K2RIW
cooling method was a simple case of "Not Invented Here".
All the work with has been done to show that the K2RIW cooling method
does provide more than adequate cooling of the base seals. I and several
others still have the temperature sensitive paints from the 1970s, and
thousands more amplifiers using this cooling method have been built over
the following 40 years.
OTOH is relatively easy to raise the back pressure required for a
given flow with
obstructions
73
Roger (K8RI)
It's very easy to raise the back pressure, but not so easy to notice.
Amplifier builders have a good understanding of "resistance" in
electrical circuits, and most of us can understand "flow resistance" in
hot-water circuits as well. Air flow circuits really aren't all that
different - the fundamental requirement is to drive a sufficiently large
current of air through the tube.