On Sat, 6 Mar 1999 16:54:34 +0000 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>
>Carl wrote:
>
>G3SEK wrote:
>>>FWIW, I've been using Pyrex glass chimneys with 4CX250Bs for well
>>>over
>>>20 years. They were cut from stock tubing that was a close slip fit
>
>>>over
>>>the anodes, the ends ground square, and simply glued on to the base
>
>>>with
>>>silicone rubber. Works fine.
>
>>
>>
>>There is a big requirement difference between tiny little tetrodes
>and
>>large glass tubes and then going up to the 8877 and above level.
>>
>That's what "FWIW" meant: "for what it's worth", i.e. whatever comes
>after may not apply in all cases.
>
>>Too often hams do not understand the air requirements of their
>particular
>>choice and are mesmerised by misinformation and pure BS. Please
>also
>>understand that in many cases the published air requirements are for
>CCS
>>AM duty.
>>
>Slight change of subject here?
Not if one knows how to read spec sheets and differentiate between the
various modes and classes of operation.
.
>
>>Pyrex at the hard working 4-1000A level is a poor choice...period.
>
>So what material do/should they use for chimbleys for 4-1000As... or
>do
>you suggest not using a chimney and blowing across the tube?
I believe it has been sufficiently established that a quartz glass is the
type used in commercial chimneys.
At <1500W blowing across a 4-1000A may not be a bad idea. Not recommended
with the old military sockets tho. Trimming away every two cut-outs,
leaving one, etc., in the base also helps reduce air flow requirements.
Another problem with building a 4x1 amp today is availability of decent
tubes. It has reached the point that at flea markets you buy a pull of a
pull! Or the original NIB is 40 yrs old and a gas bottle.
One might as well consider a quad of 4CX250B's.
>
>> With a
>>4CX250B almost anything will work., even a fruit jar.
>
>>I try to keep things simple in my replies and reference common
>interests
>>and power levels; not useless 200KW tales and CB level amps. Those
>who
>>have a problem with that already know my answer.
>
>With a 4CX-anything, it had better be a fruit jar of exactly the
>right
>internal diameter, or else the air will bypass the finned cooler.
Since when do many hams bother with details?
BTW, check out the flea market trash bins for old GE Master Pro UHF TX
strips. The 8072 tube has a ceramic chimney that is identical to the
4CX250 version. The socket is the same as the 8874/3CX800 and the screen
ring can be used with the 250 series tubes OR shorted out and used as a
3CX800 grid ring. One dive into the dumpster yields hams gold !
73 Carl KM1H
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