>
>Hi Chris,
>
>You are quite correct. I took a closer look at my L-7 last night, and
>indeed the fan
>is sucking air out rather than blowing it in. When I first looked at it, I
>guess I just assumed the air was moving the other way. Also, it looks like
the only
>openings in the L-7 chassis, are the fan outlet on the back and the air inlet
on the
>bottom under the tube sockets. So indeed, the L-7 chassis is pressurized
(negative
>gauge pressure in this case), whereas the chassis on the SB-220, if memory
serves
>correct, is not (I
>believe the case on the Heath is made with perforated sheet metal).
>
>Still seems like the L-4B air system would be more effective with the
>glass chimneys and the blower, but I can't say for sure as I have never done
any of the
>calculations.
? Measurement of surface temperature with an IR thermometer is simpler
than esoteric calculations that depend on fairly sophisticated
measurements. I trust hands-on measurements more than I trust
calculations because I have attended VHF antenna gain contests where the
calculated gain and the measured gain were rather different. // The
simplist estimation of surface temperature is the tube's marking ink.
Red or green ink turns toast brown at high temperature. In a typical
SB-220 (blowing fan) or TL-922 (sucking fan), the marking ink on the
3-500Zs is still in 'mint' condition after even a decade of use. However,
in amplifiers which use an 'air-system' socket and a centrifugal blower,
the marking ink typically turns brown after a few months of use. At
even higher temperatures, the toasted ink begins to disappear.
cheers, Mike.
>Another exercise for the student (me) I guess.
>
>73 de Mike, W4EF........................................
>
>is made with ----- Original Message -----
>From: "Chris Pedder" <g3vbl@netcomuk.co.uk>
>To: "Michael Tope" <W4EF@dellroy.com>
>Sent: Thursday, May 24, 2001 10:55 AM
>Subject: Re: [AMPS] This guy is gotta be kidding!
>
>
>> At 21:31 23/05/2001 -0700, you wrote:
>>
>> >Chuck,
>> >
>> >Also notice the cooling. The L4B uses a squirrel cage blower to
>> >pressurize the chassis and force air up thru the tube sockets to
>> >a set of glass chimneys. The L-7 uses a muffin blowing air
>> >horizontally across the glass bottles as is done in the Heathkit
>> >SB-220.
>> >
>> >Mike, W4EF...............
>> Mike,
>>
>> I'm not at home with my L7 but I don't think this is entirely true.
>> Although I think the L7 cooling is less good than the L4B, it's not quite
>> as you describe/imply. The air is drawn past the tube pins and through the
>> socket before being exhausted through the fan at the back.
>>
>> Love my L7 and wouldn't mind getting hold of an L4B but they are not too
>> common in U.K.
>>
>> Best wishes,
>>
>> Chris
>>
>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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