[AMPS] ?: Cooling

[David Kirkby]

Radio WC6W wrote:
> 
> A friend asked me this the other day... and, since I don't know, I
> thought I'd ask the list.
> 
>   If a 4CX250 (or similar) tube is mounted with the screen ring directly
> grounded by means of a flange (or perhaps finger stock) in a cavity, and
> with the anode cooler provided the recommended air supply, then is any
> additional cooling required on the base side, to keep the seal
> temperatures within specs?
> 
>   Or would conduction cooling from the screen ring to the chassis be
> sufficient?
> 
> 73,
>   Marv  WC6W
My guess (and I admit I have not tested it) is that such an arrangement
would not provide sufficient cooling. It would probably be fine for the
screen, but there is electrical isolation (and so thermal isolation)
between the screen and cathode/heater. The base seals will probably
overheat. 

Eimac did sell a base (sk660/sk660a if memory serves me correct) that
would allow sufficient base seal cooling with the socket mounted on the
chassis. This base used beryllium oxide (BeO), which is a reasonably
good conductor of heat, but an electrical insulator - its also
carcinogenic. These bases are no longer made, but you might just be able
to find one. They don't have the screen decouping caps built in - or at
least the one I have does not. 

On the whole, electrical insulators are poor thermal conductors, with
BeO and diamond being exceptions to this rule. 

There are many other problems if you ground the screen. You then need
negative supplies for cathode and grid, plus the heater will have to
float at around 350-400 V negative. 

If your reason for this is to reduce noise in standby, you might look at
the Y799/Y808/CCS1 and similar conduction cooled tubes. However, they
are a lot of hassle, and require a huge heatsink. Reducing fan speed
might be a better option. 

-- 
Dr. David Kirkby Ph.D,
email: drkirkby@ntlworld.com 
former email address: davek@medphys.ucl.ac.uk
web page: http://www.david-kirkby.co.uk       
Amateur radio callsign: G8WRB

--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com