[Amps] Coupling a blower to an air system socket

[Carl]

----- Original Message ----- 
From: "Ian White" <gm3sek at ifwtech.co.uk>
To: "'Roger (K8RI)'" <k8ri at rogerhalstead.com>; <amps at contesting.com>; "'Paul 
Hewitt'" <wd7s at earthlink.net>
Sent: Sunday, March 17, 2013 4:05 AM
Subject: Re: [Amps] Coupling a blower to an air system socket


> >On 3/16/2013 11:10 PM, Paul Hewitt wrote:
>>> Greetings Ian
>>> Besides the lower back pressure advantage of blowing into the anode
>>> compartment, this method also cools the tank components.  This helps
>>> reduce themal tuning drift in very hi-Q tanks.
>>> 73, Paul
>
> That is also true - the complete opposite of layouts like the Dentron
> DTR-2000 where the 8877 blasts HOT air at the tank circuit.


How true but the MLA-2500, SB-220 family, L-7 and others do it the same.
With DTR-2000L 6M conversions I route the exhaust air thru a hole in the 
cover and it especially helps with a lot more B+.


>>
>>One thing to remember about external anode tubes. They  often have
> cooling
>>requirements listed as so many cfm at a given back pressure,
>>
>>I do not know of any way to achieve the required air flow at a reduced
> back
>>pressure other than an exhaust fan reducing the exhaust pressure thus
> making it a
>>little easier to get more cooling air through.
>>I believe Emtron and OM both use this approach on some models.
>>
>
> Back to physics. The only fundamental requirement for cooling the tube
> is the air flow rate in cfm. Data sheets also quote the associated back
> pressure, but that is not a fundamental requirement - it also depends on
> the manufacturer's chosen air-system layout. In Eimac data sheets this
> was always for blowing upward through the specified air system socket
> and chimney, which places all the flow resistances in series and greatly
> increases the back pressure.
>
> Some history: the original Eimac air system socket was designed for the
> 4X150A, which had  fragile glass seals.


I believe the  4-250A, 4X500A, and 3X2500A3 were the first socketed tubes to 
use forced air cooling and are in the 1946 catalog. In 1947 the 4-400A, 
4-1000A, 4x150A, and several other forced air tubes are listed.


The recommended method of
> blowing upward was entirely appropriate for the 4X150A because that tube
> certainly did need the full blast of air directly onto the base wafer.
> The 4X150A was very soon superseded by ceramic-metal tubes starting with
> the 4CX250 - but Eimac still specified the same cooling method.
>
> Even when faced with the evidence of the K2RIW and similar amplifiers,
> Eimac continued to insist that only their own specified cooling method
> would do, because the base seals of a metal-ceramic tube required just
> as much air flow as the anode. Until it suited them to change, that is.
> Enter the 8877 and the 3CX800, with a wafer socket that completely
> covers the base seals. In other words, Eimac's rejection of the K2RIW
> cooling method was a simple case of "Not Invented Here".


Does that surprise you one little bit Ian?

Carl
KM1H