The dissipation spec for a tube is what it is. cooling doesn't change
it. Cooling matters for the pin seals.
The mfr. spec. for 3-500 pins is a bit above 200 degrees C. (I can't
recall the exact temperature but I know the plate and bottom pins are
over 200 C.) That's pretty high for what hams normally do and one
reason why Ten Tec gets away with 80 CFM fans in the Centurion amps
for what is marketed as a SSB CW amp. They do de-rate it for RTTY
for 10 minutes at 600 watts. Yes it is true that they have a low
volume fan in there because it is quiet and there are so many idiots
who piss and moan if an amp is too loud--just read on-line Centurion
reviews where all these guys are raving about how quiet the Centurion
is--that they and other manufacturers are driven to this if they want
to make a product that hams will buy.
But there are other factors to consider too with that amp. The stock
h.v. p.s. uses a 500 ma. CCS transformer and the fan is positioned to
draw air in over it and blow it out through the RF deck. The current
spec. for the plate transformer is part of the reason for the 600 W.
continuous RTTY spec. I believe. As I recall if you do the math, 600
w. is right around 1/2 amp on the p.s. So you also help it when you
increase air volume. OTOH, the filament choke sits directly under
the tube socket deck right in front of the lower arc of the fan,
blocking a lot of air flow to the pins. Not good in my opinion, but
it is hard to do anything about it without increasing the length of
the leads from the choke to the cathode pins.
The air moves over the envelopes but it also moves over the anode heat
sinks and plate choke. It is a pretty good cabinet cooling design
except that as usual with commercially made amps, the cabinet is a bit
too small and the fan diameter could be more if you ask me. Of
course it is not as good as having the tubes in chimneys in a large
rack with a bigger 1 A plate transformer.
To the origninal questioner, I operate a Centurion on AM with 250 to
300 w. for lengthy transmissions (10 minutes or more) until I get a
plate modulated AM rig. I installed a 180 CFM dc fan and also opened
up the cover by cutting out the vents and covering the rectangular
holes with expansion steel. When the air is roaring through the amp
(and at 180 CFM it is pretty loud) you need to open up the cabinet so
it can freely move in and out. On a hot summer day the air coming out
of the amp hits 115 deg. F. in an 80 degree shack. Not too bad.
(MIne also has a Dahl h.v. transformer.). The graphite plates get
orange. You are probably okay with your new fan but I'd start out at
800 w. and keep an eye on things. Observe your plate current demand
and keep in mind your CCS spec. on your h.v. supply is your other
limiting factor and may be more critical than your tubes--I don't know
what kind of stock B+ tran they use now.
73
Rob
K5UJ
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