[Amps] tube blower time out

[John T. M. Lyles]

Arne, Rich,
I would agree with Rich's statement with the caveat "for tubes with 
glass envelopes and smaller
ceramic metal tubes". However, Eimac tubes like 4CX3500A and up 
benefit from having a 15 - 30 second blower run out period cooling 
the base of the tube. ( From postings here, I believe that some 
amateurs - as well as commercial amplifier builders - use larger 
tubes too.)

Years ago at BE, we instrumented the filament stem of a 
4CX20,000A/8990 in a 30 kW FM amplifier with thermocouple. When the 
power was switched off instantly along with air, the stem temperature 
remained high (sorry i don't have the quantities anymore). We 
designed all systems from then on with timers on the blower after 
filament off is done.

In all larger tubes with air cooling for the socket, I use longer 
periods of cooling. For the Thales TH555A for instance (ok it is a 
250 kW tube), the manufacturer recommended 20 minutes. I compromised 
to 15 for convenience. It is painful to convince people to wait this 
long before opening up the amplifier, but tube life is worth $$. For 
the water cooled BJ1643J2 450 kW industrial triode, no need for 
cooling the base with air, as it has water tubing braised to the 
socket connectors!

To answer Arne though, I would just set up at 555 timer or perhaps a 
CMOS one shot multivibrator (see logic books) to time out, holding 
the relay on the blower closed for a period after shut down.

73
john
K5PRO




>Date: Tue, 13 Jan 2004 04:38:00 -0800
>From: R.Measures <r at somis.org>
>To: "ARNE  GJERNING" <gjerning at flash.net>,
>	"AMPS REFL" <amps at contesting.com>
>Subject: Re: [Amps] Fan delay turn-off
>Message-ID: <20040113124018.ADBE43197FF at dayton.akorn.net>
>Content-Type: text/plain; charset="US-ASCII"
>MIME-Version: 1.0
>Precedence: list
>Message: 1
>
>**  Arne -- The only instance where a blower delay would be needed would
>be where the operator does not realize that 10 (or so) seconds of standby
>cooldown after transmitting on RTTY is a good idea before shutting down.
>
>>Been trying to find article on delay circuit to turn off blower after
>>filament mains switch is off.  I seem to remember QST had a circuit not long
>>ago but just can not seem to locate it.  Anyone remember which issue this
>>was in?
>>
>  >73 de Arne N7KA