That was a stupid design. Did the blower use two windings or two caps to
change the speed or just a resistor? In the resistor type just shorting out
the resistor would provide the same functionality AND provide a fail safe mode.
If the switch failed at least the fan would be providing air to the system.
K.I.S.S. is the way to go here.
Randy AB9GO
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bill Coleman
Sent: Tuesday, September 08, 2009 10:30 AM
To: Carl
Cc: David J Windisch; amps@contesting.com
Subject: Re: [Amps] 3CX800 and blower
Carl wrote:
> Yes, according to the Eimac spec sheet.
>
> It also sounds like a good reason to install a 2 speed switch as is in a few
> commercial amps with that tube.
>
> It can be as elaborate as a thermal switch that runs the blower full speed
> when necessary, to a simple toggle switch and a resistor.
>
> Carl
> KM1H
>
> ----- Original Message -----
> From: "David J Windisch" <davidjw@cinci.rr.com>
> To: <amps@contesting.com>
> Sent: Monday, September 07, 2009 10:26 AM
> Subject: [Amps] 3CX800 and blower
>
>
>
>> Does a 3CX800A7 require airflow any time the filament is lit, like, say,
>> 3-500Z glass tubes with tender seals and 70-watt filaments?
>>
>> I'm thinking of leaving the (unobtanium) blower turned off, to lower wear
>> and tear, until I need the power.
>>
>> Yep, I'm aware of the potential hazard of a senior moment: I have in mind
>> a no-blower-no-operate interlock scheme.
>>
>> Tia & brgds,
>> Dave, W8FGX
>> _______________________________________________
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>> Amps@contesting.com
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>>
>
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>
>
>
A quick story regarding a 2-speed switch... I had a 1st generation
TenTec Titan (pair of 3CX800s), its 2-speed fan switch is located on the
power supply. I regularly operate wearing headphones & one day was
running RTTY. I set the switch to high speed with my toe as the power
supply was on the floor under the op table. A while later after many
contacts I went to change bands, grabbed the bandswitch on the Titan to
find it extremely hot, the case of the RF deck was too hot to touch.
Yep, mid-way on the HI/LO fan switch was NO fan.
This never happened again as I wired around the switch to force high
speed all the time. I was very lucky, the amp is still around and the
tubes still going after 20+ years.
I still wear headphones all the time because my present amps make lots
of noise but both are protected by air flow switches. The switch in
one is a simple microswitch with a long actuating arm & a brass vane. No
big deal to fabricate.
I'm not a 'fan' of two-speed fan switches!
73, Bill N2BC
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