km1h @ juno.com wrote:
>On Wed, 25 Mar 1998 15:35:20 +0000 Mike Willis <firstname.lastname@example.org>
>>The K2 style cooling system has been used a lot with the 4CX250B. It
>>not provide an air blast to the base. Does this matter ? I have never
>>trouble with it in my W2GN.
>Not at the 500-600W level and low duty cycle. If you were to rebuild with
>Svetlana 4CX400A's and go for 1000W + output then the cooling system
>should be redone. The base heating does not change since the filament is
>the same but the overall heating does and gradually makes the complete
>assembly intolerably hot.
>If used for only a few short bursts then the original may be adequate.
The K2RIW cooling system DOES work with a pair of 4CX400As at 1kW
I tested it a few weeks ago in a 6m amp, using temperature-sensitive
paint at 200C and 250C. After prolonged key-down testing at maximum
rated condx, the 200C spots on the anode seals changed color, but they
did NOT reach 250C which is the maximum rated temperature.
The base and screen-ring seals didn't even reach 200C.
The base cooling requirements are pretty much the same with 4CX250s or
400As. The key to successful base cooling is to increase the sizes of
the outlet holes in the grid compartment and lower chassis. Sorry to
"shout", but this is CRITICAL!
The original K2RIW design had miscellaneous holes and slots in the grid
box which vented into the lower chassis, and then a 3/4in screened vent
out of the lower chassis. These airflow impedances appear in series, and
together they regulate the proportion of the air that goes down through
the base. K2RIW recommended one-quarter of the air going downwards, but
his 3/4-in hole covered with mesh has an effective area of only about
0.3 square inches - and that's not big enough! Various constructors have
used different hole sizes and different kinds of screening mesh without
realising they have a big effect on base cooling.
A grid-side exhaust hole of about 1.1 square inches OPEN area gives
about one-third of the air to flow downwards, and that's about right.
The ARCOS amps achieve this by a pattern of 90 small holes, each 1/8in
diameter, and I've run an ARCOS K2RIW at 1kW output for years on EME.
Take care if you use one large hole covered in mesh - small mesh
probably has an open area of only 50-60% so you'll need to increase the
hole size to compensate.
It's very easy to measure the relative proportions of volume flow that
are going up the chimneys and down into the base. Simply attach a thin,
light plastic bag to the anode air outlet and time how long it takes to
inflate from squashed flat. Then repeat the test on the lower air outlet
- the bag should take 2-3 times longer to inflate. (It's only a rough
measurement, so don't worry about small air leaks.)
Don't worry about the increased downward airflow reducing the anode
airflow. The pressure in the anode compartment is so low (about 0.25in
water gauge) that most blowers are operating in a near-level part of
their characteristic curve where they can deliver a lot more air with a
very minimal decrease in pressure.
A squirrel-cage blower with a 3.5in diameter wheel running at 2800rpm
(50Hz mains) is fine for a pair of 4CX250s at normal output, and it's
just about big enough for a pair of 400As at 1kW out. If you're using
60Hz mains, the increased blower speed will be a big benefit. For even
cooler operation, use a blower with a 4in wheel.
Finally, increase the blower inlet hole to 2in diameter because the
1.5in hole is a very significant obstruction - especially when covered
with mesh. (W2GN got a lot of things right when he re-engineered the
original K2RIW for the ARCOS line of amplifiers).
I'll probably put all this information on my web pages.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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