--- In ham_amplifiers@..., Bill Turner <dezrat@...>
wrote:
The only absolutely guaranteed measure of correct airflow is to
measure the temperature of the tube seals as specified by the tube
manufacturer, under full load for long enough for the tube to reach
max temp at that load. Any airflow beyond that is not necessary but
may give one peace of mind.
Bill, W6WRT
#### Well.. Eimac specs ceramic metal tubes for a seal temp of
250 deg C [482 deg F] On a triode... this is where the lower
portion of the anode just touches the ceramic "stem". The top part
of the anode can be much hotter. The issue is the seal temp on
the stem. The other concern is the seal temp where the fil
connection is located.
### I wouldn't say that just because XXX amount of airflow
provides say 249 deg C.... that that is good enough. Eimac bases
their airflow specs on 225 deg C.. allowing a 25 deg C "safety
factor". You also have to allow for dirty air filters,
partially clogged anode fins, etc. Most designers of commercial
gear [ real broadcast gear... not Ameritron ham stuff] will take
the Eimac pressure requirements... and add 20% right off the bat.
That will factor in ducting from blower to plenum... and also dirty
air filters, right angle bends etc.
### Then they have to allow for variations in inlet air temp. Very
few will use Air conditioning to cool inlet air on a hot day.
Then you have to factor in elevation. At 5000' you need 20+% more
air... and also 20+% more pressure. Then the last kicker is...
you have to procure a blower. Now blowers are rated at SEA level
only..... so one has to go through the various formulae
for "blowers for elevated locations".... to find a sea level rated
blower... that when de-rated.. will work at elevation.
### The tubes need XXX LBS of air per minute.... called mass
volumetic airflow rate. The higher you go... the thinner the
air... hence the MORE of it you need CFM wise... just to get the
same WEIGHT of air per minute.
### Now having said all that... hams are always worried about
noise... the real commercial stuff and engineer's who design it...
could care less about noise. Apparently on the AL1200-1500...
they all come with a 4 x speed blower. That should handle
everything from hardcore RTTY ops to casual ssb ops.... and also
handle seasonal variations from winter to summer.
### Broadcaster's have an entirely different scenario to deal with.
They don't RX... so local noise level is not an issue. They don't
qsy... so are stuck on one freq... only have one ant array to worry
about. The swr is always flat. It's called simplex. TX... NO
RX. Also no T/R relays to mess with, etc.
### personally, I wouldn't run any metal tube with a seal temp of
225 deg C... even if Eimac sez that's ok. That doesn't allow any
wiggle room when stuff goes wrong... like off resonance for hrs on
end....... or having enough air for ssb... but not enough for FM.
I like Robert Bonner's idea.... use a variac.. or other motor speed
control... and dial it up/down as needed. Some other scheme's I
have used are a resistor in one leg of blower.... and shorted out on
TX... so it's part of the T/R relay scheme. You don't need all the
air on RX.
Later... Jim VE7RF
------------ ORIGINAL MESSAGE ------------
On Sat, 11 Nov 2006 12:26:27 -0300, GGLL <nagato@...> wrote:
how do one determine correct airflow?.
