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RE: Hot Tubes (Was "Tubin

> Date:          Wed, 12 Feb 1997 23:25:26 -0700
> From:          Tesla List <tesla-at-poodle.pupman-dot-com>
> To:            Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject:       RE: Hot Tubes (Was "Tubin
> Reply-to:      tesla-at-pupman-dot-com

> Subscriber: robert.michaels-at-online.sme-dot-org Wed Feb 12 23:13:44 1997
> Date: Thu, 13 Feb 1997 00:06:28 GMT
> From: Robert Michaels <robert.michaels-at-online.sme-dot-org>
> To: tesla-at-poodle.pupman-dot-com
> Subject: RE: Hot Tubes (Was "Tubin
> 
> 
> [ ... ]
> T>am able to get a wonderfully bizarre brush discharge of five inches.
> T>These babies are a sensitve lot though. Ok, so now for the qustions: How
> T>hot is to hot for the 803's? I can run them for about two minutes before
> T>they get to hot to touch, this is with a 2400 Volt -at-~500mA (actually not
> [ ...]
> 
> T>David Trimmell
> 
> 
> 
>         How hot do like it, baby?  Most transmitting-class vacuum
>         tubes can =at minimum= run all day with their plates dull red.
> 
>         Some are designed to run with bright-orange-hot plates.
>         The general rule seems to be: if the plates are metal, then
>         stick with red-hot as a maximum.  If the plates are graphite,
>         then bright-orange is okay.   They can (and should) be air-
>         cooled (like with a fan) for max. life.   The higher power
>         tubes fit (and require) special ventilated sockets.
> 
>         Hey -- these aren't transistors, you know.   They date from
>         an era when men were really men, and you could literally
>         warm your lunch on your electronic equipment.
> 
>                                 In a fever,
> 
>                                 Robert Michaels
>

David, All,

Robert gives a correct generalization about *most* transmitting tubes 
with glass shells.  Another point which should be made is it is 
imperative to keep the temperature of the glass to metal seals where 
the plate caps and grid caps (if applicable) are located, below some 
specific temperature which will be available in the manufacturer's 
datasheet or a good tube manual.  Often it is recommended that a 
blower be installed to direct cooling aiflow directly upon these 
critical areas of the tube.  For the popular 833-A triode for 
example, here is a quote from the RCA manual.

"With forced air cooling, an air flow of 40 cubic feet per minute 
from a 2-inch diameter nozzle directed vertically on the bulb between 
grid and plate seals is required to limit the temperature between 
these seals to +145 degrees Centigrade."

In general, if such a tube is used somewhat below maximum power capability
with good natural convective airflow available around it, or at high power for
short run times at the low frequencies encountered in Tesla coil work, the tube
will probably be O.K. without a blower.

rwstephens